Photothermal Effect of Modulating Laser Irradiation on the Thermal Diffusivity of Al2O3 Nanofluids
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Modulated continuous wave (CW) lasers cause photothermal effect that leads to rapid optical absorption and generation of thermal waves around the irradiated nanostructures. In this work, we examined the effect of modulated CW laser irradiation on the particle fragmentation process to enhance the thermal diffusivity of nanofluids. A facile and cost-effective diode laser was applied to reduce the agglomerated size of Al2O3 nanoparticles in deionized water. The thermal wave generation, which was determined by the modulated frequency of the laser beam and the optical and thermal properties of the nanofluid, is also briefly discussed and summarized. The influence of laser irradiation time on nanoparticle sizes and their size distribution was determined by dynamic light scattering and transmission electron microscopy. The thermal diffusivity of the nanofluid was measured using the photopyroelectric method. The data obtained showed that the modulated laser irradiation caused the partial fragmentation of some agglomerated particles in the colloids, with an average diameter close to the original particle size, as indicated by a narrow distribution size. The reduction in the agglomerated size of the particles also resulted in an enhancement of the thermal diffusivity values, from 1.444 × 10−3 to 1.498 × 10−3 cm2/s in 0 to 30 min of irradiation time. This work brings new possibilities and insight into the fragmentation of agglomerated nanomaterials based on the photothermal study.
KeywordsPhotothermal effect Continuous wave laser Fragmentation of nanoparticles Thermal diffusivity of nanofluids
Amplitude of PE signal
Metal oxide nanofluids have attracted a lot of attention due to their enhanced thermal properties which allows them to play specific roles in the development of heat transfer equipment. Metal oxides nanofluids is well known to possess enhanced thermo-physical properties such as thermal diffusivity, thermal conductivity, and convective heat transfer coefficients compared to those of base fluids like oil or water. Al2O3 is an interesting oxide, as a material for enhancing the heat transfer, because of its high thermal conductivity. The thermal conductivity of nanofluids act as important properties in developing an energy-efficient heat transfer equipment, mainly used in industrial field such as automotive, electronics equipment, and medical applications. The thermal properties of nanofluids are sensitive to the size and shape of the nanoparticles (NPs) and their base fluids [1, 2, 3, 4, 5]. This poses a problem as NPs have a tendency to aggregate quickly and causes a decrease in thermal properties of the nanofluids [6, 7, 8]. Recently, laser-produced nanoparticles methods have been used to modify and generate NPs directly in the base fluids [8, 9, 10] to be used in chemical, optical and thermal engineering, phototherapy, catalysis, and heat transfer. The size and dispersion of it can be controlled by varying laser parameters, such as the laser wavelength, pulse duration, number of laser pulses, and pulse energy [11, 12]. In general, the interaction between the laser and the particles not only caused photothermal ablation but also generated thermal waves (TWs) around the nanostructures, and their surrounding medium, which lead to a reduction in size of the particles or the formation of NPs with a specific size distribution. Studies on the optical fabrication of NPs by laser irradiation showed that the laser ablation of solid targets [12, 13, 14, 15] and fragmentation from suspended microcrystalline powders [16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26] can be employed by either using powerful pulsed lasers or low-power intensity CW laser sources. Pulsed lasers have been used in many studies for the laser ablation of solid targets in liquids. Although laser irradiation is a useful technique to assist the formation of NPs in nanofluids, the efficiency of the laser irradiation process is quite sensitive to the pulse duration. However, in the case of pulsed laser irradiation, the NP size and distribution were significantly influenced by the number and irradiation times of the laser pulses. This implies that it was still difficult to achieve more particles production with control over the size distributions of the produced nanoclusters. In recent years, CW lasers have been used in several studies for the fabrication of NPs [27, 28, 29, 30]. There are several advantages in using CW laser sources as opposed to other optical sources, as they are generally less expensive, smaller, and have a more portable setup that can be potentially combined with other devices, especially as a photothermal therapy source for medical application and the reshaping and fabrication of nanomaterials [30, 31]. Recently, many experimental and theoretical investigations aimed at understanding the mechanism of laser irradiation have been performed [24, 31, 32, 33, 34, 35, 36]. On the basis of calculations and experimental confirmations, the laser ablation and fragmentation of NPs can be driven by the photothermal (PT) effect [37, 38, 39, 40, 41]. The PT effect allows for the optimization and monitoring of the efficiency of the laser irradiation with different optical sources in different experimental designs [42, 43, 44, 45, 46, 47, 48, 49]. Modulated CW laser is generally used in applications involving the PT effect. It can be a good PT source of light given an optimal modulation frequency. An increase in the efficiency of the thermal waves and the signal to noise ratio (S/N) can be observed, making it more suitable for the NPs fragmentation process. Moreover, a careful optimization of the experimental conditions can establish control over size distributions of the produced nanoclusters and thermal properties of nanofluids. However, no detailed study exists in literature for the PT effect of modulating CW laser on the formation and size of NPs and their thermal properties.
In the paper, a CW diode laser was used for the fragmentation of clustered Al2O3 particles to enhance the thermal diffusivity of the nanofluids, under various irradiation times. The basis of the thermal wave generation of the modulated CW laser beam was briefly summarized and the effect of the modulated beam frequency and physical parameters were discussed. The results of the laser fragmentation process were analyzed using transmission electron microscopy (TEM) and dynamic light scattering (DLS) analysis. Finally, the effect of laser treatment on the thermal diffusivity of the nanofluids was investigated. The photopyroelectric (PPE) technique was used as a valid method for measuring the thermal diffusivity of the nanofluids with very high precision and resolution.
Thermal Wave Generation of the Modulated Laser Beam
Preparation of Nanofluids
The nanofluids were prepared by dispersing 0.05 g Al2O3 NPs (11 nm, Nanostructured and Amorphous Materials, Inc.) into 25 ml deionized (DI) water. One volume percent polyvinylpyrrolidone (PVP) (K25, MW–29000, Aldrich Chemistry) was added to stabilize the nanofluids; Al2O3 NPs in water have a strong tendency to form aggregates [54, 55]. The suspension was stirred in about 1 h then the mixture was subjected to probe sonication for 30 min (VCX 500, 25 kHz, 500 W) to ensure homogeneous particle distribution. After the suspension was mixed thoroughly for 30 min, the hydrodynamic size of the agglomerated particles in the solution was monitored using DLS.
Laser Fragmentation Process
Thermal Diffusivity Measurements
where A = (πf/α)1/2 to obtain this expression, V(f, l) is the complex PE signal, Vo and φ are the amplitude and phase of PE signal, f is the modulation frequency, and α is the thermal diffusivity of sample. From the slope fitting parameter A = (πf/α)1/2 of phase and ln(amplitude) as a function of cavity scan, thermal diffusivity of liquid can be calculated .
Results and Discussion
Thermal Wave Enhancement
Modulation frequency of the modulation light
From Eq. (5), there should be an optimum modulation frequency to maximize the thermal wave amplitude. Unlike other waves, thermal wave is very heavily damped with a decay constant equal to the thermal diffusion length of the medium of propagation . The thermal waves originating from no deeper than the thermal diffusion length in the material contribute to the heat propagation . The thermal waves are reflected and transmitted at the interface and the amplitude of the thermal waves is attenuated within one thermal diffusion length of the sample. With increasing modulation frequency according to Eq. (5), the thermal diffusion length decreases, and only light absorbed within the surface layer contributes to the signal, while the thermal waves will propagate deep into a solid if the material has a high thermal diffusivity or if the thermal wave frequency is low. In the experiment, one should carefully choose the modulation frequency in order to get a sharp resonant peak (actually a trough). The modulation frequency is chosen in the spatial range. If the frequency is too low, the signal is strong, but the peak is too flat for precise determination of its maximum. While if the frequency is too high, the peak is quite sharp, but the signal-to-noise (S/N) ratio is compromised, which makes identification of the peak position difficult.
Optical absorption of the nanofluids
Specific heat capacity of the nanofluids
Thermal diffusivity of the nanofluids
Heat is transferred from the solid particles to the surrounding medium followed by thermal wave expansion, where the amplitude of the thermal waves (TWs) is a strong function of the thermal diffusivity. As shown in Fig. 2, a larger thermal diffusivity is usually preferred for higher thermal diffusion lengths and the thermal wave amplitude below the surface decays slowly. Therefore, the large thermal diffusivity of the base fluid is crucial for effective heat transfer from the solid particles to the fluid, thus, maximizing thermal wave generation. In this work, water with a high thermal diffusivity (0.00145 cm2/s) was a good base fluid for efficient thermal wave generation. The thermal diffusivity of water increased with an increasing amount of NPs, due to increasing Brownian motions . The higher thermal diffusivity and smaller specific heat of the Al2O3 nanofluid compared to water allowed it to be excellent thermal wave generator.
Laser Fragmentation of the Al2O3 Nanoparticles
Thermal Diffusivity Measurements
Summarized results for thermal properties of Al2O3 nanofluids at different laser irradiation times
120.6 ± 0.8
122.6 ± 0.6
121.6 ± 0.7
1.444 ± 0.008
119.4 ± 0.6
121.8 ± 1.0
120.6 ± 0.8
1.468 ± 0.011
118.4 ± 0.9
120.4 ± 0.6
119.4 ± 0.7
1.498 ± 0.009
The thermal diffusivity showed an enhancement compared to the base fluid. However, for the nanofluid without irradiation, the thermal diffusivity was (1.444 ± 0.008) × 10−3 cm2/s, which was lower than base fluid. This could be due to the low thermal diffusivity of PVP in the nanofluids. The thermal diffusivity gradually increased around 3–6% after laser irradiation, which was defined as an aging effect [56, 57]. The increase in the thermal diffusivity with longer irradiation time was a consequence of the decrease in the clusters and agglomerate sizes, due to the fragmentation of the larger NPs [7, 8, 9, 10]. Generally, the density of the number of particles or volume fractions of the particles increased and it was evident that the particle size reduction increased the nanoscale mixing effects, such as Brownian motions . Therefore, this could help to enhance the thermal diffusivity of the nanofluids. However, the increase in the number of particles in the solution had an influence on the rate of laser fragmentation, due to the attenuation of laser light in the liquid at high concentrations.
In principle, the interaction between the CW laser beam (in our experiment 103 W/cm2) and the Al2O3 clusters is governed by thermal effects which depends on the characteristics of the laser radiation and the nature of the particle. Hence, considerable research has been directed towards decreasing the size of the particles using various nanosecond (ns) and femtosecond (fs) lasers running at different pulse duration [13, 14, 15, 16, 17, 18, 19, 21, 25, 26, 27]. Coincidentally, the exact same result was obtained through our experiments. As a result of the nanofluids, in the laser irradiation, time affected mainly the particles rather than their size. This was probably because of the effect of the laser irradiation on the fragmentation of the agglomerated particles to the smaller NPs thus increasing the homogeneous particle distribution of the Al2O3 nanofluids. These results demonstrated the surprisingly narrow distributions, with size dispersions in the order of the mean size, which was confirmed by measuring TEM and Nanophox results. This suggested that the NPs were excited and heated by irradiation of the modulated CW laser with some heat loss to the surrounding water, while the absorption of the laser energy by the particles could cause further fragmentation of the particles to smaller possible sizes thus increasing the total number of particles in the solution . In addition, the distribution of particle also decreased with an increase in the laser irradiation time, which has been reported with other materials, such as metal [11, 13, 14, 17] and metal oxide [9, 10, 29].
In conclusion, we confirmed that the modulated continuous wave laser can be used as a good photothermal light sources to generate the thermal waves for fragmentation of the clustered Al2O3 particles and enhancing the thermal diffusivity of the Al2O3 nanofluids. Modulated CW laser technique shows an enormous promise for accurate characterization of the particle size distribution of Al2O3 nanofluids. There are some controlled experiments to optimize the thermal wave generation efficiency, such as the size of the particles, modulation frequency, thermal properties of particles, and base fluid. The results showed that the effect of laser irradiation on the distribution size was more on the size of particles. The thermal diffusivity of the Al2O3 nanofluid increased to 3–6% with the increase of irradiation times, due to the fragmentation of the NPs which in turn increased the total number of particles in the solution. Therefore, from this work, it predicated that inexpensive and compact CW diode lasers can be successfully designed and employed for the fragmentation of NPs in nanofluids.
Io Source intensity
ω Angular frequency of modulated light
f Modulation Frequency
∇T Temperature gradient
q Energy flow
e thermal wave diffusion coefficient
φ phase of PE signal
μ Thermal Diffusion Length
k Thermal Conductivity
α Thermal Diffusivity
The authors would like to acknowledge the Ministry of Education (FRGS Grant 2016, Vot. No. 5524942) and Universiti Putra Malaysia (IPS grants, Vot No. 9493000) for the financial support.
Availability of Data and Materials
All data are fully available without restriction.
MN carried out the experimental work, synthesis, characterization and analysis, and wrote the paper. BM, SR, AZ, and RSA supervised the experimental work and revised the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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- 5.Nguyen CT, Roy G, Galanis N, Suiro S (2006) Heat transfer enhancement by using Al2O3-water nanofluid in a liquid cooling system for microprocessors. In: Proceedings of the 4th WSEAS Int. Conf. on Heat Transfer. Thermal Engineering and Environment Elounda, Greece, pp 103–108Google Scholar
- 9.Kim S, Choi S, Kim D. Thermal conductivity of metal-oxide nanofluids. Particle size dependence and effect of laser irradiation. ASME. J Heat Transf 2006; 129(3):298–307Google Scholar
- 13.Mendivil Palma MI, Krishnan B, Castillo Rodriguez GA, Das Roy TK, Avellaneda DA, Shaji S (2016) Synthesis and properties of platinum nanoparticles by pulsed laser ablation in liquid. J Nanomater 2016 11 pagesGoogle Scholar
- 28.Liu Z, Yuan Y, Khan S, Abdolvand A, Whitehead D, Schmidt M, Li L (2009) Generation of metal-oxide nanoparticles using continuous-wave fibre laser ablation in liquid. J Micromech Microeng 19:5Google Scholar
- 31.Long L, Huang Y, Zhang J. Experimental investigation and numerical simulation on continuous wave laser ablation of multilayer carbon fiber composite, Proceedings of the Institution of Mechanical Engineers Part L. J Mat Des Appl 2017; 231(8):674–682Google Scholar
- 35.Al-nassar SI, Adel KM, Ahmed OS, Mahdi ZF (2015) Study the fragmentation phenomena of TiO2 nanoparticles produced by liquid-phase laser ablation method using computer simulation technique. Mat Today 2(4):3718–3727Google Scholar
- 38.Gao F, Kishor R, Feng X, Liu S, Ding R, Zhang R et al (2017) An analytical study of photoacoustic and thermoacoustic generation efficiency towards contrast agent and film design optimization. Photo-Dermatology 7:1–11Google Scholar
- 41.Stylogiannis A, Prade L, Buehler A, Aguirre J, Sergiadis G, Ntziachristos V (2018) Continuous wave laser diodes enable fast optoacoustic imaging. Photo-Dermatology 9:31–38Google Scholar
- 50.Coufal H, Mandelis A (1991) Pyroelectric sensors for the photothermal analysis of condensed phases. Taylor & Francis 118(1):379–409Google Scholar
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