Introduction

Water-repellent and hydrophobic coatings are increasingly common in daily life. Hydrophobic surfaces, which are also found in nature, play an important role in research and industry, and are applied in a variety of applications such as clothing, automobiles and window glasses etc [1]. Among them, the surface of the fluoropolymer hydrophobic coatings increases the contact angle of water droplets and makes them easy to roll off from the surface due to their low surface energy and low friction, respectively [2, 3]. In particular, a fluoropolymer hydrophobic coating called Cytop is commonly used in electrowetting-on-dielectric (EWOD) technology because of these properties. EWOD technology has recently gained attention in the Micro Electro Mechanical Systems (MEMS) application due to its fast response time and low power consumption among the droplet control technologies [4] such as surface acoustic wave (SAW) [5, 6], mechanical drive [7, 8], and thermal capillary [9,10,11]. For these reasons, a variety of applications using EWOD technology, such as lab-on-a-chip [12,13,14], liquid lens [15,16,17,18,19], liquid shutter [20], electrowetting display (EWD) [21,22,23], smart self-cleaning lens [24], and droplet-based energy generator [25,26,27,28,29] have been developed.

However, it has been noted that the hydrophobic coating, which plays a crucial role in EWOD devices, degrades its hydrophobicity due to prolonged water exposure and repeated EWOD actuations, etc [2, 3, 30,31,32]. To overcome these limitations, research has been conducted to recover damaged hydrophobic coatings. Zeng et al. conducted a study on recovering the Teflon AF hydrophobic coatings applied to EWOD-based chemiluminescence detectors. Teflon AF damaged by the chemiluminescence reaction was heated at various temperatures to quantify the recovery characteristics of the coating as a function of time and temperature [32]. Trapuzzano et al. damaged a variety of fluoropolymer coatings commonly used in the EWOD industry through long-term water exposure, and then demonstrated that certain coatings could recover their hydrophobicity by heating [2]. However, these studies focused on the damage and recovery methods of the hydrophobic coating and did not provide information on the long-term durability of the coating. In addition, open-air EWOD devices such as smart self-cleaning lens [24] and droplet-based energy generator [25,26,27,28,29] require consideration of damage variables that differ from water immersion. Therefore, this study evaluates the durability of open-air EWOD devices by simulating raindrop damage to hydrophobic coatings and comparing the durability of different hydrophobic coating thicknesses.

Figure 1 is the concept of this study. The Cytop (CTX-809SP2) hydrophobic coatings with different thicknesses are damaged by a raindrop simulation. (Fig. 1a1-2) The damaged hydrophobic coatings are then heated on a 200 ℃ hot plate for 24 h to recover their hydrophobicity (Fig. 1a3-4) [33]. This process of heating and recovering damaged hydrophobic coatings is called the damage-recovery cycle (Fig. 1a1-4). In a previous study, the contact angle (CA) and contact angle hysteresis (CAH) of droplets placed on the hydrophobic coating were evaluated to determine the recovery of the hydrophobic coating by heating [2]. In this study, we not only evaluate the CA and CAH of droplets placed on the hydrophobic coating but also analyze the performance of EWOD actuation to verify the recovery of the hydrophobic coating (Fig. 1b). This allows to analyze the trend of damage and recovery as a function of the thickness of the hydrophobic coating, and to verify the number of recovery limitations of the hydrophobic coating by repeating the damage-recovery cycle. Therefore, this study is expected to provide useful information on how to evaluate the durability of hydrophobic coatings in open-air EWOD devices and the selection of their thickness (Fig. 2).

Fig. 1
figure 1

Schematic diagram of a damage-recovery cycle of hydrophobic coating; b changes in electrowetting-on-dielectric (EWOD) performance due to damage and recovery of hydrophobic coating

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Fig. 2
figure 2

Wettability changes in EWOD: a initial state and b after voltage is applied between a conducting droplet and an electrode

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Theoretical background

The theoretical basis of EWOD was established in the early 2000s [34]. The relationship between voltage and CA, which is given by the so-called Lippmann-Young equation, theoretically describes the physical phenomenon of EWOD. Upon applying a voltage between a conductive droplet and a dielectric layer-coated electrode, an electric double layer builds up near the solid–liquid interfaces in the dielectric layer spontaneously [35, 36]. Since the accumulation of the opposite charges around the three-phase contact line (TCL) near the solid surface, reduction of the interfacial tension causes the droplet to spread out under the applied electric field, as shown in Fig. 2 [35]. Berge et al. substituted the Lippmann equation, which describes the relationship between applied voltage and droplet contact angle into the Young's equation, which describes the relationship between interface and droplet contact angle, and derived the following equation [37]:

$${\text{cos}}\theta = {\text{cos}}{\theta }_{0}+ \frac{{\varepsilon }_{0}{\varepsilon }_{D}{\varepsilon }_{H}}{2({\varepsilon }_{D}{d}_{H}+{\varepsilon }_{H}{d}_{D})\gamma }{V}^{2}$$
(1)

Here, \(\theta\) is the CA of droplet with the external voltage V, \({\theta }_{0}\) is the initial CA without applied voltage, \({\varepsilon }_{0}\) is the vacuum permittivity, \({\varepsilon }_{D}\) is the dielectric constant of the dielectric layer, \({\varepsilon }_{H}\) is the dielectric constant of the hydrophobic layer, \(\gamma\) is the interfacial tension between the droplet and the surrounding fluid, \({d}_{D}\) is the thickness of the dielectric layer, and \({d}_{H}\) is the thickness of the hydrophobic layer. This equation shows that the thickness of the hydrophobic and dielectric layer affects the change in droplet contact angle with voltage.

Experimental methods

An experimental setup is designed to damage multiple hydrophobic coating samples with falling droplets simultaneously. Figure 3 shows an experimental setup that simulates raindrops to damage hydrophobic coatings. The 3D-printed holder can hold fifteen EWOD samples reliably. The holder is designed to be at an angle of 45° to the ground to allow water to flow away effectively, minimizing damage to the hydrophobic coating from water immersion. Moreover, the holder can be easily mounted in a fixed position on the 10 mm thick acrylic water tank, which not only makes it easy to remove and add samples, but also ensures that the experiment is always performed in the same position. A top water tank is placed 800 mm above the holder, and the water is flowed through a flow regulator, allowing water drops to fall into the center of the sample. Through the flow regulator, 70 microliters of droplets were dispensed at a rate of one drop per second. Then, when the water in the top tank goes below a certain level, the water level regulator activates the water pump to keep the water level constant. This experimental setup allows for the evaluation of different variables in samples simultaneously. And it ensures that the same experimental environment is always maintained, which reduces the gap between damage-recovery experiments and evaluations to examine long-term durability.

Fig. 3
figure 3

Experimental setup for simulating raindrops

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The EWOD-structured samples are fabricated for the durability test of hydrophobic coatings. Within every deposition process, test samples are cleaned with acetone and then isopropyl alcohol, followed by rinsing with deionized water. After rinsing, they are first dried using nitrogen and then completely dried in a 120 °C oven. Indium Tin Oxide (ITO) electrodes are deposited on a cleaned 6″ diameter soda lime wafer (0.7 mm thick) using a sputtering process. It is then diced into 25 mm × 25 mm squares. On top of the diced wafers, 1 μm thick \({{\text{SiO}}}_{2}\) is deposited as a dielectric layer using plasma enhanced chemical vapor deposition (PECVD), and the hydrophobic coating, Cytop (CTX-809SP2), is spin coated in 0.1, 0.5, and 1.0 μm thicknesses. The spin-coated Cytop is dried at room temperature for 30 min and then prebaked at a temperature of 50 °C for 30 min to remove air bubbles within the Cytop. The Cytop is then heated at 80 °C for 60 min to evaporate the remaining solvent. Finally, a final baking at 250 °C for 60 min is performed to improve the surface adhesion of the Cytop.

The CA and CAH of droplets placed on the sample surface are measured by a contact angle goniometer (Smartdrop, Femtobiomed Inc.) to analyze the hydrophobicity of the experimental samples, as shown in Fig. 4. The CA is measured by averaging the contact angles of the three-phase contact line on both sides of a 5 \(\upmu\)L droplet generated on the sample’s surface. And the CAH of each sample is measured using the captive method. The volume of a 5 \(\upmu\)L droplet is precisely controlled by the syringe. As the syringe then increases the volume and the contact area with the sample of the droplet by dispensing water, the contact angle is measured simultaneously, called the advancing contact angle (\({\uptheta }_{{\text{adv}}}\)). On the contrary, as the syringe sucks in water, the volume and the contact area with the sample of the droplet decrease, and the contact angle is also measured simultaneously, called the receding contact angle (\({\uptheta }_{{\text{rec}}}\)). The advancing and receding contact angles are measured through each process, and the CAH of each sample is calculated as the difference between the advancing and receding contact angles.

Fig. 4
figure 4

Measurement of contact angle and contact angle hysteresis by a contact angle goniometer (Smartdrop, Femtobiomed Inc.)

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Results and discussion

Damage

The samples with different thicknesses of hydrophobic coatings are first damaged by falling droplets. Figure 5 shows the change in CA and CAH due to falling droplets for hydrophobic coatings of 0.1, 0.5, and 1.0 μm thickness over 10 days. Regardless of the hydrophobic coating thickness, CA and CAH measure the same before damage, averaging 117° and 13°, respectively. After 10 days of damage, the CA of all samples decrease by about 17° and the CAH increase by about 31°. All samples show a decrease in CA at a rate of 5.7° per day and an increase in CAH at a rate of 10° per day during the initial 3 days. This experiment validate the proposed hydrophobic coating damage method and demonstrate that a 10-day experimental period is sufficient to saturate the damage. It is also experimentally observed that the thickness of the hydrophobic coating does not affect the rate of damage by falling droplets.

Fig. 5
figure 5

Graph of the change in contact angle and contact angle hysteresis for the samples damaged by falling droplets over 10 days

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The EWOD actuation performance of damaged and undamaged hydrophobic coatings is compared. All the EWOD experiments are performed using a function generator (33210A, Agilent Co.) and amplifier (PZD700, Trek Co.) with a frequency of 1 kHz to measure the change in CA of the droplets by sequentially increasing the voltage from 0 \({{\text{V}}}_{{\text{rms}}}\) to 100 \({{\text{V}}}_{{\text{rms}}}\) and decreasing it back to 0 \({{\text{V}}}_{{\text{rms}}}\), in 10 \({{\text{V}}}_{{\text{rms}}}\) increments. The EWOD actuation performance is measured by the ratio of the initial contact angle to the contact angle recovered when the voltage is removed. Figure 6 shows the variation of the experimental measurements of CA and the theoretical value from the Lippmann-Young equation as a function of the voltage of the sample before and after damage. The hydrophobic coating of the undamaged sample is found to have the same trend as the theoretical value under EWOD actuation. In addition, the contact angle of the dewetted droplets after EWOD actuation is recovered by about 98 % on average compared to the initial contact angle, indicating normal EWOD performance. On the other hand, the damaged samples show a significant decrease in droplet wetting and dewetting performance as a function of voltage, regardless of the thickness of the hydrophobic coating.

Fig. 6
figure 6

EWOD hysteresis graph of the damaged samples and the undamaged samples; the solid line is for increasing voltage, and the dashed line is for decreasing voltage

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Recovery

The samples are then heated on a hot plate at 200 °C for 24 h to recover their hydrophobicity damaged by falling droplets, followed by CA and CAH measurements, as shown in Fig. 7. Regardless of the hydrophobic coating thickness, CA and CAH are measured to average 114° and 16° after heating. The experiment shows that CA and CAH recovered about 98 % and 81 %, respectively, compared to before damage. This proves that the damage to the hydrophobic coating caused by the falling droplets can be recovered by annealing at 200 °C. On the other hand, the recovery rate of CA and CAH over 24 h does not differ regardless of hydrophobic coating thickness. This confirms that the thickness of the hydrophobic coating and the rate of hydrophobicity recovery are independent.

Fig. 7
figure 7

Graph of the change in contact angle and contact angle hysteresis for heat-treated samples over 24 h

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Damage-recovery cycle

The EWOD actuation performance is evaluated by repeating the damage-recovery cycle to determine the tendency of the damage-recovery of the hydrophobic coating depending on its thickness. Figure 8 shows the change in CA as a function of voltage for the undamaged sample and the sample that conducted the first damage-recovery cycle. The samples after the first cycle recovered by about 96 % when comparing the CA of the dewetted droplets after the EWOD actuation to the initial CA. Comparing the undamaged sample, where the CA of the dewetting droplet recovers to about 98 % of the initial CA, the sample after the first cycle shows a decrease in EWOD performance of about 2 %. This experiment shows that the samples after the first damage-recovery cycle showed no difference in EWOD performance compared to the undamaged samples, and the difference in the degree of damage based on the hydrophobic coating thickness was not observed in the first damage-recovery cycle.

Fig. 8
figure 8

EWOD hysteresis graph of the undamaged samples and the heat-treated samples after first damage; the solid line is for increasing voltage, and the dashed line is for decreasing voltage

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After that, we check the EWOD actuation performance after the second damage-recovery cycle. Figure 9 shows the change in CA as a function of voltage for the undamaged sample and the sample that conducted the second damage-recovery cycle. For the 0.1 μm thick sample after the second cycle, the CA of the dewetted droplet after the EWOD actuation recovered by about 87 % compared to the initial CA before the EWOD actuation, while the 0.5 μm thick sample recovered by 88 %. The 1.0 μm thick sample recovered about 95 %, performing 8 % better than the 0.1 μm thick sample and 7 % better than the 0.5 μm thick sample. This experiment confirms that a thicker hydrophobic coating results in superior long-term durability of EWOD actuation.

Fig. 9
figure 9

EWOD hysteresis graph of the undamaged samples and the heat-treated samples after second damage; the solid line is for increasing voltage, and the dashed line is for decreasing voltage

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Finally, we check the EWOD actuation performance after the third damage-recovery cycle. Figure 10 shows the change in CA as a function of voltage for the undamaged sample and the sample that conducted the third damage-recovery cycle. After the third cycle, no dewetting of the droplets is observed after the EWOD actuation, which means breakdown of the hydrophobic coating, regardless of its thickness. Therefore, it concludes that the damage-recovery of the hydrophobic coating in this falling droplets experiment can not fully perform for three cycles, regardless of the thickness of the hydrophobic coating.

Fig. 10
figure 10

EWOD hysteresis graph of the undamaged samples and the heat-treated samples after third damage; the solid line is for increasing voltage, and the dashed line is for decreasing voltage

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The CA and CAH of water droplets placed on the surface of the hydrophobic coating are measured after each cycle to determine the relationship between the observed EWOD actuation performance and the change in hydrophobicity of the coating, as shown in Fig. 11. Similar to the results of the previous experiments, the hydrophobicity of the hydrophobic coating in the first cycle shows little difference from the hydrophobicity of the undamaged coating, and in the second cycle, the thicker the hydrophobic coating, the better the hydrophobicity shows. After the third cycle, the CA has recovered 90 % of the initial CA, but the CAH has increased by about 375 % of the initial CAH, which is consistent with the results of the EWOD performance.

Fig. 11
figure 11

Graph of the change in contact angle and contact angle hysteresis of samples over repeated damage-recovery cycles

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The surface morphology of the 1.0 µm samples are analyzed using atomic force microscopy for each cycle. Figure 12 shows the change in surface morphology of the hydrophobic coating and its roughness changes. Ridges appeared on the surface of the damaged coating, represented by white areas, increasing the surface roughness. This is likely caused by the constant impact of falling droplets and exposure to water. Through the reflow which occurs by heating the damaged coating, the white area seen in (a2) is reduced in (a3).

Fig. 12
figure 12

Atomic force microscopy (AFM) images of surface changes in the 1.0 µm thickness of CYTOP in each damage-recovery cycle

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The roughness gradually increased with each repeated damage-recovery cycle. The high roughness of the surface makes it difficult for water droplets to slide easily. Therefore, it is likely to have had a significant impact on the increase in CAH. However, it is difficult to explain the degradation of CA only due to a change in surface morphology. Chemical changes on the surface will also play an important role in CA degradation [30].

Conclusion

This study evaluates the long-term durability of hydrophobic coatings for open-air EWOD devices. Cytop (CTX-809SP2), a fluoropolymer-based hydrophobic coating, was subjected to a continuous damage-recovery cycle, and the EWOD actuation performance was analyzed as a function of hydrophobic coating thickness in each cycle. Regardless of the thickness of the hydrophobic coating, the performance of the EWOD actuation has degraded with each repeated cycle. However, after the second damage-recovery cycle, the thickest 1.0 μm sample showed 8 % and 7 % higher EWOD performance than the other samples, respectively. The second cycle showed that the thickness of the hydrophobic coating affects the hydrophobicity damage caused by falling droplets. Furthermore, after the third damage-recovery cycle, EWOD performance degraded significantly for all samples, experimentally demonstrating the number of recovery limitations of the hydrophobicity of the coating. This study verified the number of recovery limitations of different thicknesses of Cytop through raindrop simulation, which will provide useful information for durability evaluation methods and thickness selection for open-air EWOD devices and for further research on hydrophobic coating durability. Although thicker hydrophobic coatings showed the best durability against falling droplets, it should be noted that the required voltage for EWOD actuation increases with increasing thickness. Therefore, future research should analyze and optimize the correlation between the long-term durability of open-air EWOD devices and the voltage for EWOD actuation. This research is expected to provide a basis for future research and applications of EWOD technology.