Accepted Manuscript Drop evaporation on superhydrophobic PTFE surfaces driven by contact line dynamics S.M.M. Ramos, J.F. Dias, B. Canut PII: DOI: Reference:

S0021-9797(14)00829-7 http://dx.doi.org/10.1016/j.jcis.2014.10.064 YJCIS 19958

To appear in:

Journal of Colloid and Interface Science

Received Date: Accepted Date:

31 July 2014 28 October 2014

Please cite this article as: S.M.M. Ramos, J.F. Dias, B. Canut, Drop evaporation on superhydrophobic PTFE surfaces driven by contact line dynamics, Journal of Colloid and Interface Science (2014), doi: http://dx.doi.org/10.1016/ j.jcis.2014.10.064

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Revised version 10-21-2014 1

Drop evaporation on superhydrophobic PTFE surfaces driven by contact line dynamics S.M.M.Ramos1*, J. F. Dias2, B. Canut3 1

Institut Lumière Matière, UMR 5306 Université LYON1-CNRS, Université de Lyon 69622 Villeurbanne cedex, France 2 Implantation Laboratory, Physics Institute, Federal University of Rio Grande do Sul CP 15051, CEP 91501-970, Porto Alegre, RS, Brazil 3 Institut des Nanotechnologies de Lyon; CNRS UMR 5270; F-69622 Villeurbanne cedex, France

In the present study, we experimentally study the evaporation modes and kinetics of sessile drops of water on highly hydrophobic surfaces (contact angle ~ 160°), heated to temperatures ranging between 40° and 70°C. These surfaces were initially constructed by means of controlled tailoring of polytetrafluoroethylene (PTFE) substrates. The evaporation of droplets was observed to occur in three distinct phases, which were the same for the different substrate temperatures. The drops started to evaporate in the constant contact radius (CCR) mode, then switched to a more complex mode characterized by a set of stick-slip events accompanied by a decrease in contact angle, and finally shifted to a mixed mode in which the contact radius and contact angle decreased simultaneously until the drops had completely evaporated. It is shown that in the case of superhydrophobic surfaces, the energy barriers (per unit length) associated with the stick-slip motion of a drop ranges in the nJ.m-1 scale. Furthermore, analysis of the evaporation rates, determined from experimental data show that, even in the CCR mode, a linear relationship between V2/3 and the evaporation time is verified. The values of the evaporation rate constants are found to be higher in the pinned contact line regime (the CCR mode) than in the moving contact line regime. This behavior is attributed to the drop’s higher surface to volume ratio in the CCR mode.

*Corresponding author. E-mail : [email protected]; Phone : (33) 472431218

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1. Introduction Superhydrophobic surfaces have been studied with an increasing interest for both fundamental research [1-3] and practical applications [4-6] such as anti-fogging or self-cleaning surfaces, micro- and nano-devices [7-9], and thermal/energy systems [10, 11]. Many such applications are confronted by the evaporative behavior of a sessile droplet. Thus, understanding and controlling the evaporation dynamics of water droplets on superhydrophobic surfaces is of critical importance to the optimization of their large range of applications. Droplet evaporation kinetics on hydrophilic or hydrophobic surfaces have been extensively studied, both numerically and experimentally [12-21]. Several evaporation modes have been evidenced: a constant contact angle (CCA) mode [12, 22], in which the contact area of the droplet on the substrate vanishes; a constant contact radius (CCR) mode [12, 23, 24], in which the contact angle vanishes; and a combination of both of these modes [12, 25, 26], in which both contact angle and radius decrease. In previous studies, it is commonly agreed by the scientific community that most of the evaporation occurs at the triple line, which is usually assumed to be stationary in the different evaporation approaches. On superhydrophobic surfaces or surfaces with larger contact angles, the evaporation kinetics of droplets may be influenced by various parameters such as: a generally free (or only weakly pinned) contact line, and a droplet geometry that, with a large contact angle, leads to an increase in the concentration of water close to the contact line. Both features modify the dynamics of the droplet evaporation and lead to a decrease in the evaporation mass flux. In the last few years, several experimental studies of droplet evaporation on superhydrophobic surfaces were performed [27-33]. Nevertheless, most of these works have dealt essentially with the natural evaporation process, whereas the influence of substrate temperature on the evaporation process has not been investigated. Indeed, until now, the relationship between temperature and the wettability regimes (homogenous or heterogeneous wetting) had been studied only for contact angles up to 90° [34]. The evaporation mechanisms on such surfaces also remain unclear and more extensive and systematic investigations are still lacking in this field of research. The present paper reports on an experimental study of water droplet evaporation on highly hydrophobic PTFE surfaces with a small contact angle hysteresis. These surfaces were initially constructed by combining ion irradiation effects and thermal annealing. The evaporation dynamics of small droplets on processed surfaces, heated to temperatures ranging between 40° and 70°C, were then investigated. The influence on the evaporation process and

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evaporation rate, of contact line mobility (pinned or moving) and substrate temperature, are discussed in quantitative terms. The values of the evaporation rate constants are determined at several different temperatures, the observed stick-slip motion of the drop is analyzed, and the energy barriers associated with such events are estimated. 2. Experimental procedure Surface processing Commercially available sheets of 1.0 mm thick PTFE were used as substrates. Coupling between the low surface energy of the PTFE and judicious micro-structuration of substrates appears as an interesting way to process superhydrophobic surfaces. The PTFE substrates used in the present study were tailored by combining the effects of ion irradiation and thermal annealing, at a temperature close to the melting point of this polymer (melting point of PTFE: Tm = 327°C). Prior to their irradiation, the samples were initially cleaned with ethanol in an ultrasonic bath for ∼30 min and then dried at room temperature. The specimens were then irradiated at room temperature, with 80 keV  ions at a nominal fluence of 1017 ions.cm-2 produced by the ion implanter of the Institute of Physics at Porto Alegre (Brazil). The choice of this fluence value was guided by previous studies [35], which evidenced the interest of a high implantation fluence for the tailoring of target surfaces. In the present study, we fixed the spatial distribution of the irradiated ions and focused our attention on the influence of the geometric characteristics of the features produced by the combined effects of ion irradiation and thermal annealing on the wettability of processed substrates. Following their irradiation, the substrates were annealed in air at 325 °C for 90 min and then slowly cooled, at the “Institut Lumière Matière” (Lyon, France). The resulting surface morphology of the final samples was characterized by Scanning Electron Microscopy (SEM), using a FEI Inspect instrument operated at 5 kV. Wetting characterization The sessile drop method was used to characterize the wetting properties of the as-processed surfaces. For such experiments a homemade device was used, allowing the substrate temperature and humidity rate to be controlled independently. The samples were thus introduced into a glass chamber and the liquid drop was gently deposited onto the substrate by an automatic injection pump. The three-phase contact line of the water drop was advanced or receded by adding or withdrawing a small volume (~ 2 μl) of de-ionized water. The steadystate advancing (θA) and receding (θR) contact angles were measured, allowing the

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determination of the contact angle hysteresis defined as Δθ= θA - θR. To avoid any perturbation from external flux, the chamber remained closed during the experiments. The measurements were performed optically with an accuracy of 1°, on at least four drops deposited at different locations on each sample. In order to provide a reference surface, the wetting properties of a bare, smooth PTFE sample were also characterized. All measurements dealing with wetting characterization were performed under ambient conditions (temperature: 22 ± 1 °C and relative humidity: 45 ± 2%,).

Evaporation kinetics For the investigation of evaporation kinetics, the same experimental device was used. In this part, the droplets of de-ionized water were initially at room temperature when deposited on the heated substrates. The heating temperature was measured by two ways : (i) in the expererimental device, it was directly measured in the copper support where the teflon substrates were fixed. This temperature will be referred as the substrate temperature ; (ii) directly at the surfaces of the heated samples (five different locations were considered) by using an infrared thermometer. The maximal difference measured between the setted temperature and the surface temperature was of 1.8° for the highest temperature values. The uniformity of the heating was observed in about 95% of the surface area. The sessile water droplets having an initial volume of ~ 3 μl were thus evaporated from substrates heated to four different temperatures (40, 50, 60 and 70 °C). The static contact angle θE(ev) was measured just after the drop deposition. A relative humidity of 50 ± 5% was maintained inside the chamber during all of the experiments. To analyze the evaporation dynamics, the droplets were observed during their total evaporation time and side-view images were recorded at 1 s intervals. Several experiments were performed for each surface, with reproducible results. The temporal variation of the contact angles and contact radii were measured directly by using “Drop Snake”, a plugging of image J, based on the B-spline to fit the drop shape. In this general method the contact angle is obtained by a piecewise polynomial fit. Other parameters such as the contact radius and the interface tilt angle are also available. [36, 37].

3. Results Superhydrophobic surfaces elaboration

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Figure 1 shows typical top-view SEM images of the micro-structured PTFE. It can be seen that the surface morphology consists of a random distribution of rod-like structures, which are either isolated, or joined together in a bundle. More than 30 objects were analyzed, in order to determine the dimensions of these features. Each of the individual structures is characterized by a width (w) of 3.5 μm, a thickness (e) of ~ 2.6 μm, a length of ~ 40 μm, and by the presence of a small number of 0.9 µm wide streaks at their surface. These structures thus have a very high aspect ratio. From a topographical point of view, this is a fundamental condition to obtain a superhydrophobic behavior. Two characteristic distances were also extracted from the SEM images: d1 = 23 ± 3 μm, describing the interspacing of two adjacent isolated features and d2 = 5 ± 1μm, corresponding to the distance between the two constituent structures of the bundle of rods. According to previous studies [35] based on the irradiation effects on the PTFE, the length of similar structures can reach a value of 100 μm, whereas their width can be reduced to the scale of a nanometer. These parameters are strongly influenced by the annealing conditions. The advancing and receding contact angles measured, at room temperature, on processed surfaces were θA = 160° and θR = 154° (see inset in figure 1). These large contact angles, associated with a small contact angle hysteresis (Δθ= 6°), indicate that the liquid drop remains on the top of the roughness and consequently that superhydrophobic surfaces were achieved from PTFE processing. This type of configuration is well described by the Cassie-Baxter (CB) approach [38], which assumes that the liquid drop sits on top of a composite surface that consists of solid and air pockets, i.e. that the liquid does not fill the grooves on the rough surface. The CB model leads to the following relationship: cos θ* = −1 + Φ (1 + cos θflat) (1) where θ* is the apparent contact angle measured on the structured surface and the parameter Φ is the solid fraction in contact with the liquid (Φ is dimensionless and smaller than unity). To determine the value of Φand complete the wetting characterization of the PTFE surfaces, the water contact angles were also measured on a flat, smooth Teflon surface. The values obtained were: θAflat = 118° and θRflat = 105°, which are in good agreement with those found in the literature [39]. From the contact angles measured on the bare and structured surfaces, the value of Φ ψwas determined to be ~ 0.13. This shows that the base of the drop is mostly in contact with air, and provides an explanation for the very low values of hysteresis. A crude

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estimation of the latter parameter was also made from SEM images, leading to Φ~ 0.17, which is in good agreement with the aforementioned value.

Evaporating drops Figure 2 shows typical images of a drop evaporating on superhydrophobic PTFE heated to 40°C. The variations in contact angle and contact radius during the evaporation process are shown in Figure 3 for four different temperatures. Qualitatively, it can be seen that evaporation occurs according to triple line dynamics. In the case of surfaces characterized by a small contact angle hysteresis, a singular behavior, intermediate between the constant contact radius and the contact angle regimes, was observed at all temperatures, although the kinetics were of course faster at higher temperatures. As shown in Figure 3a, in the case of a droplet at 40°C, the evaporation at superhydrophobic PTFE surfaces occurs in three stages. The first of these has a duration is about 20-30% of the whole evaporation process, and the contact radius of the droplet remains constant while the contact angle gradually decreases until the receding angle of the surface is reached. At this critical value, depinning of the contact line occurs. The contact line is then observed to jump to lower values. This is accompanied by a temporary increase in the contact angle, followed by renewed pinning of the contact line. The contact angle then decreases further to a new receding contact angle, and the relative contact radius remains practically constant until the next jump occurs. A ‘‘stickslip’’ cycle is thus established. The first of these events characterizes the transition from stage I to stage II in the evaporation process. During stage II the contact angle decreases significantly, from about 140° to 120°-115°, whereas the motion of the contact line exhibits multiple stick-slip events, which also lead to a decrease in contact radius. This type of motion could be related to the weak interaction between the droplet and the substrate, resulting in weak pinning of the triple line, which thus promotes non-hysteresis sliding of the droplet. It is interesting to note that the CCA evaporation regime was not observed during our experiments. Contrary to the results reported elsewhere in the literature [31, 32], the samples used in the present study had a smaller surface area per volume during this stage of the evaporation process. Therefore, just before the end of stage II, a transition from the Cassie-Baxter state to the Wenzel state [40] takes places. In the later state, contact at the liquid-solid interface is maximal (the liquid penetrates completely into the roughness valleys), describing a homogeneous wetting. Finally, in regime III, a mixed mode is observed during which both the radius and the contact angle decrease simultaneously, until the end of the evaporation process. Depending on the

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substrate temperature, the length of this phase represents approximately 10-15% of the drop’s lifetime.

4. Discussion Two main aspects of the aforementioned kinetics of drop evaporation on a structured PTFE surface are discussed as following: the first of these deals with the stick-slip phenomenon and contact line moving observed during the evaporation process; the second concerns the evaporation rate on highly hydrophobic surfaces and its dependence on the evaporation modes. Stick-slip phenomena We now focus more closely on the stick-slip events observed in Figure 3. From the thermodynamic point of view, this phenomenon results from the larger number of metastable apparent contact angles of the drop. Thus, to move from one local minimum to the next, the drop has to overcome an energy barrier. The theoretical approach, proposed by Shanahan [41] is based on a certain excess of free energy as a criterion for contact line movement. This has provided a satisfactory description of the stick-slip motion occurring on smooth or some rough surfaces, with a liquid drop in a Wenzel configuration40. According to the referred work the excess free energy (per unit length) of the triple line associated with the stick-slip event can be expressed as: మ ೃሺ೐ೡሻ   ೃሺ೐ೡሻ ಴ మ ಴

(2)

where γ is the surface tension of the liquid, θR(ev) is the receding angle of the surface calculated as the constant angle with which the triple line recedes in phase II, δRC is the local slipped distance to the stick-slip event, and RC is the local pinned contact radius before the drop slips. This approach was adopted for the quantitative analysis of the stick-slip events observed in our experiments. We thus measured δRC, θR(ev) and RC during the main individual events, and at various temperatures. The measured values were used to estimate the energy values reported in Table 1. The experimental results can be summarized as follows: - The average slip distance of the stick-slip events is about 25 μm, which is close to the interspacing between two neighboring microstructures at the PTFE surface. This observation

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suggests that the stepwise reduction in contact radius is directly related to jumping of the contact line between surface features. - At the primary «stick » period, the initial contact angle decreases by a temperaturedependent value ranging between 6°- 10° C (e.g., 6° at 40°C and 10° at 70°C), suggesting an increase in the value of Δθ(ev) (Δθevap =θE(ev) - θR(ev)). However, the contact angle hysteresis is directly related to the depinning force (per unit length) acting on the contact line, which depends on both the contact angle values and the surface tension of the liquid. As the latter parameter decreases when the temperature increases, the depinning force remains relatively constant when the temperature of the substrate varies. Thus, superhydrophobic surface properties, such as low adhesion and low friction at liquid-solid interfaces, are not significantly modified by heating of the substrate. Concerning the computed results shown in Table 1, the δG value appears to be about 10-8 J.m1

, and to be few sensitive to variations in substrate temperature. This value is about one order

of magnitude smaller than that reported by Shanahan and Sefiane [42] for the evaporation of ethanol droplets on randomly rough PTFE substrates, and two orders of magnitude smaller than those reported by Bromashenko [43] for the evaporation of water on various polymers. It should also be noted that the aforementioned studies deal with a homogeneous wetting configuration (Wenzel state) in which the surfaces are partially wetted by the liquid and have a high contact angle hysteresis. The low values obtained for δG in the present study can be ascribed to the high hydrophobicity of our substrates. In the Cassie-Baxter state the drop meets a composite surface that consists in a great part of gas pockets (gas fraction in contact with the liquid : ~ 83% ). The small solid-liquid interfacial area clearly reduces the energy barrier. On the other hand, considering the random character of the surface roughness we cannot exclude that other stick-slip events happen in other directions different than the view direction. Such events might also contribute to reduce the energy barrier of sequential stickslip that can be observed through a single side view. To confirm this hypothesis more quantitative investigations are required in the future experiments. Finally, the dimensions of the quantity δG suggest that it could be related to the line tension, as has been proposed in earlier studies [42, 43]. Whether they be determined by experiment or derived from theoretical models, the values of line tension reported in the literature generally lie in a very broad range, between 10-11 and 10-5 J.m-1 [44]. As the values of δG determined in the present study also lie in this range, it is likely that the line tension is intimately related to the free energy barrier associated with the stick-slip events.

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Evaporation rate Another striking feature analyzed in this study is the evaporation rate of small drops on highly hydrophobic surfaces. According to the theory of drop evaporation, based on vapor diffusion across the droplet boundary, the evaporation rate is strongly correlated to the evaporation mode [12, 17, 22 - 24]. A linear relationship was thus reported between the volume of a sessile droplet and its total evaporation time in the CCR mode, which is usually found on hydrophilic surfaces [23, 24]. On hydrophobic surfaces or in the CCA mode, a power law /

relationship between the volume of a drop and its evaporation time, such as  /  



, where V0 is the droplet’s initial volume and k is the evaporation rate constant, that depends on the temperature and the surrounding relative humidity, has often been reported [17, 22 ]. Evaporation on superhydrophobic surfaces usually includes the CCR and/or CCA modes, but can also include a combination of both modes that may result in a more complex behavior. Since the influence of gravity is negligible in the case of small drops, the fluid adopts a spherical cap shape and the volume V depends only on the contact angle and the contact radius, described by the following expression:

 , 

 య  మ   

య 

(3)

Figure 4 displays the rate of volume change of water droplets (i.e. evaporation rate) on substrates heated to different temperatures, clearly showing that the volume decreases nonlinearly with time. As indicated in this figure, the changes in the droplet volume are influenced by the dynamics of the contact line, in which two phases can be identified: pinned contact line (CCR mode) and moving contact line. Nevertheless, according to previous studies [32, 33], the evaporation rate on superhydrophobic surfaces can in general be well described by the linear law:  /  . To evaluate the relevance of this law in the context of the present study, we analyzed, for each regime, the change in volume raised to the power of 2/3 during the evaporation of a droplet. Typical results are presented in Figure 5, showing that on the pinned contact line phase, a linear regression of the experimental data leads to a quite good fit. This confirms that on a highly hydrophobic surface, even in the CCR mode, the timedependent decrease in drop volume follows the power-law model. As could be expected, the evaporation rate constants obtained from the slope of the linear fit to the experimental data

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increase with temperature. The slopes values corresponding to these fits vary from 2.0×10 -3s-1 to 7.6×10 -3s-1 on substrates heated to 40°C and 70°C, respectively. In the case of drops evaporating on the moving contact line phase, involving multiple stickslip events, the expected relationship between the volume of a droplet and its total evaporation time is less clear. In the present case, both the contact angle and the radius decrease. However, the behavior observed from our experimental results (see figure 5b) is similar to that described above for the pinned contact line regime. Although the stick-slip motion of the contact line leads to some transient variations in the experimental curve, the time-dependent reduction in droplet volume is found to be in good agreement with a power law representation, thus confirming the predicted behavior for the evaporation of drops on hydrophobic surfaces. The slopes of the lines fitting the experimental data are found to lie in the interval between 1.0×10 -3 s-1 and 3.8×10 -3 s -1 for samples heated to 40°C and 70°C, respectively. The evaporation rate constants are thus found to be smaller than those obtained in the CCR mode. This trend can be explained by the fact that during evaporation in the CCR regime, only the contact angle gradually decreases, whereas in the moving contact line regime (characterized in the present study by multiple stick-slip events) both the contact angle and the radius decrease. A direct consequence is that in the CCR mode, despite a higher average value of contact angle, the drops have a greater surface area per volume, thus making the evaporation process more efficient. It is of interest to note that the ratio between the values of the evaporation rate constants determined at the minimum and maximum temperatures is almost the same for the two contact line configurations. To summarize, as shown in Figure 5c-d, our experimental results reveal that whatever the evaporation regime of droplets (CCR mode or a set of stick-slip events), on superhydrophobic PTFE surfaces the overall evaporation rate is characterized by a linear relationship between V2/3 and the evaporation time. The temperature has only a limited influence on the evaporation process and the wetting dynamics. This result can be ascribed to the different factors such as: (i) a very small solid-liquid interfacial area (Ac= πRc2) which, associated to the low thermal conductivity of PTFE, significantly reduces the thermal conduction; (ii) The possible existence of a temperature gradient across the droplet, due to its high height-to-base aspect ratio, which should be amplified with the increase of the substrate temperature. This point should be deeply investigated in future experiments. Conclusion

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Highly hydrophobic surfaces were produced using Teflon substrates, on which θ∗ > 150° and Δθ ~ 6° under equilibrium conditions. The drops were found to be in the Cassie-Baxter state. By focusing our study on the evaporation modes and kinetics of sessile drops, two dominant phases that depend on the dynamics of the contact line were evidenced: a pinned contact line where the contact radius decreases during the evaporation process and a moving contact line where the contact radius decreases step-by-step and the contact angle also decreases. The evaporation rate and the constants of the evaporation rate were determined from the temporal evolution of drop volume at different temperatures. It is shown that the contact line configuration (pinned or moving) does not affect the linear relationship between a droplet’s evaporation time and its volume raised to the power of 2/3. Differently, the values of the evaporation rate constants are found to be higher in the pinned contact line regime (the CCR mode) than in the moving contact line regime. This difference is attributed to the fact that in the CCR mode, the droplets have a greater surface-to-volume ratio. Finally, the energy barriers computed for the stick-slip motion of a drop show that the average slip distance is of the order of the interspacing between two adjacent surface microstructures, and that the energy barrier ranges in the nJ.m-1 scale which is at least one order of magnitude lower than values previously observed for drops in a homogeneous wetting configuration.

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[43] E. Bromashenko, A. Musin, M. Zinigrad ; Colloids and Surfaces A : Physicochemical and Engineering aspects, 385 (2011) 235-240. [44] V. Raspal, K. O. Awitor, C. Massard, E. Feschet-Chassot, R. S. P. Bokalawela, and M. B. Johnson, Langmuir 28 (2012) 11064 – 11071, and references there in.

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T (°C)

γ (mN.m-1)

θE(ev)(°)

θR(ev) (°)

Rc (mm)

δRc (μm)

40

69.6

151

50

67.9

151

60

66.2

153

70

64.2

154

145 138 125 113 141 139 132 144 141 133 144 143 146 120

0.46 0.41 0.36 0.31 0.41 0.39 0.36 0.38 0.37 0.31 0.39 0.33 0.30 0.27

22 20 30 30 15 30 30 23 13 20 50 20 20 32

δG (J.m-1) 1.4 ×10 -8 2.3×10 -8 8.3×10 -8 1.4 ×10 -7 8.3×10-9 4.2×10 -8 5.7×10 -8 1.1×10-8 7.4×10-9 3.0×10-8 8.4×10-8 1.7×10 -8 1.8×10 -8 1.4×10 -7

Table 1 : Substrate temperature T, surface tension γ, typical measured values of contact and angles during the evaporation process (θE(ev), initial quasi-equilibrium contact angle, θR(ev) contact angle with which the contact line recedes before the corresponding slip events), the local pinned contact radius RC before the its slips, the local slipped distance δRc and the energy barrier (per unit length), δG.

a b

The contact angle measurements were done optically with an accuracy of ±1°. The uncertainty in the RC values was estimated of 5%.

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Figure Captions : Figure 1 : SEM images of structured samples. Two kinds of detectors were used. (a) standard detector ; (b) association of standard and InBeam detectors. Inset: An instantaneous image of the drop being advanced on the surface.

Figure 2 : Typical snapshots of the sideviews of an evaporating sessile droplet on the microstructured Teflon surface. They illustrate the drop in two contact line configurations.

Figure 3 : Temporal evolution of the contact angle and the contact radius of a sessile droplet at various substrate temperatures from 40°C to 70°C. In (a) the distinct evaporation configurations (CCR, constant contact radius mode ; moving contact line with stick-slip events ; mixed mode) are divided by vertical dotted lines.

Figure 4 : Temporal evolution of droplet volume during evaporation on substrates heated at different temperatures varying from 40°C to 70°C. The transition from pinned to moving contact line is indicated by a color arrow for 40° and 50°C, respectivelly. The uncertainty in the volume values was estimated of 10%.

Figure 5 : Temporal variation of droplet volume raised to two-third power with substrate temperatures at pinned (a) and moving (b) contact line configurations. The dashed lines represent the best fits of the data. Evolution of V2/3 versus time of sessile droplets on surfaces at 40°C (c) and 60°C (d), respectively. The continuous line describes the best fit obtained on each of two evaporation configurations.

17

(a)

(a)

50 μm

(b)

10 μm

Figure 1

18

(a) Pinned contact line

t = 0 min

4 min

7 min

(b) Moving contact line

t = 8 min

13 min

22 min

Figure 2

28 min

31 min

19

160

40°C 0,5

140

Con tact 0,4 Rad ius (m 0,3 m)

δRC

Cont 120 act Ang 100 le (°) 80

δRC δRC δRC

60 40

0,2

Stick-slip

CCR

Mixed

20 0

500

1000

1500

2000

Time (s)

160

160

Con140 tact 120 Ang100 le 80 (°) 60

50°C

0,5

160

60°C

140

0,5

120 0,4 100

100

80

0,3

0,2

20

0,3

400

800

1200

1600

80

60

60

40

0,2 40

20

0

tact Rad 0,4 ius 0,3 (m m)

120

0,4

40

70°C 0,5Con

140

0

200

400

600

Time (s)

Figure 3

800

1000

0,2

20 0

200

400

600

800

20

3,5

• 40°C • 50°C • 60°C • 70°C

3

Vo lu me (μl)

2,5 2 1,5 1 0,5 0 0

500

1000

Time (s)

Figure 4

1500

2000

21

2

2,4

Pinned CL 2,2

• 40°C ; • 50°C ; • 60°C ; • 70°C.

V 2/3

(10

2

-6

1,8

m2 )

1,6

Moving CL

V

• 40°C ; • 50°C ; • 60°C ; • 70°C .

1,5

2/3

(10 -6

m2 )

1

0,5

1,4

0

1,2 0

100

200

300

0

400

500

Time (s)

1000

1500

Time (s)

2,5

2,5

40°C

V

Moving CL

2

Moving CL

2

2/3

1,5

(10

-6

m2 )

60°C

V

2/3

(10

2000

1,5

-6

m2 )

1

0,5

1

0,5

0

0 0

500

1000

1500

2000

0

Time (s)

200

400

600

Time (s)

Figure 5

800

1000

22

Graphical Abstact

160

PTFE

10 μm

C 140 on 120 ta ct 100 an 80 gl e 60 (°)

0,5

40

0,2

Co nta 0,4 ct rad ius 0,3 (m m)

20 0

500

1000

time (s)

1500

2000

23

Highlights • • • • •

Highly hydrophobic surfaces were produced using Teflon substrates. The evaporation kinetics of water droplets were studied at different temperatures. The evaporation process is driven by the contact line dynamics (pinned/moving). The energy barriers associated with the stick-slip motion of droplets is ~ nJ.m-1. The evaporation rate constants are enhanced in the pinned contact line regime.