Water Drop Evaporation on Mushroom-like Superhydrophobic Surfaces

Article pubs.acs.org/Langmuir

Water Drop Evaporation on Mushroom-like Superhydrophobic Surfaces: Temperature Effects Rodney Marcelo do Nascimento,† Cécile Cottin-Bizonne, Christophe Pirat, and Stella M. M. Ramos* Institut lumière matière, UMR5306 Université Claude Bernard Lyon 1-CNRS, Université de Lyon, 69622 Villeurbanne cedex, France ABSTRACT: We report on experiments of drop evaporation on heated superhydrophobic surfaces decorated with micrometer-sized mushroom-like pillars. We analyze the influence of two parameters on the evaporation dynamics: the solid−liquid fraction and the substrate temperature, ranging between 30 and 80 °C. In the different configurations investigated, the drop evaporation appears to be controlled by the contact line dynamics (pinned or moving). The experimental results show that (i) in the pinned regime, the depinning angles increase with decreasing contact fraction and the substrate heating promotes the contact line depinning and (ii) in the moving regime, the droplet motion is described by periodic stick−slip events and contact-angle oscillations. These features are highly smoothed at the highest temperatures, with two possible mechanisms suggested to explain such a behavior, a reduction in the elasticity of the triple line and a decrease in the depinning energy barriers. For all surfaces, the observed remarkable stability of the “fakir” state to the temperature is attributed to the re-entrant micropillar curvature that prevents surface imbibition.

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INTRODUCTION Microstructured materials can show a superhydrophobic behavior with extremely high contact angles, very low adhesion between the liquid and the surface, and the ability to self-clean. These interesting and unusual interfacial properties make superhydrophobic surfaces important candidates for a wide range of applications from microfluidics and lab-on-a-chip devices to drag reduction and thermal or energy systems.1−5 In many of such applications, understanding and controlling the behavior of water droplets (e.g., droplet mobility, degree of penetration of liquid into the microstructures, etc.) and the contact line dynamics is of critical importance for an optimal designing of drop-based devices. The wetting properties of periodically or random textured superhydrophobic surfaces have been extensively studied6−10over the last two decades. Many review articles have been reported,11,12 discussing the metastable character of superhydrophobicity and the optimal surface design able to obtain this property. In particular, great efforts have been made to improve the understanding of the droplet motion and the physical mechanism of pinning or depinning dynamics on such surfaces. In the past few years, different scenarios have been proposed to precisely describe how the triple line propagates (parallel to itself or by forming kinks) between the surface microstructures.1,13−17 Despite this, the full determination of the depinning mechanisms on super-repelent surfaces still remains a challenge, in particular, when it deals with evaporation of sessile drops. It is known that the evaporation kinetics of droplets is strongly influenced by both the surface properties and the surrounding parameters (like temperature, pressure and humidity). Among these parameters, the effect of the surface © XXXX American Chemical Society

topography on the droplet evaporation dynamics has been, up to date, the most investigated.18−21 Many of these studies have been focused on the different evaporation modes, which appear to be controlled by the triple line mobility (pinned or moving),19,22−25 whereas others have targeted the understanding of specific transport mechanisms governing the drop evaporation processes on these specific surfaces.1,4,21,26 Nevertheless, the greatest part of all of these works has dealt essentially with the natural evaporation process. The influence of substrate temperature on the sessile droplet evaporation and on the contact line dynamics remains largely poorly investigated.27−29 The effect of temperature on the drop motion and the receding line dynamics remains thus unclear. We report an experimental study of water droplet evaporation on heated superhydrophobic surfaces with different areal densities of microstructures. We separately investigate the role of two parameters on the evaporation dynamics: the solid− liquid fraction determined from structural parameters of surfaces and the substrate temperature, ranging between 30 and 80 °C. In all cases the drop evaporation process appears to be controlled by the triple line dynamics (pinned or moving). In the moving regime the droplet motion is characterized by successive stick−slip events. The influence on the evaporation process and on the dynamics of the receding contact line (i) of substrate temperature and (ii) liquid−solid fraction is discussed in terms of capillary and friction contributions. Received: December 4, 2015 Revised: January 22, 2016

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DOI: 10.1021/acs.langmuir.5b04445 Langmuir XXXX, XXX, XXX−XXX

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configuration (liquid removed from the drop) for the receding contact angle θr measurement. For the investigations of evaporation kinetics, the initial contact angle and contact radius are defined as θi and Ri respectively, while contact angle and contact radius at time t are introduced as θ(t) and R(t) respectively.

EXPERIMENTAL SECTION

The surfaces are decorated with micrometer-sized mushroom-like pillars etched from a silicon substrate on a regular square lattice characterized by a height h, a hat diameter D, a hat thickness t, a foot diameter Db, and center-to-center distance λ. (See the details of the surface features in Table 1 and in Figure 1.) By varying the gap d

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RESULTS AND DISCUSSION Characterization of the Dynamic Wetting Properties. We first characterize the advancing θa and the receding θr contact angles on the different surfaces for T = 40 °C as reported in Table 2. It appears from such results that contact

Table 1. Dimensions of the Pillars According to the Inset of Figure a dimensions (μm)

D

h

7.8 sample

d (μm)

ϕs

S2 S4 S6 S8

2.2 4.2 6.2 8.2

0.48 0.33 0.25 0.19

Db

7.0 5.1 n × 109 (m−2) 10.00 6.94 5.10 3.91

t 1.4 λ (μm)

Table 2. Advancing and Receding Contact Angles Measured for the Four Investigated Surfaces and for a Temperature of the Substrate of 40°C

10 12 14 16

sample S2 S4 S6 S8

d, distance between microstructures; ϕs, solid−liquid fraction, n, density of pillars; λ = D + d spacial periodicity. a

θa (deg) 152 153 155 159

± ± ± ±

1 1 1 1

θr (deg) 140 145 149 153

± ± ± ±

2 1 2 1

angles increase with the distance between two adjacent microstructures, whereas the values of the contact-angle hysteresis, Δθ, decrease, reaching a minimum value of 6° for surface S8. The large contact angles associated with the small contact angle hysteresis indicate that the water drop does not penetrate significantly into the rough grooves, it remains suspended on top of the asperities with a gas phase trapped in between. Such a behavior characterizes a “fakir” configuration8 typical of the superhydrophobic surfaces. From the experimental measurements of the contact angles, we determined, for each surface, the depinning force (per unit length) of the contact line, defined as Fdyn = γ(cos θr − cos θe)dyn30 where cos θe = (cos θa + cos θr)/2 and γ is the liquid− gas surface tension which in our experiments decreases from 71.2 mN·m−1 at 30 °C to 62.7 mN·m−1 at 80 °C. The evolution of the depinning force as a function of the areal density of microstructures, n, is displayed in Figure 2 for two different

Figure 1. Scanning electron microscope image of a sample of type S4. The mushroom hat diameter D is 7.8 μm. Inset: post features. between pillars, four designs were used with d = 2.2, 4.2, 6.2, and 8.2 μm. The resulting solid fractions and post densities are reported in Table 1. All surfaces (hereafter labeled S2−S8) were rendered hydrophobic by deposition of fine Teflon-like coating from a C4F8 plasma prior experiments. This specific geometry has been chosen to ensure enhanced stability of the fakir state throughout the drop evaporation by taking advantage of the reentrancies. Contact-angle measurements of ultrapure water drops (of volume ∼3 μL) were carried out through the standard sessile drop method with a tailor-made setup. For each experiment, a drop, initially at room temperature, was gently deposited from a needle connected to a syringe pump on the surface enclosed in a glass chamber. A white and homogeneous lighting system was positioned behind the sample to make the drop to appear black. Side-view images of the drops were recorded with a CCD camera for subsequent contact angle and contact radius measurements. The temperature of the substrates was varied from 30 to 80 °C, and the relative humidity was set to 45 ± 5% at the beginning of the experiments. Indeed, during the evaporation process the humidity rate inside the box tends to increase; however, considering the small volume of the drops used, the increase corresponds to a maximum of ±10% the initial value. At least five different measurements were performed on different areas of each sample. When a drop is deposited on the surface at a given temperature T, the dynamic contact-angle hysteresis is defined as the difference between the advancing and receding contact angles Δθdyn = θa − θr, where the advancing contact angle θa is reached as the contact line (CL) moves consecutively to addition of fluid, while it is the opposite

Figure 2. Dynamic hysteresis as a function of the pillars density measured at T = 40 and 60 °C.

temperatures. From this Figure, it appears that Fdyn grows linearly with n, suggesting that each pillar pins the contact line individually. We have also studied the influence of the temperature on the depinning force and plotted the result for 40 and 60 °C. It is shown to have a noticeable influence on the depinning force. For sample S2 (with the highest n value), the relation between the depinning forces at different temperatures is found to be Fdyn(40°C) ≈ 2 Fdyn(60°C). This result cannot be explained only by the temperature effect on the surface tension. Keeping in mind that the depinning force is due to two B

DOI: 10.1021/acs.langmuir.5b04445 Langmuir XXXX, XXX, XXX−XXX

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force due to the evaporation arising at the transition is introduced as Fevap = γΔcos θevap with Δcos θevap = cos θdep − cos θi.19,22,31 The measured values of θi could indicate that for the evaporation experiments, when the drop is released from the needle, the liquid slightly penetrates the texture around the mushroom hats. In the following, we make the assumption that this does not significantly affect the values of Fevap. We note that whatever the temperature and the type of substrate, θi is around 140°, while θdep is always >90°, which clearly indicates that a full fakir-to-Wenzel transition never arises. During the evaporation, the CL must overcome a depinning energy barrier to recede, which translates in the depinning forces Fevap. To check whether the PCL to MCL transition is thermally activated or at least influenced by the temperature variation, Fevap is plotted versus T in Figure 4 for the samples S2−S8.

contributions, capillary and friction at liquid−solid interface, our result evidences that the substrate temperature not only modifies the liquid surface tension but also changes the contact angles and consequently the deppining force and the depinning energy. The temperature increase thus leads to a reduction in the frictional term. The slope of the straight line fitting the experimental points corresponds to the total energy W dissipated by one pillar when the contact angle moves back and forth on it. From our experiments, we found the following energy values: W40° ≈ 4 × 10−13 N·m and W60° ≈ 6 × 10−14 N·m. The thermal energy brought by the substrate heating should reduce the energy required to the contact line depinning. Evaporation Kinetics. The triple line dynamics was systematically investigated during the whole drop evaporation. In Figure 3a is shown a typical evaporation that exhibits three

Figure 4. Fevap = γ Δcos θevap as a function of the substrate temperature T during evaporation for S2 and S8. There is no net dependency with temperature.

From the results, it is not possible to identify any clear decrease in Fevap when T is increased. On average, taking into account the error bars, Fevap appears to be quasi-constant. Such a behavior suggests an increase in the friction at liquid−solid interface with the temperature, accounting for the dependence of γ with the temperature. Moreover, it is worth noticing that the ratio of both forces per unit of length is inversely proportional to the corresponding ratio of the linear density of posts (λS8/λS2 = 1.6). This result emphasizes that the pinning is only due to the presence of the posts: The lower the solid− liquid fraction, the weaker the pinning effects. Furthermore, we study the effect of the geometry by comparing Fdyn and Fevap at a given temperature. It is observed that Fdyn and Fevap increase with n, as evidenced in Figure 5. Nevertheless, a marked difference between dynamic and evaporation depinning forces is evidenced that can be interpreted as a partial imbibition that can develop throughout the evaporation. Indeed, a larger friction during evaporation can

Figure 3. (a) Evolution of the contact angle θ (black) and of the contact radius R (green) of an evaporating sessile drop on the surface S8 at 40 °C. The same behavior is observed for the whole set of temperatures from 30 to 80 °C. (b) Evolution of R/Ri as a function of the normalized time at 40 °C for different solid−liquid fractions ϕs. Inset: Evolution of CA.

distinct regimes for T = 40 °C: (i) The drop starts to evaporate with a constant contact radius, that is, a pinned contact line (PCL); (ii) then, the contact line starts to recede. This moving contact line (MCL) regime is characterized by stick−slip events associated with oscillations of the contact angle. (iii) Finally, a mixed mode (MM) takes place, in which the contact radius and the contact angle decrease simultaneously until the drops evaporates completely. A similar kinetics was observed at all temperatures. The normalized contact radius R/Ri and the contact angle θ are plotted against t/tf, with tf the evaporation time in Figure 3b and inset. It is shown that the kinectics of evaporation exhibits remarkable dependence on the microstructure arrangement. The surfaces provide an initial angle θi between 140 and 150°. When the friction is increased (higher ϕs) the PCL to MCL transition is delayed together with a lower θdep. Pinned to Moving Contact Line Transition. The effect of the surface geometry on the dynamics of evaporation on the PCL-to-MCL transition is investigated. First, the depinning

Figure 5. Normalized depinning forces Fdyn and Fevap as a function of n, including dynamic force measurements for twait = 5 and 7 min after drop deposition. T = 40 °C. C

DOI: 10.1021/acs.langmuir.5b04445 Langmuir XXXX, XXX, XXX−XXX

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the contact radius decreases stepwise. Qualitatively, it appears that the average slip distance of the stick−slip events is close to the interspacing between two neighboring microstructures at the surface. This observation suggests that the stepwise reduction in contact radius is directly related to jumping of the contact line between surface features. This type of motion is accompanied by a temporary oscillation in the contact angle. Figure 6b shows significant variation (∼10° for a substrate at 30 °C) between the angle measured at the onset of the receding motion (the maximum angle) and the angle measured right before the snapping of the receding line (the minimum angle). After the detachment of the receding line, the angle of the contact line would abruptly increase as a new cycle of receding line motion began on the adjacent feature. In addition, it is worth noticing that also for the investigated surface the contact-angle oscillations occur around 10°. This result associated with the multiple stick−slip events exhibited by the contact line indicates that during the evaporation process in the MCL regime the water droplet remains in the “fakir” state. Such a robustness of the superhydrophobicity property results from the re-entrant surface curvature of the micropillars. Another striking feature analyzed in this study concerns the influence of the substrate temperature on the evaporation dynamics. The temperature effect on the contact radius evolution is displayed in the inset of Figure 6a. It shows that for a water droplet at 80 °C, the stick−slip events quasidisappear and the contact radius decreases more smoothly. The temporal evolution of the contact angles is plotted in Figure 6b for different temperatures. As can be observed, among the two parameters (amplitude and period) characterizing the angle dynamics, the amplitude of oscillations is clearly the most affected by the increase in the substrate temperature. The oscillations are drastically reduced and become smooth (see Figure 6b, T = 40 °C and Figure 6b, T = 80 °C). The temperature effect on the angle oscillations is similar to one observed in the stick−slip events. In both cases the dynamics described by contact angles and radius appears to be smoothed by a temperature effect. We can think of two mechanisms that may be responsible for such behavior. From a thermodynamic point of view, it means that at higher temperatures the energy barrier that the CL has to overcome to recede is reduced. Consequently, a small evaporation force allows us to overcome this barrier and leads to a smoother motion of the contact line. From a point of view of the line elasticity theory,30,32 the decrease in the surface tension when the temperature increases also leads to a smooth depinning of the CL. Under such conditions, the strong attenuation observed in the contact angles can also be attributed to the reduction in the elasticity of the receding contact line. In such approach the elastic restoring force, which tends to bring the line back to the undisturbed original position, is proportional to the amplitude of the contact line deformation. It is consequently affected by the temperature variation.

be responsible for this difference. It is known that the stability of the metastable fakir state is straightened by the reentrancies. During the evaporation, the drop is likely not fully suspended on the top microstructures. Rather, because of the specific form of the posts, the liquid−gas interface might connect on the reentrancies at a lower position, increasing the adhesion force and holding the CL more before depinning. On the contrary, less time is given for the liquid to protrude inward the cavities when Fdyn is obtained. Thus, it is relevant to probe how the dynamic hysteresis and the corresponding Fdyn depend on the sample history. For this, a drop is inflated on the surface until θa is reached. After a waiting time twait on the substrate of few minutes, the drop is filled again until it reaches the same volume. Then, θa is measured again as well as θr. The new θa* results in an increase in Fdyn,twait (Figure 5). When the waiting time increases the wetting dynamics get closer to the evaporation dynamics. Thus, a moderate imbibition can take place in time at the hat of the pillar at a rate that could depend on the sample features. It is clear that the drop remains in the fakir state because the contact line is only weakly pinned. Moving Contact Line Regime. As aforementioned, the second phase of drop evaporation on the investigated surfaces occurs in a MCL regime, which is characterized by a stick−slip motion of the receding line and contact-angle oscillations. Such events could be related to the weak interaction between the droplet and the substrate, resulting in weak pinning of the triple line, which thus promotes small contact-angle hysteresis. Focusing initially on the influence of the surface morphology on the evaporation dynamics, we report in Figure 6a the temporal evolution of the contact radius on surfaces with four different pillar concentrations. As can be seen, for all surfaces,

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CONCLUSIONS We have studied the evaporation of droplets on heated mushroom-like superhydrophobic surfaces. In particular, we have focused on the influence of the temperature and of the solid fraction. From our experiments, it clearly appears that the kinetics of drop evaporation is controlled by the contact line dynamics (pinned or moving regimes), which results from a competition between capillary effects and friction. Our main findings are twofold: (i) The depinning angle decreases when

Figure 6. (a) Evolution of the contact radius R during evaporation evidencing stick−slip events in the MCL regime at T = 30 °C for the four designs. Inset: Evolution of the normalized R for the surface S8 at T = 30 and 80 °C. (b) Oscillation of the contact angle during MCL at different temperature for sample S8. D

DOI: 10.1021/acs.langmuir.5b04445 Langmuir XXXX, XXX, XXX−XXX

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Langmuir the friction at the liquid−solid interface increases (higher ϕs), while the droplet motion described by multiple stick−slip events and contact-angle oscillations clearly evidence that the water droplet remains in the fakir state during the evaporation in both regimes. (ii) The thermal energy brought by the substrate heating promotes the contact line depinning in the first stage of the evaporation process. The stick−slip events and the angle oscillations are smoothed by the temperature increase, which from one side reduces the elasticity of the contact line and on the other side reduces the depinning energy barrier; however, even for the highest temperatures, the average contact angle remains >90° and the receding line is only weakly pinned. This remarkable stability of the superhydrophobic state, in regards to the temperature increase, makes such mushroomlike superhydrophobic surfaces very promising candidates for thermal applications involving the gas phase when the fakir state is required.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address †

R.M.d.N.: de São Carlos Institute of Physics, University of São Paulo, PO Box 369, 1356-6590 São Carlos, SP, Brazil.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.M.d.N. acknowledges support given by CAPES 9494-13-8 and FAPESP 2013-21970-8 Brazilian Agencies. This work was partly supported by the French RENATECH network.

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DOI: 10.1021/acs.langmuir.5b04445 Langmuir XXXX, XXX, XXX−XXX