A review of recent advances in the effects of surface and interface properties on marine propellers

Marine propellers are important propulsion devices for both surface ships and underwater vehicles. Increasingly severe environmental problems have required further performance enhancement for propellers. Nowadays, traditional methods to improve propeller performances through geometrical and structural optimizations have been extensively investigated, while the underlying mechanisms of the effects of surface and interface properties on marine propellers are still far from being fully understood. This paper presented a comprehensive review of recent advances in the effects of surface and interface properties, such as surface roughness and surface wettability, on marine propellers with an emphasis on the significant improvements in both hydrodynamic and cavitation performances, hoping to arouse more in-depth investigations in the field of surface/interface science and technologies on marine propellers, and also promote the state-of-the-art technologies, such as superlubricity technology, into practical applications.


Introduction
Marine propellers are the primary propulsion devices for both surface ships and underwater vehicles (such as submarines and torpedoes).The design and performance improvement practices for marine propellers have gone through a long history.From inventions of marine propellers with original structures burgeoned in the 19th century to the standard series data of marine propellers formulated nowadays, the structures and performances of marine propellers have been significantly changed and improved through centuries' effort [1].Nowadays, marine propellers have developed into several different structural configurations for different purposes (Fig. 1), such as for adapting to changing working conditions or for high efficiency and low noise performance.
However, due to the increasingly serious environmental problems such as climate change, global warming, and noise pollution, it is imperative for people to consistently seek new ways for further performance improvement of the propellers to reduce the impact on the environment.It has been reported that marine shipping alone makes up about 12% of the world's total transportation energy consumption [7], and the produced CO 2 emission will be increased by 50%-250% up to 2050 if not being controlled [8].As the propulsion work accounts for a large portion of the total effective work of a ship, the energy consumption of the shipping industry can be further reduced if the propeller propulsion efficiency η 0 is further improved.Apart from the energy efficiency, as ships and underwater vehicles are developing toward faster speed, the related noise pollution has become a substantially serious problem for both marine creatures and the staff on board [9,10].The noise emitted from a ship mainly originates from three sources, which are the machinery noise, the hydrodynamic noise, and the propeller cavitation noise, and propeller cavitation noise is often the dominant one in the high-speed range [11].It has been investigated that the overall sound pressure level | https://mc03.manuscriptcentral.com/friction(SPL) of a propeller can be 10-30 dB higher when the propeller cavitation is developed [12], which will not only do great harm to the marine environment but also pose a great threat to the sound stealth performance for naval vessels.Therefore, propeller cavitation and the radiated noise also need to be effectively controlled.
Traditionally, efforts to improve the performances (both the efficiency and the cavitation properties) of marine propellers have been focused on geometrical and structural optimizations.The blade number, blade area ratio, blade rake and skew, blade section geometry, and other geometrical parameters can all have influences on propeller performances.It has been found that the efficiency of the propeller decreases with the increase of the blade number, and that the cavitation characteristics can be enhanced as the total propeller thrust is shared by more blades so that the load per area can be significantly lowered [13].Similar to the effects of the blade number, the increase in blade area ratio can also improve the propeller cavitation performance while decreasing the hydrodynamic efficiency [14,15].Also, the overall blade rake or the tip rake have shown benefits in noise reduction [16,17], cavitation performance improvement [18,19], and efficiency enhancement [20].As for the blade skew, it has been found that the intensity of the cavitation decreases with the increase of the skew extent [21], and that the cavitation volume of a highly skewed propeller can be significantly reduced with the amplitude of the pressure fluctuations greatly attenuated [22].Also, by mimicking the wing section geometries of birds, the maximum efficiency of the propeller has been found to increase by 2.22% [23].Apart from the geometrical parameters to improve the propeller performance, the structural configurations can also influence the performance.The tip leakage flow in the ducted propeller and the pumpjet propulsor can induce significant efficiency loss and cavitation occurrence [24], and many investigations on tip structure modification to control tip leakage flow have been conducted, such as the C Fig. 1 Different types of marine propellers.(a) Base propeller (information on https://www.italianpropellers.com/propellers-marine/yacht-boat-propellers).(b) Controllable pitch propeller (CPP) is often applied to changing working conditions (information on SCHOTTEL website: https://www.schottel.de/en/portfolio/products).(c) Ducted propeller is often applied to heavy load conditions.Reproduced with permission from Ref. [2], © Elsevier Ltd. 2021.(d) Pumpjet propulsor is usually equipped on submarines or torpedoes.Reproduced with permission from Ref. [3], © The Author(s) 2018.(e) Tandem propeller can provide more thrust than the single base propeller.Reproduced with permission from Ref. [4], © The Author(s) 2010.(f) Contra-rotating propeller has better torque Q balance performance than the single base propeller.Reproduced with permission from Ref. [5], © Publishing House for Journal of Hydrodynamics 2010.(g) Rim-driven propulsor can reduce vibration noise and provide high efficiency.Reproduced with permission from Ref. [6], © Publishing House for Journal of Hydrodynamics 2012.(h) Podded propulsor can improve the maneuverability of ships (information on SCHOTTEL website: https://www.schottel.de/en/portfolio/products).
www.Springer.com/journal/40544| Friction groove on the rotor tip [25,26], the T-shape tip [27], and the overhanging grooves on the blade tip [28], which all have shown benefits in controlling the tip leakage effects.Moreover, biomimetic trailing edge structures have also demonstrated significant effects in noise control [29,30].
However, as the demand for the propeller design becomes higher and more stringent, traditional methods are meeting their top ceiling in further efficiency improvement and cavitation control.Therefore, finding new strategies for further performance improvement is of vital importance.As the hydrodynamic performance and the cavitation performance have close relationships to the surface/interface properties, such as surface roughness and surface wettability, surface/interface science and technologies may provide new ways for solving the dilemma.More specifically, hydrodynamic performance of the propeller is closely related to the solid/liquid interfacial viscous drag.Drag can originate from the marine biofoulings or the inherited viscous drag at the interface.Therefore, methods to prevent biofoulings and to reduce the viscous drag can greatly help to increase the efficiency of the propeller.Nowadays, various types of antifouling methods have been developed, such as the bioinspired antifouling methods [31] and the polymer-based antifouling coatings [32].Choosing appropriate antifouling methods for marine propellers can greatly reduce the energy loss and performance deterioration induced by biofoulings.Also, the superhydrophobic surfaces have shown significant drag reduction ability under both laminar [33] and turbulent [34] flow conditions, which are drawing more and more attention for underwater drag reduction, such as underwater vehicles, hydrofoils, and propellers, although the problem concerning the long-term stability of the air layer is still unresolved [35].Apart from the hydrodynamic performance, the cavitation performance has also been found to have a close relationship to surface/interface properties, such as surface roughness and surface wettability, and there have emerged several passive control methods for cavitation control in recent years [36].However, the practical effects and the underlying mechanisms on marine propellers are still waited to be thoroughly investigated.
This review aims to cover the state-of-the-art surface and interface technologies that are applied or can be applied to marine propellers, and provide a whole map of the effects and underlying mechanisms of solid-liquid interface properties on marine propellers.It will be developed as follows.Section 2 will firstly introduce the basic performances of marine propellers, and then introduce the fundamental surface and interface concepts related to the performance enhancement of marine propellers.Section 3 will be devoted to the effects of surface and interface properties on propeller hydrodynamics, mainly focusing on the antifouling coatings and superhydrophobic surfaces.Section 4 will center on the effects of surface and interface properties on propeller cavitation from three aspects, i.e., cavitation inception, cavitation development, and cavitation erosion.After that, Section 5 will focus on the challenges faced by the technologies toward practical applications and discuss the possible solutions and strategies.Finally, contents of this review will be summarized, and the conclusions will be drawn in Section 6.By providing those new ideas and technologies to the traditional and conventional field of marine propellers, this review is hoped to arouse more in-depth investigations on practical applications of surface and interface technologies to the field of marine propellers, and also to the fluid machinery field in the future.

Fundamentals of marine propellers and surface/interface properties
Hydrodynamic performance and cavitation performance are two fundamental and important aspects of marine propeller performance.Understanding those basic performance characteristics of marine propellers is the first step to understand the effects of surface and interface properties.Therefore, before going deep into the effects of the surface and interface properties, basic performance characteristics of propellers will be first briefly introduced, which will be followed by the illustration of the fundamental concepts of surface and interface properties related to the performance enhancement.

Fundamental performance mechanisms of marine propellers
Performance characteristics of marine propellers can be generally learned from three aspects, i.e., the hydrodynamic performance, the cavitation performance, and the noise performance.Hydrodynamic performance refers to the thrust T, the torque Q, and the corresponding propulsion efficiency η 0 produced by the propeller at a given advance coefficient J (J is the ratio of the advance velocity V a to the production of the rotation speed n and the diameter of the propeller rotor) [68].The thrust and Fig. 2 Sketch of velocity and force relationship on a propeller blade element.V a is the advance velocity, n is the rotation speed, r is the radius of the propeller blade, V R is the overall incoming velocity,  is the hydrodynamic pitch angle, and dL and dD are the lift and drag produced by the element, respectively.
torque generation mechanism can be referred to Fig. 2.Under the V a and n, there will be a lift force dL and a drag force dD acting on the propeller blade element δr, so the derivative thrust dT and torque dQ will be generated.By integrating dT and dQ over the whole blade, the total T and Q of the propeller can be obtained, and therefore the η 0 can be obtained by Eq. ( 1): It can be learned from Fig. 2 that the drag force contributes positively to the Q of the propeller while oppositely to the thrust of the propeller, and therefore it will act oppositely to the total efficiency η 0 .Usually, the practical η 0 of marine propellers is under 0.80, with a large portion of the energy loss resulted from the solid-liquid interfacial drag [69].In some cases, the drag related loss can make up about 20% of the total input power [69].Therefore, it is of vital importance to find new ways to reduce frictional drag on marine propellers to achieve further efficiency enhancement.
Cavitation refers to a state, where the pressure decreases to a critical value, and the liquid vaporizes into cavitation bubbles [68].It is typically characterized by the cavitation number 0  .
where 0 p is the reference pressure, and v p is the critical vapor pressure.Often, when a local pressure is under the v p , cavitation will occur, which is called cavitation inception.
Marine propellers can demonstrate four typical types of cavitation (Fig. 3).Cavitation can not only increase vibration and noise, but also induce blade surface damage and decrease the hydrodynamic performance [68].Therefore, there is an urgent need to prohibit the occurrence of cavitation or to control the cavitation extent.
In addition, the radiated noise of marine propellers should also be controlled.Detailed investigations on noise reduction methods of marine propellers have been reviewed by Ebrahimi et al. [73], which will not be covered in this review.

Fundamental surface/interface properties related to marine propellers
Two most widely investigated surface and interface properties related to marine propellers are surface topography and surface wettability, the concepts of which will be briefly introduced.Surface topography represents the geometry of the surface, and it is closely related to both the efficiency and the cavitation performance of the propeller.The most commonly used concept to characterize surface geometry is the surface roughness, which is represented by the profile of a cross section of the surface [74].Based on the cross-section profile (Fig. 4), both one-dimensional and two-dimensional topography parameters can be developed to characterize the surface.
One-dimensional parameters are frequently used in practice.Two of them are the arithmetic mean deviation R a and the root-mean-square deviation R q , which are defined by Eqs. ( 3) and (4), respectively: where the x axis is determined by equalizing the upper area and the bottom area, z(x) is the height at position x, l is the measuring length, N is the measured point number, and i z is the measured height at the ith point.
Apart from the height distribution of the surface profile, the irregular wavelength of the profile also needs to be determined, which is usually characterized by the arithmetic mean interception length of centerline S ma .S ma is the average intercept of the interceptions at the centerline within the measured length l, and in Fig. 4, S ma is the mean value of S 1 -S 10 .
It is worth noting that the one-dimensional surface parameters cannot define a surface geometry well.To better reflect the surface roughness and geometry, two-dimensional parameters are developed.One of them is the arithmetic mean slope a z  of the profile, which is the arithmetic mean value of the slope dz/dx in the l.By combining one-dimensional and two-dimensional parameters, the topography of the surface can be better described.The detailed effects of both one-dimensional and two-dimensional topography parameters on propeller hydrodynamic performance and cavitation performance will be discussed in Sections 3 and 4.  Surface wettability is related to the surface energy and is usually characterized by the contact angle  .In Young's equation [75], the  on smooth and homogeneous solid surfaces (Fig. 5(a)) is defined by Eq. ( 5): where SV  is the surface tension of the solid surface, SL  is the liquid-solid interfacial tension, and LV  is surface tension of the liquid.The solid surface with 90    is called a hydrophilic surface, and that with 90 Considering the existence of surface roughness, two wetting states are developed, which are the Wenzel state [76] and the Cassie-Baxter state [77], as shown in Fig. 5(b).In the Wenzel state, liquid can fully fill the grooves on the rough surface, while in the Cassie-Baxter state, there exist trapped air pockets on the surface, leading to the formation of liquid-solid-air composite interfaces.Normally, the Wenzel state is considered to present large contact angle hysteresis and sticky property, while the Cassie-Baxter state exhibits the behavior of low contact angle hysteresis and non-sticky property [78].Both surface roughness and surface hydrophobicity influence the wetting behavior of the liquid on solid surfaces.By combining low surface energy materials and properly designed multiscale surface roughness, superhydrophobic surfaces with a large  and a small sliding angle can be designed and manufactured, with great potential for antifouling and drag reduction [79].

Relationships between surface/interface properties and marine propeller performances
Surface and interface properties can impose significant influences on propeller performances.Two most frequently investigated surface/interface properties are surface roughness and surface wettability, and the effects can be divided into five subparts, as shown in Fig. 6.Two of them are closely related to the hydrodynamic performance of marine propellers, i.e., (1) the effects of biofouling and antifouling coatings and (2) the effects of surface wettability on interfacial drag reduction.Three of them are closely related to the cavitation performance of marine propellers, i.e., (1) the effects of surface roughness and surface wettability on cavitation inception (the occurrence of cavitation), (2) the effects of surface roughness and surface wettability on cavitation development, and (3) the effects of surface roughness, surface wettability, and some other surface and interface properties on cavitation erosion (the result of cavitation).
To better understand it, the effects of surface and interface properties can be learned from two aspects.
Firstly, surface roughness and surface wettability can greatly influence the hydrodynamic performance of marine propellers.As introduced in Section 2.1, the drag loss of the propeller makes up a large proportion of the efficiency loss, and it often originates from two aspects, i.e., the increased surface roughness by unevenly adhered and distributed marine creatures on propeller surface and the inherited viscous solid-liquid interfacial drag of the propeller surface when propeller blades rotate in water.For marine biofoulings, by implementing antifouling coatings or other techniques, marine biofoulings can be effectively Fig. 6 Effects of surface and interface properties on marine propeller performances.
www.Springer.com/journal/40544| Friction controlled [80].However, the effects of the implementation of antifouling coatings itself on propeller performances, whether being positive or negative, still remain to be explored.Different from the idea to reduce the viscous drag by eliminating extra surface roughness induced by marine biofoulings, the idea to control viscous solid-liquid interfacial drag, which originates from the no slip boundary wall condition and the viscosity of the liquid, is totally different.One idea is to apply different surface hydrophobicity on solid walls [81], and then the no slip wall condition can be altered into a slipping wall condition with different slipperiness.Then, the velocity gradient in liquid layer adjacent to the wall can be decreased, and therefore the shear stress and interfacial drag on the wall can be reduced.
Secondly, surface roughness and surface wettability can have great impacts on the cavitation performance of marine propellers.The occurrence of cavitation requires three necessary conditions: The first one is the existence of cavitation nuclei, the second one is the low-pressure environment, and the third one is a time period to enable the cavitation nuclei to grow and expand [36].For the first necessary condition, the formation and existence of cavitation nuclei are closely related to the surface roughness and surface wettability due to the significant effects of surface roughness and surface wettability on the process of heterogeneous nucleation [82,83].For the second one, the prediction of cavitation lies in the prediction of the minimum pressure, and the pressure distribution and the minimum pressure around an object have close relevance to the boundary layer on solid surface [84], which also has a close relationship with surface roughness.For the third one, properly designed surface roughness or surface micro textures can also prohibit the development of the cavitation in the time period investigated.In addition, the result of cavitation, e.g., cavitation erosion, also has a close relationship with surface and interface properties.By implementing proper surface processing techniques, the resistance to cavitation erosion can also be enhanced.
Despite the close relationships existed between surface and interface properties and propeller performances, effective modification methods and fundamental mechanisms underlying are still far from being fully understood due to the complexity of marine propellers.Unraveling the influencing factors and the underlying mechanisms of surface and interface properties on propeller performances will greatly accelerate the developments of more effective techniques to improve propeller performances.Based on the above analysis, the effects of the surface and interface properties, mainly surface roughness and surface wettability, on the five subparts of hydrodynamic performance and cavitation performance will be elucidated thoroughly in Sections 3 and 4.

Effects of surface/interface properties on propeller hydrodynamic performance
As stated in Section 2.3, surface roughness and surface wettability can influence the hydrodynamic performance of marine propellers, and the mainly related problems are marine biofoulings and the viscous interfacial drag.Section 3 will be divided into two parts.One is devoted to marine biofoulings, and the other is for the inherited interfacial drag reduction.For marine biofoulings, the effects of marine biofoulings on propeller performance will be firstly introduced, and then the complex effects and the possible mechanisms of antifouling coatings on marine propeller performances will be presented and discussed.
For the inherited interfacial drag reduction, the drag reduction mechanisms and effects of the frequently used superhydrophobic surfaces will be firstly presented, and then some representative results and underlying mechanisms of utilizing either the concept of slip or the superhydrophobic coatings on propeller blade surfaces will be explained and discussed in detail.

Effects of biofouling roughness and antifouling coatings
Marine biofoulings can lead to significant increase in power consumption and great loss in efficiency due to the increased surface roughness and wall shear stress [85][86][87].Apart from the biofoulings on ship hull surface, biofoulings and the corresponding effects on marine propellers can also be severe.Owen et al. [88] quantitatively investigated the effects of different biofouling extents on a propeller efficiency loss using | https://mc03.manuscriptcentral.com/friction the computational fluid dynamics method.They found that compared with those of the smooth condition, light slime and heavy slime of the biofouling can lead to a 3.88%-18.22%efficiency loss of a five-blade propeller for the advance coefficient J ranging from 0.6 to 1.2, and that calcareous fouling can result in a 11.11%-30.33%efficiency loss in the same J range.Sezen et al. [89] found that apart from the effects of thrust reduction, torque increase, and efficiency loss induced by biofoulings, the radiated noise of a four-blade propeller has also been significantly increased in a frequency range under 1 kHz, which is very detrimental to the sound stealth performance of the propeller.Continuous efforts have been made to control or eliminate biofoulings, and the most frequently utilized method is the employment of the antifouling coatings.With the increasing awareness of environment protection, the toxic tributyltin (TBT) compound coatings were gradually substituted by the tin-free self-polishing co-polymers and the more ecofriendly antifouling coatings, such as the fouling-resistance coating, the fouling-release coating, and the amphiphilic coating [90][91][92][93], which are mainly based on the surface wettability properties.For example, the frequently used fouling-release coating is developed based on the combined effects of low surface energy and low Young's modulus of the polymeric materials to weaken the interfacial interactions between the bio-foulants and the deposited surface, so that the bio-foulants can be easily removed under moderate hydrodynamic shear forces [90].
To fully evaluate the effects of antifouling coatings, Atlar et al. [94] have firstly carried out numerical investigations on the effects of different surface roughness on propeller hydrodynamics.They found that with the increase of surface roughness, the efficiency loss of the propeller can range from 3% to 6%.However, a fouling-release-coated propeller blade surface can be equivalent to a new or well-polished blade surface so that the efficiency loss can be eliminated.Furthermore, Candries et al. [95] performed experimental investigations on drag characteristics of two antifouling coatings, which are the tin-free self-polishing copolymers (SPC) and the fouling-release coatings, using a rotor apparatus.They found that both the application methods of the same coating and the type of the coating can affect the drag characteristics of the test cylinder.They found that compared with the smooth test cylinder, the average frictional drag increased by 4.3% for the sprayed fouling-release coating, 5.7% for the rollered fouling-release coating, and 8.0% for the tin-free SPC.After comparing the surface topography of the coatings (Figs.7(a)-7(c)), they found that although the one-dimensional height surface roughness parameter of the tested fouling-release surfaces is higher than that of the tin-free SPC surface, the fouling-release surfaces have more "open" texture with smaller a z  of the profile compared with the tin-free SPC surface, therefore leading to a lower frictional drag, which indicates that the surface topography has a profound influence on the drag characteristics.After a re-analysis of 46 different coated surfaces, Candries et al. [95] confirmed that by combining both the one-dimensional surface roughness parameter R a (the arithmetic mean deviation) and two-dimensional surface roughness parameter a z  (the arithmetic mean slope), and the new characteristic roughness parameter h = R a a /2 z  can better correlate the relationship between surface roughness and frictional drag of coated surfaces.
The above results have demonstrated the effects of antifouling coatings and the underlying mechanisms on drag characteristics of different antifouling coatings.However, the practical effects of antifouling coatings on practical propellers still need to be explored.Korkut and Atlar [96] experimentally investigated the effects of fouling-release coatings on the efficiency, cavitation, and noise characteristics of a commercial marine propeller.They found that the thickness of the coating can significantly influence the propeller performance, as shown in Fig. 8(a).The four-layer antifouling coated propeller (350 m thick) has shown extremely low efficiency, high noise levels, and unrealistic cavitation patterns compared with the uncoated propeller.However, for the three-layer antifouling coated propeller (250 m thick), although the employment of antifouling coating may lead to a slight 1% loss in efficiency, the extent of both the tip vortex cavitation and sheet cavitation of the coated propeller can be decreased compared with those of the uncoated propeller, as shown in Figs.8(b) and 8(c).Also, the propeller noise under low frequencies can be reduced under both uniform and non-uniform conditions, which is favorable for the overall sound stealth performance.
Furthermore, Bagheri et al. [97] investigated the effects of the fouling-release coating inner sleek 700 (IS700, which is a hydrophobic silicone) on a five-blade propeller, and they found that the coating can not only postpone the cavitation inception but also reduce the cavitation extent without obvious effects on the propeller hydrodynamic performance.More  | https://mc03.manuscriptcentral.com/frictionimportantly, the IS700 coating can reduce the noise level of 2-5 dB under conditions of non-cavitation, cavitation inception, and cavitation development conditions, especially in low-frequency regions (Fig. 9), and that the noise reduction rate under fully developed cavitation was slightly lower than those under the cavitation inception and non-cavitation conditions.In addition, scale effects of the thickness of the fouling-release coating on propeller performance were also found in Ref. [97].They found that the thrust and torque characteristics of the propeller were not significantly affected by the application of the three-layer coating with a 200 m thickness.However, a coating with a thickness over 300 m can lead to a propeller efficiency decrease.From the above results, it can be found that antifouling coatings on marine propellers can postpone the cavitation inception, decrease the extent of tip vortex cavitation and sheet cavitation, and also decrease the noise level under a low-frequency range, without significant opposite effects on the hydrodynamic performance when being properly applied.However, the underlying mechanisms are still not very clear due to the relatively few investigations carried out on this topic up to now [97].The possible underlying mechanisms may be related to both the blade section geometry and the surface/interface properties.On the one hand, the application of antifouling coatings may lead to the deviation of the blade thickness and the blade section geometry from the originally designed geometry, which are underserved for the hydrodynamic and cavitation performance.Also, this might be the reason for the scale effects of coating thickness.On the other hand, the wettability and the roughness of the antifouling coatings may also have contributed to the hydrodynamic and cavitation performance.As sheet cavitation often originates from the transition from laminar to turbulence in the boundary layer [96], antifouling coatings with special surface/interface properties may have helped to delay the transition in the boundary layer, and therefore delayed the cavitation inception [97].Also, as cavitation is one of the main sources of noise [97], delaying the occurrence of the cavitation and reducing the extent of the cavitation can help to reduce the noise level.At the same time, the viscoelastic property of the antifouling coating may also have helped to damp out the energy of cavitation, and hence reduced the noise level [96].However, detailed relationships are still not clear, and more in-depth investigations are still urgently needed.

Effects of surface slip and superhydrophobic coatings
With regard to the effects of surface roughness and surface wettability on the interfacial drag reduction, the superhydrophobic surface is one of the most representative methods.When water flows over a superhydrophobic surface, air pockets will be trapped in the micro/nano scale roughness on the superhydrophobic surface so that effective slip will


Over the years, drag reduction effects of superhydrophobic surfaces have been extensively investigated under both laminar and turbulent conditions.Detailed discussions can be found in two critical reviews by Lee et al. [33] and by Park et al. [34] for laminar flow conditions and turbulent flow conditions, respectively.Some representative research results are selected and listed in Table   it can be found that most of the drag reduction results are investigated on flat plate surfaces or under pipe flow conditions in the laboratory environment, while the practical effects of the superhydrophobic surfaces with large areas on curved objects under flow conditions of high speed and changing directions, such as on marine propellers, still need further investigations.
As stated in Section 3.1, research on the effects of coatings on marine propellers has been rare.However, in the recent five years, pioneering works focusing on the effects of superhydrophobic surfaces on marine propellers have gradually emerged.Investigations have firstly focused on the effects of superhydrophobic coatings on hydrofoils to investigate the underlying mechanisms under different working conditions.Lee et al. [107] compared the flow field of a superhydrophobic hydrofoil and a non-coated hydrofoil, and found that under the flow condition of intermediate angles of attack α, the flow separation on the suction side of the superhydrophobic foil can be delayed; while under the fully separated flow conditions, the effects of the superhydrophobic surface can be negligible.Sooraj et al. [108] also investigated the effects of the surface hydrophobicity on the flow field of the hydrofoil (Fig. 11), and found that at relatively low α (10°), the superhydrophobic coating can help to keep the flow attached on the foil, and that at middle α a Sample dimension denotes the characteristic dimension of the sample, such as the length dimension of the flat plate and the chord length dimension of the foil.b Reynolds number is defined as follows: V R denotes the incoming velocity, and υ denotes the kinetic viscosity of water; in | https://mc03.manuscriptcentral.com/friction(15°), the superhydrophobic coating can delay the flow separation, exhibiting a large drag reduction effect of 40% at Reynolds number of 30,800.However, at large α (20°), there has been no significant effect on the flow field due to the appearance of large flow separation region at the suction side of the hydrofoil, which is consistent with the results found in the experiments by Lee et al. [107].The underlying mechanisms of the effects on the flow field have been mainly attributed to the large u s at the superhydrophobic surface, which can effectively delay the flow separation and thin the boundary layer, thereby leading to a reduced wall shear stress and viscous drag.The above investigations on hydrofoils have laid solid foundations for the investigation of marine propellers.Katsuno et al. [109,110] numerically investigated the effects of different surface hydrophobicity (indicated by different ) on a three-blade propeller (Fig. 12(a)).They found that with the increase of the , the efficiency of the propeller gradually increases, with  = 100 m corresponding to a more than 10% efficiency gain, as shown in Fig. 12(b), which demonstrated the great potential for significant efficiency improvement of marine propellers through surface and interface technologies.However, it has been found that there existed a gain limit above the 100 m , which indicated that a  above 100 m may not be needed for marine propellers.Also, the increase in efficiency might be accompanied by an increase in the suction pressure as well, which may lead to enhanced cavitation.The above results indicate that a trade-off between the efficiency gain and the cavitation performance has to be evaluated when applying "slip" coatings.Later, Choi et al. [111] experimentally investigated the effects of the superhydrophobic coating on the wake flow field of a two-blade propeller (Fig. 13(a)) under low Re = 96,000 using the stereoscopic particle image velocimetry technology.They found that the wake turbulent kinetic energy (TKE, which indicates the strength of the turbulence) can be reduced for about 20% when the superhydrophobic coating is applied to both sides of the propeller blade surface, and that the TKE reduction rate can be influenced by the location and the wetting state of the superhydrophobic surface.They found that the TKE can be reduced when the superhydrophobic coating was either applied to the pressure side or the suction side, although larger TKE reduction has been achieved when the coating was applied to the pressure side compared with that for the suction side (Fig. 13(b)).Also, they found that although the Wenzel state (without air pockets, only the effect of roughness) can reduce the TKE in the wake, the reduction rate has been slightly less compared with that of the Cassie state (with both trapped air pockets and roughness) (Fig. 13(c)).These results indicated that flow structures in the wake were mainly affected by surface conditions of the pressure side due to the fact that the pressure side can push the water flow to the downstream direction.The TKE reduction effect has been attributed to the partial slip boundary condition at the blade surface, in which the slip boundary condition can decrease the near wall velocity gradient and reduce the accumulation of the vorticity, and then results in a weakened strength of the vortices.
More recently, Pan et al. [112] performed both numerical and experimental investigations on the effects of the surface hydrophobicity on the hydrodynamics of a three-blade propeller.They found that the hydrodynamic performance of the propeller was closely related to the surface slip rate (the area proportion of free slip surface to all the surface counted), advance coefficient, and the rotational speed.A relative 4.70% efficiency gain (equals an absolute 3.02% gain) has been achieved at a rotational speed of 1,200 r/min under J = 0.9 with a 75% surface slip rate.By coating the propeller with polyvinylidene fluoride (PVDF), the propeller with surface slip property can be obtained (Fig. 14(b)), and the flow field of it has shown less high-speed region downstream of the propeller hub (Fig. 14(d)) compared with that of the uncoated one (Fig. 14(c)), which might be the reason for the enhanced efficiency.
The above numerical and experimental investigations have demonstrated the beneficial effects of superhydrophobic surfaces on propeller performance and provided new avenues for further performance improvement.However, there are still many challenges waiting to be solved.Firstly, the stability of the trapped air pockets [113].In Refs.[107,108,111], the Reynolds numbers have been limited to the relatively  | https://mc03.manuscriptcentral.com/frictionlow magnitude of 10 4 .However, in practical applications, propellers often work at high Reynolds numbers of about 10 5 or even 10 6 .Effects of superhydrophobic surfaces on marine propellers under high Reynolds numbers should be examined.Secondly, most of the investigations are conducted numerically or under laboratory environment, which is still not comparable to the natural environment.Under natural environment, water may contain chemicals and particles and may also exhibit different temperatures, pressures, and salinity [34].These multi-aspect factors imposed by the natural environment may also influence the practical effects of superhydrophobic surfaces, which also need further investigation.Last but not least, the effects of superhydrophobic surfaces on the overall performance of marine propellers are also worthy of investigation [34].As indicated by Katsuno et al. [109], improvements in the efficiency of the propeller may deteriorate the performance of cavitation.Therefore, overall effects of superhydrophobic coating on propeller performances should be comprehensively evaluated, and the possible benefits or the losses should also be clearly clarified.

Effects of surface/interface properties on propeller cavitation performance
As mentioned in Section 2.3, cavitation is a complex phenomenon involving three processes, i.e., cavitation inception, cavitation development, and cavitation erosion, and they are closely related to each other.Cavitation inception is the very first occurrence of cavitation, which will happen under proper cavitation nuclei and low-pressure environment.After the inception, cavities will gradually develop and demonstrate different forms, as shown in Fig. 3, and some of the cavitation forms may also lead to cavitation erosion, which can be regarded as a result of cavitation inception and cavitation development.Therefore, the three cavitation processes are closely related to each other, and one factor influencing cavitation inception may accordingly influence the successive processes of cavitation development and cavitation erosion.Investigations on effects of surface and interface properties on cavitation characteristics have lasted for a relatively long time since the 1960s.However, due to the complexity of cavitation, in the early stages, www.Springer.com/journal/40544| Friction it can only be speculated that the surface condition or the material might be attributed for the observed unusual phenomena of cavitation [114].Only in recent years have people gone deeper into the relationship between surface/interface properties and cavitation characteristics, and methods in utilizing surface/interface properties for cavitation control have gradually emerged.Section 4 will focus on the beneficial effects of surface and interface properties such as surface roughness and surface wettability on the three cavitation processes, hoping to provide valuable insights for effective cavitation control and inspire new generation design of marine propellers.

Effects of surface/interface properties on cavitation inception
Cavitation inception is the first and foremost process of cavitation.The delay or prohibition of the cavitation inception is of vital importance for cavitation control and performance enhancement, which should be put more emphasis on.Effects of two factors of the surface/interface properties, i.e., surface roughness and surface wettability, will be discussed in Section 4.1.
Influence of surface roughness on cavitation inception has been considered to be harmful in early investigations.Holl [115] and Arndt and Ippen [116] have firstly investigated the relationship of surface roughness and cavitation inception.Holl [115] found that the isolated surface irregularities can induce significant pressure reduction and increase the incipient cavitation number 0  .Arndt and Ippen [116] found that the isolated roughness is more susceptible to cavitation than the distributed roughness of equivalent roughness height on a surface, and they correlated the incipient cavitation number 0  , as defined by Eq. ( 2), with the skin friction coefficient f C in the form of 0 , which means that a higher f C caused by the surface roughness will lead to a higher 0  and an earlier cavitation inception.Apart from the protruding roughness, surface crevices embedded into the solid surface have also been found to have a close relationship to cavitation inception.Originating from the crevice model proposed by Harvey et al. [117], studies on the role of surface crevices in cavitation inception have been continuingly carried out [82,[118][119][120][121].It has been found that the surface crevice can serve as a place for bubble nucleation [82,121], and demonstrate a self-excited cyclic property under a diffusion-driven shear flow field [118][119][120], which is considered to be one of the mechanisms of sheet cavitation inception on marine propellers [122].
The above investigations indicated that surface roughness can lead to earlier cavitation inception and should be avoided as much as possible.However, recent studies have demonstrated that properly designed surface roughness applied under certain flow conditions can also delay or prohibit the cavitation inception.Tao et al. [123] applied particles with a 60 m average diameter on the LE of both the pressure side and suction side of a thin hydrofoil to form a 4 mm wide surface roughness strip, and they found that although the sheet cavitation inception at low incidence angles (0°-2°) was triggered to occur earlier under the same conditions by the LE roughness, cavitation inception at relatively high incidence angles (4° and 5°) has been significantly delayed.This unexpected inception delay was related to the change of the boundary layer structure due to the existence of the LE roughness.Similar to the roughness applied by Tao et al. [123], Chen et al. [124] also adopted sand particles in a 60 m diameter to form roughness strip at the LE of the hydrofoil.However, the roughness strip was located at 4 mm downstream of the stagnation line on the suction side of the hydrofoil, which was 4 mm wide and 0.15 mm in height.They found that both the incipient 0  of the smooth hydrofoil and the roughened hydrofoil increased with the increase of the incidence angle, and that the incipient 0  of the hydrofoil with the LE roughness was always smaller than that of the smooth hydrofoil when the incidence angle ranged from 6° to 13° (Fig. 15), indicating a delayed cavitation inception due to the existence of the LE roughness.The possible mechanism has been speculated to be that the existence of the LE roughness has increased the TKE near the LE area and induced an earlier transition, leading to an increased local pressure coefficient and hence a decreased incipient 0  and uppressed cavitation.
Apart from the irregular surface roughness investigated above, surface roughness with regular patterns have also demonstrated the ability to postpone cavitation inception.Chen et al. [125] investigated the effects of a regular roughness pattern, i.e., micro vortex generator (mVG), on the cavitation inception behavior of the hydrofoil.They found that the location of the mVG with same geometric parameters can also influence the cavitation behavior.The mVG-1 located upstream of the laminar separation point (Fig. 16(a)) always promoted earlier cavitation inception in the investigated α (4°-12°), while the mVG-2 located in the laminar separation point zone (Fig. 16(a)) demonstrated cavitation inception delay with α in the range of 6°-8°, as partly shown in Fig. 16(b).By performing detailed computational fluid dynamics calculation and analysis, they found that the mVG-1 can not only create the fingerlike vortex inducing the vortex cavitation but also increase the  length of the laminar separation bubble (LSB) and decrease the surface pressure nearby, therefore leading to an earlier cavitation inception.Contrary to the mVG-1, mVG-2 can reduce the length of the LSB and increase the surface pressure around the LSB, thereby delaying the inception of sheet cavitation under α = 6°-8°.
The above investigations have mainly focused on the sheet cavitation inception.However, in practical conditions for marine propellers, tip vortex cavitation is often the foremost to occur.Prohibition of the tip vortex cavitation for marine propellers can obtain significant benefits in noise reduction and critical speed without cavitation, and should be taken with more consideration.Researchers [126][127][128] have found that the properly applied tip roughness can effectively mitigate the tip vortex cavitation.McCormick [128] found that by applying the surface roughness on the pressure side of a hydrofoil, the tip vortex cavitation incipient number can be reduced by approximately 20%.However, Küiger et al. [127] found that the roughness placed on the suction side has shown to be the most effective to mitigate the tip vortex for a propeller blade under the translational flow, and this effect has been attributed to the increased viscous wall-friction between the vortex and the roughened tip, which helped to reduce the angular momentum of the tip vortex and mitigate the tip vortex.Asnaghi et al. [126] and Svennberg et al. [129] have performed numerical and experimental investigations on the effects of surface roughness on the tip vortex cavitation of an elliptic hydrofoil.They found that the application of roughness on the LE, tip region, and trailing edge of the suction side has been the optimum pattern to mitigate the tip vortex and limit the performance degradation.The results demonstrated that the tip vortex inception number of the optimum roughness pattern has been decreased by 33% compared with that of the smooth foil, with drag force increased by less than 2%.Moreover, by optimizing the location of surface roughness, Asnaghi et al. [130] have successfully mitigated the tip vortex cavitation and decreased the 0  by 37% with merely a 1.8% performance degradation on a five-blade marine propeller in model scale condition (Fig. 17).In full scale condition, tip vortex cavitation can also be mitigated by 22% with a 1.4% performance degradation.
Similarly, Sezen et al. [131] found that surface roughness placed on the suction side of the propeller can help to mitigate tip vortex cavitation volume in both model scale and full-scale conditions.However, they reported a significant efficiency loss (5%-10% in model scale and 2%-5% in full scale) due to the existence of the applied roughness, which indicated that for practical applications, more efforts have to be put into the optimization of the surface roughness, such as height and distribution, to achieve a balance on cavitation control and the possible performance degradation.
Apart from the surface roughness, surface wettability can also have great impact on cavitation inception.Gupta [114] investigated the cavitation inception of several hemispherical-nosed bodies made of different materials, and found that the hydrophobic surfaces made of Teflon and polyethylene can lead to earlier cavitation inception, while the hydrophilic surfaces made of glass and stainless steel showed no contribution of surface nuclei to the onset of cavitation.This has been one of the relatively early investigations on the effects of materials on cavitation inception, and the   | https://mc03.manuscriptcentral.com/frictionresults were speculated to be related with surface nuclei induced by surface hydrophobicity, which can trigger early cavitation inception.Later, Belova et al. [132,133] confirmed through calculation that the energy barrier for bubble nucleation on the hydrophobic surface was much lower than that on the hydrophilic surface, and they found that the hydrophobic sites have been more intensely impacted compared with the hydrophilic sites.Also, by utilizing the high-speed imaging technique, Belova-Magri et al. [83] found that the bubbles were preferentially located on the hydrophobic areas instead of the hydrophilic areas on a hydrophobic/hydrophilic patterned surface, which provided the direct evidence that surface wettability can significantly influence the formation of cavitation nuclei.Later on, the experimental and numerical investigations performed by Onishi et al. [134] and Ezzatneshan et al. [135], respectively, have confirmed that the cavitation inception for the hydrophilic surface has been delayed compared with that of the hydrophobic surface.Hao et al. [136] also found that the material of the hydrofoil can significantly influence the cavitation flow.For cavitation inception, they found that the incipient 0  of the aluminum foil is the highest compared with those of the hydrofoil of stainless steel and the one painted with epoxy coating.Inspired by the relationship between the surface wettability and the cavitation inception, Ye et al. [137] have successfully suppressed bubble formation on a high-density polyethylene (HDPE) surface by coating a hydrophilic polydopamine polymer layer.Petkovšek et al. [138] have utilized the direct laser texturing method to modify the surface topography and wettability.They found that the slightly increased surface roughness induced by laser texturing can help to reduce the incipient 0  , and therefore delay the cavitation inception.However, significant increase of the surface roughness may lead to an enhanced cavitation intensity, indicating that there existed a proper range of surface roughness to delay and control cavitation inception.In addition, they found that the laser-oxidized surface can exhibit different hydrophobicity after exposed into the atmosphere environment for different time periods (Fig. 18(a)), and that the hydrophilic surface induced by laser texturing can help to delay cavitation inception (Fig. 18(b)).The above investigations have demonstrated the feasibility to delay or control cavitation inception by modifying surface topography and surface wettability.As demonstrated, properly designed surface topography and hydrophilic surfaces can help to delay cavitation inception.The laser texturing method can modify surface topography and surface wettability simultaneously, which may provide a fast modification strategy for cavitation control and function as an efficient way in cavitation inception delay for marine propellers.

Effects of surface/interface properties on cavitation development
Section 4.1 mainly talked about the effects of surface roughness and surface wettability on cavitation inception.However, in some conditions, such as the propeller behind a hull, the unsteady inlet flow field for the propeller can easily trigger cavitation inception and induce sheet cavitation or even unsteady cloud cavitation [139], which are hard to avoid and can lead to unwanted vibration, noise, and even cavitation erosion.Therefore, it is of significant importance to find methods for sheet cavitation or cloud cavitation control.Surface roughness is one of the key parameters that influence the cavitation development (mainly on the control of cloud cavitation).Cloud cavitation is a typical type of detrimental cavitation, and can be considered as the developed sheet cavitation with strong unsteadiness.Kawanami et al. [140] investigated the effects of surface obstacles placed on the hydrofoil surface on cloud cavitation.They found that the cloud cavitation was formed due to the collapse of the sheet cavity triggered by a reentrant jet rushing from the trailing edge of the sheet cavity, and that it could be controlled by the obstacle placed at proper location on the hydrofoil surface.However, the work of Kawanami et al. [140] only focused on the dynamics of cloud cavitation, while little effort has been devoted to the inception.In this condition, the macroscale obstacle placed on the surface may instead lead to earlier cavitation inception and increased drag, and therefore worsened the cavitation condition and the hydrodynamic performance.There have been several investigations concentrating on the suppression of cloud cavitation instabilities by placing MVGs on the surface.However, earlier cavitation inception was observed as well [141,142].Therefore, it is very important to find ways to effectively control cloud cavitation, while at the same time without worsening the cavitation inception behavior.
Hopefully, there have been several investigations focused on this area.Kadivar et al. [143][144][145] have done a series of numerical and experimental investigations on the effects of patterned surface microstructures on unsteady cavitation control.They believed that to control the unsteady developed cavitation, the foremost and the most important thing to do was suppressing the cavitation inception.They found that by utilizing the cylindrical cavitating-bubble generators (CCGs) or the wedge-type vortex generators on the suction side of the hydrofoil surface, the cavitation inception can be suppressed, and that the large-scale cloud cavities can also be mitigated, leading to a substantial decrease in the amplitude of pressure pulsations [143,145].Also, the CCGs have also demonstrated an enhanced lift to drag ratios [144].The main underlying mechanisms have been attributed to that those microscale surface structures can work as vortex generators, increase the local pressures in the forepart of them, suppress the LSB, reduce the instabilities of the boundary layer, and therefore delay the cavitation inception [143].These methods may be further developed and utilized on marine propellers.
Apart from the patterned structures presented above, the irregular surface roughness can also influence the cloud cavitation.Coutier-Delgosha et al. [146] found that the surface roughness on the surface can significantly reduce the cloud cavity length and the pressure fluctuations.Churkin et al. [147] found that the extent of the cloud cavitation was not only related to the surface roughness height parameter R a but also closely related to the surface roughness width parameter S ma .They found that although the attached cavity length may expand with an increase of surface roughness, the cavitation extent of a surface with higher R a and higher S ma can be equivalent to that of a surface with lower R a and lower S ma , which indicates that it is not enough to use merely one one-dimensional surface roughness parameter to describe the surface when dealing with the three-dimensional cavitation phenomena.However, the irregular surface roughness still has to be evaluated on whether or not it will promote earlier cavitation inception when demonstrating possible suppression effect on the cloud cavitation.
Apart from surface roughness, surface wettability can also impose significant effects on cavitation development.Kim and Lee [148] numerically investigated the surface hydrophobicity on cloud cavitation.The surface hydrophobicity was expressed by the surface velocity slip rate.They found that with the increase of the slip rate, the cloud cavity gradually became longer, and the shedding frequency gradually decreased, indicating a decreased cloud cavitation instability.However, Onishi et al. [134] also compared cavitation growth on hydrofoils with different wettability (Fig. 19), and they found that under the same operating condition, the cavitation extent of the hydrophilic hydrofoils can be much weaker than that of the relatively hydrophobic hydrofoils, although the differences in cavitation behavior of different hydrofoils may gradually disappear with the increase of the angle of attack α.
The seemingly opposite results in the above investigations may lie in that the numerical investigation carried out by Kim and Lee [148] merely considered the dynamics during the development of the cloud cavitation, while it did not put much attention to the | https://mc03.manuscriptcentral.com/frictionoccurrence of the cavitation.As demonstrated in Section 4.1, surface with more hydrophobicity may lead to earlier cavitation inception, therefore leading to more intense cavitation extent.However, it might be hard to consider or simulate cavitation inception accurately by merely using numerical methods, which may therefore lead to the above different conclusions.

Effects of surface/interface properties on cavitation erosion
Cavitation erosion is one of the severest consequences of cavitation.Apart from suppressing the cavitation inception or cavitation development to decrease the possibilities of cavitation erosion, many protection methods against cavitation erosion have been developed, and it can be generally classified into three categories.One is hardening the surface, such as the laser-based surface engineering processes [149][150][151] to increase the cavitation erosion resistance.The other is softening the surface, such as coating the surface with composite materials [152][153][154] to alleviate the impact energy.Different from the above two methods, the third method can be regarded as the cushioning effect or the "repulsive force" effect.For instance, Al-Hashem et al. [155] achieved a 47% reduction of cavitation erosion weight loss rate by applying cathodic protection to the nickel-aluminum bronze material, and this reduction effect was attributed to the cushioning effect produced by the cathodic gas to reduce the bubble collapse impact.Innovatively, Li et al. [156] achieved increased resistance against cavitation erosion on the titanium surface by applying passive potential, and the mechanism was attributed to the electrostatic repulsion formed by the negative charges at the outer side of the passive film/solution interface to repel the microjets containing microparticles with negative charges from the surface, leading to increased resistance to cavitation erosion of the surface.More recently, Gonzalez-Avila et al. [157] reported a type of biomimetic micropatterned surface manufactured with SiO 2 /Si substrate to trap air when immersed in water, and they have successfully demonstrated that the entrapped air can protrude from the surface to repel the cavitation bubble away from the surface, and thus prevent the cavitation erosion damage, as illustrated in Fig. 20.This work has presented effective means to counteract the negative effects of cavitation erosion, and provided a new avenue for mitigating cavitation erosion through the application of inexpensive and environment friendly materials.However, it has to be noticed that this cavitation erosion experiment was finished in quiet water and focused on one bubble collapse, while marine propellers often work in flowing water conditions, and therefore they often suffer from hydrodynamic cavitation and cavitation erosion under multiple bubbles or bubble clouds.In this regard, there might still be a long way to apply this strategy on marine propellers.Specifically, the performance of the gas-entrapping microtextured surfaces (GEMSs) under the collective effects of multiple bubbles in flowing seawater conditions, which contains particles and chemicals, still needs to be evaluated and optimized.Also, the fabrication of the gas-entrapping microstructures on different material surfaces, the methods to resupplying gas to the microcavities after deactivation, and the proper locations for applying the microstructures may still have to be investigated and explored.
Apart from the above surface treatment technologies, surface wettability also has a significant effect on cavitation erosion, as discussed in Section 4.1 in the work of Belova et al. [132,133].In addition, several other investigations have demonstrated that severer cavitation erosion will occur on more hydrophobic surfaces than that on hydrophilic surfaces [158][159][160], which further indicates that hydrophilic surfaces are more beneficial for the prevention of cavitation inception and erosion.

Discussion
This review aims to provide recent advances in the effects of surface/interface properties on performances of marine propellers, mainly concentrating on the methods to improve the hydrodynamic performance and cavitation performance.Concerning the hydrodynamic performance, for one thing, the application of the antifouling coatings can help to prohibit the propeller performance degradation induced by marine fouling, and bring extra benefits in cavitation performance.For another thing, the application of the superhydrophobic coatings can provide significant improvements in efficiency and benefit in modifying the flow field.With regard to the cavitation performance, hydrophilic surfaces and properly designed surface roughness or surface microstructures can provide remarkable mitigation effects on cavitation | https://mc03.manuscriptcentral.com/frictioninception, development, and cavitation erosion.Also, innovative methods such as the formation of electric double field at the solid/liquid interface and the construction of biomimetic GEMSs to repel the bubbles can provide new avenues for the design of high-performance marine propellers from the perspective of surface/interface characteristics.
Despite the above progress in improving propeller performances, there are still several challenges waiting to be solved in the perspective of practical applications, on which investigations should be concentrated in the future.
Firstly, the durability of the applied coatings or methods.As demonstrated by Cong et al. [161], the antifouling coatings applied on propeller blades may easily suffer from cavitation erosion, and the damaged areas on propeller blades may then otherwise lead to the performance deterioration of the propeller.Also, the superhydrophobic coatings applied to the propeller blades may also suffer from the depletion of the entrapped gas and lead to a decreased performance enhancement or even performance degradation.Therefore, methods to enhance the durability or the sustainability of the applied coatings are urgently needed.As for the antifouling coatings, materials exhibiting antifouling properties and erosion resistance should be explored.One possible solution is the ceramic coating.The ceramic coating is usually a damage resilient hard coating that can attach to propeller blade surfaces and can be applied very thinly, which indicates that it might be an ideal coating for marine propellers [162].However, the antifouling characteristics of ceramic coatings have rarely been reported and should be further investigated.As for the superhydrophobic coatings, methods to enhance the sustainability of the entrapped gas should be further explored.One possible method is to learn from nature, i.e., to fabricate biomimetic structures that can demonstrate self-recovering or gas-entrapping ability, such as the Salvinia structure, which combines hydrophilic patches on superhydrophobic surfaces [163,164] and the mushroom-shaped doubly reentrant cavities to entrap air under liquids [157,165].In addition, those gas entrapping structures can also be combined with active gas supplying methods to achieve long-term water repellent effect [33].Furthermore, methods or materials, which can demonstrate the effects of both antifouling and drag reduction, are also of significant importance.One promising candidate is the slippery liquid porous surface (SLIPS), which has shown more superior stability and performance compared with the conventional superhydrophobic surface [166], and may provide new insights into the field of performance enhancement for marine propellers.
Secondly, the adaptivity of the coatings or the surface/interface property-based methods.As indicated by the work of Korkut and Atlar [96], a thicker four-layer antifouling coating applied on propeller blades may lead to the deterioration of the performance, while a thinner three-layer antifouling coating applied on the propeller blades can demonstrate benefits in performance enhancement, which indicates that the actual effects of the antifouling coatings may be related to the scale of the coating and the size of the propeller.Also, concerning the superhydrophobic surfaces applied to hydrofoils or propeller blades, the Reynolds numbers have been limited to the magnitude of 10 4 .However, in practical applications, propellers often work at high Reynolds numbers of about 10 5 or even 10 6 .Therefore, the scale effects concerning antifouling coatings and the dependence of drag reduction effects of superhydrophobic surfaces on the Reynolds numbers for marine propellers should be further and thoroughly investigated.Also, most of the antifouling effects, drag reduction, or cavitation inception investigations are conducted under laboratory environment, which is still not comparable to the hostile natural environment [34].The chemical and particulate contaminants and the pressure fluctuation and salinity condition in the natural environment may pose a great challenge for the above methods to demonstrate satisfied performance enhancement effects.To put those innovative methods into practical application, the robustness of the surface/interface property-based methods under hostile environment should be further evaluated and enhanced.
Last but not least, the effects of the methods on the overall performance of the marine propellers should be further investigated, especially the benefits in the improvement of the hydrodynamic performance and the cavitation performance, which cannot be obtained simultaneously in most conditions.As indicated by the work performed by Katsuno et al. [109] and Asnaghi et al. [130], improvements in the efficiency of the propeller by applying slip effect on the blade surface may lead to degradation of the cavitation performance, and the mitigation of the cavitation inception may also lead to a slight decrease in efficiency.Therefore, a balance or trade-off between the improvements in hydrodynamic performance and cavitation performance must be considered and performed.The main conflicting point lies in that the superhydrophobic surface, which is commonly used for drag reduction purpose on marine propellers, may easily lead to earlier cavitation inception and more intense cavitation extent.This could be considered or partially solved from two aspects.First, this dilemma may be solved by considering the main applications of marine propellers.For most civil ship propellers, the efficiency is often the first thing to be considered, while the cavitation performance of the propellers does not suffer a strict restriction, which means that superhydrophobic surfaces can be applied to the propellers on civil ships.However, for military ship propellers, the sound stealth performance is of more concern, and the superhydrophobic coatings, which could enhance the cavitation extent, should be avoided as much as possible.Second, the above challenges can be solved by considering different parts of the marine propellers.As indicated in Fig. 1, with the development of the technologies, more and more types of propellers are containing stationary parts and rotatory parts.While the rotatory parts may easily suffer from cavitation, the stationary parts do not.Therefore, the drag reduction superhydrophobic coatings or other coatings could be merely applied to the stationary parts of the marine propeller without affecting the cavitation performance of the rotatory parts to obtain a better overall performance.

Conclusions
Marine propellers are important propulsion devices for surface ships and underwater vehicles.With the increasingly harsh environmental situations concerning climate change, energy shortage, and noise pollution, it has been a compulsory task to find new methods for further performance improvement of marine propellers.This review has provided recent advances in the effects of surface/interface properties on performances of marine propellers, which concentrated on the methods to improve the hydrodynamic performance and cavitation performance.Firstly, the benefits in applying antifouling coatings and superhydrophobic surfaces are demonstrated, and possible mechanisms and challenges are discussed.Secondly, the effects of the well-designed surface roughness or surface micro textures and the surface wettability on cavitation inception, development, and erosion have also been discussed, and the underlying mechanisms are elucidated in detail.Then, the remaining challenges faced by these technologies toward practical applications are summarized, and finally possible solutions, such as the combination of active methods and the passive methods, are proposed.
With the fast development of surface and interface science and technologies, those technologies should be gradually brought into practical applications.Just as Luo and Zhou [67] proposed in the concept of superlubricitive engineering that the state-of-the-art surface/interface technology, such as the superlubricity technology, should be applied to a broader range of engineering fields, including the fluid machinery field.The topic of marine propellers is exactly in the field of fluid machinery, and the hydrodynamic and cavitation performance characteristics of marine propellers can also be representative.Therefore, the valuable experience gained from marine propellers can be readily applied to the other types of fluid machinery.In this situation, this review is hoped to arouse more in-depth investigations in the field of surface/interface science and technologies on marine propellers, and also promote the state-of-the-art technologies, such as superlubricity technology, into practical usage.

Fig. 3
Fig. 3 Four patterns of propeller cavitation.(a) Vortex cavitation originates from the low-pressure cores of the vortices shed by the propeller and is the foremost to occur.Reproduced with permission from Ref. [70], © The Author(s) 2016.(b) Bubble cavitation appears as individual bubbles can cause great damage to blade surfaces when bubbles collapse.Reproduced with permission from Ref. [71], © The Japan Society of Naval Architects and Ocean Engineers (JASNAOE) 2019.(c) Sheet cavitation usually appears from the leading edge (LE) of the propeller blade, and it can evolve into cloud cavitation, which leads to increased noise and cavitation erosion.Reproduced with permission from Ref. [72], © Elsevier Ltd. 2018.

Fig. 5
Fig. 5 Surface wettability.(a) Illustration of surface contact angle.(b) Two wetting states on rough surfaces: the Wenzel state (left) and the Cassie-Baxter state (right).

Fig. 7
Fig. 7 Effects and surface roughness of antifouling coating surfaces.(a) Roughness profile of surfaces with fouling-release coatings applied by spraying.(b) Roughness profile of surfaces with fouling-release coatings applied by rollering.(c) Roughness profile of surfaces with tin-free SPC coatings.Repeoduced with permission from Ref. [95], © Taylor & Francis Ltd 2003.

Fig. 9
Fig. 9 Noise reduction effects of IS700 coating on propellers.(a) Noise reduction effects at J = 0.3 under non-cavitation condition.(b) Noise reduction effects at J = 0.2 under cavitation inception condition.(c) Noise reduction effects at J = 0.17 under cavitation development condition.Reproduced with permission from Ref. [97], © Bagheri M R, et al. 2017.

Friction 12 ( 2 )
: 185-214 (2024) 195 www.Springer.com/journal/40544| Frictionpresent at the air-water interfaces.The interfacial slip is often represented by the slip velocity u s at the interfaces and the induced slip length , as shown in Fig.10.The  is a virtual length, which can be obtained by extrapolating the u s below the surface, and the relation between the two parameters is u s 

Friction 12 ( 2 )
Re d = dV R /υ, d denotes the width of the rectangular pipe; in Re x = xV R /υ, x denotes the distance from the starting point of the test section to the sample; in Re = LV R /υ, L denotes the length of the sample.: 185-214 (2024)

Fig. 12
Fig. 12 Effects of surface slip on propeller performance.(a) Threeblade propeller with surface slip.Reproduced with permission from Ref. [109], © ASME 2018.(b) Effect of different slip length λ on propeller hydrodynamic performance (K T denotes the thrust coefficient, and K Q denotes the torque coefficient).Reproduced with permission from Ref. [110], © The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2022.

Fig. 13
Fig. 13 Effects of superhydrophobic coatings on propeller wake dynamics.(a) Smooth rotor (up), rotor in Wenzel state (middle), and rotors in Cassie state (down two).(b) Non-dimensionalized TKE in propeller wake with superhydrophobic surface coated on both sides (up left), pressure side (up middle), and suction side (up right) and variation of TKE on axial center line (down).(c) Non-dimensionalized TKE in propeller wake in Cassie state (up left) and in Wenzel state (up right) and variation of TKE on axial center line (down).Reproduced with permission from Ref.[111], © The Author(s) 2019.Note: D is the diameter of the propeller, R is the radius of the propeller, and Ω is the angular velocity of the propeller.

Fig. 14
Fig. 14 Effect of surface hydrophobicity on propeller performance.(a) Uncoated propeller (left) and PVDF-coated propeller (right).(b) Closed view of coated propeller.(c) Wake velocity field of uncoated propeller.(d) Wake velocity field of coated propeller.The color corresponds to the value of the in-plane velocity magnitude downstream of the propeller.Reproduced with permission from Ref. [112], © by the authors 2022.

Friction 12 ( 2 )Fig. 15
Fig. 15 Delay of cavitation inception by the LE roughness on the hydrofoil.Reproduced with permission from Ref. [124], © The Chinese Society of Theoretical and Applied Mechanics and Springer-Verlag GmbH Germany, part of Springer Nature 2020.

Fig. 18
Fig.18 Effects of surface wettability on cavitation inception.(a) Surface hydrophobicity of polished surface (S1), laser-oxidized surface after exposing in atmosphere for two weeks (S4), and laser-oxidized surface after exposing in atmosphere for one week (S4.1).(b) Cavitation behaviors on the above samples.Reproduced with permission from Ref.[138], © The Authors 2020.

Fig. 20
Fig. 20 Cavitation erosion mitigation by the repelling effect of the entrapped air on microtextured surface.(a) Cavitation process over a flat surface leading to erosion.(b) Cavitation process on biomimetic GEMS resulting in cavitation bubble repelled from the surface.(c)Expansion of entrapped gas under the pressure field generated by nearby cavitation bubble.Reproduced with permission from Ref.[157], © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science.Note: R b (t) represents the time (t) changing bubble radius, h(t) represents the time changing distance from the bubble to the surface.p ∞ represents the pressure far from the bubble, and p(x) represents the position (x) changing pressure on the surface.

Table 1
Some research results of drag reduction effects of superhydrophobic surfaces.