Article pubs.acs.org/Langmuir
Octagon to Square Wetting Area Transition of Water−Ethanol Droplets on a Micropyramid Substrate by Increasing Ethanol Concentration Huicheng Feng,† Karen Siew-Ling Chong,‡ Kian-Soo Ong,‡ and Fei Duan*,† †
School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore ‡ Institute of Materials Research and Engineering, A*Star, 2 Fusionopolis Way, Innovis, Level 9, Singapore 138634, Singapore S Supporting Information *
ABSTRACT: The wettability and evaporation of water−ethanol binary droplets on the substrate with micropyramid cavities are studied by controlling the initial ethanol concentrations. The droplets form octagonal initial wetting areas on the substrate. As the ethanol concentration increases, the side ratio of the initial wetting octagon increases from 1.5 at 0% ethanol concentration to 3.5 at 30% ethanol concentration. The increasing side ratio indicates that the wetting area transforms from an octagon to a square if we consider the octagon to be a square with its four corners cut. The droplets experience a pinning−depinning transition during evaporation. The pure water sessile droplet evaporation demonstrates three stages from the constant contact line (CCL) stage, and then the constant contact angle (CCA) stage, to the mixed stage. An additional mixed stage is found between the CCL and CCA stages in the evaporation of water−ethanol binary droplets due to the anisotropic depinning along the two different axes of symmetry of the octagonal wetting area. Droplet depinning occurs earlier on the patterned surface as the ethanol concentration increases.
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INTRODUCTION Understanding and manipulating droplet dynamics and shapes on textured substrates are of vital importance for a wide variety of applications, including biology,1,2 micro- and nanofluidics,3,4 self-cleaning,5,6 inkjet printing,7,8 and thermal management.9,10 In nature, it is widely observed that the substrate surface properties exhibit significant effects on the liquid−solid interaction, for example, the lotus effect, a well-known superhydrophobic phenomenon. As superhydrophobic surfaces find promising applications in self-cleaning,11 anti-icing,12 and water-harvesting,13 numerous studies have been carried out on this field and various kinds of methods have been proposed to generate the superhydrophobic surfaces,6,14 including microand/or nanopatterning, coating, and chemical treating. Moreover, the substrate softness also influences its hydrophobicity.15 Besides the superhydrophobic surfaces, hydrophilic surfaces are also paid more attention as they are desirable for agricultural spraying16 and boiling heat transfer.17 In addition to manipulating the substrate hydrophobicity, controlling the wetting area of a liquid on solid surfaces remains a hot topic because of the promising applications for highresolution printing in various areas.8,18 The studies have been conducted to manipulate the wetting area by mechanically and/ or chemically engineering patterns on the substrates.19,20 Grooves or ridges were fabricated on the substrates to generate anisotropic wetting of droplets.21,22 Nonuniform pillar spacings along the x and y axes also produced anisotropic wetting.23 The © 2017 American Chemical Society
impinging droplets showed anisotropic spreading on the grooved substrates,24 which is consistent with the anisotropic wetting. Various wetting areas were produced by changing the pillar arrangement on substrates for both droplets20 and liquid films.25 A pillar arrangement was also used for generating or suppressing directional splashing in droplet impingement.26,27 Hitherto, the manipulations of wetting shape and evaporation dynamics of droplets on the patterned surfaces were mainly conducted by altering the surface properties, including the shape, size, and density of artificial structures,20,28,29 chemical heterogeneity,30,31 substrate hydrophobicity,32,33 and substrate softness.34 Although the studies have been conducted for the effect of droplet composition on the shape and evaporation dynamics of droplets, the employed substrates are of plain surfaces.35,36 The influence of droplet composition on the droplet shape and the corresponding evaporation modes on the patterned substrates remains elusive, which is of essential importance because various fluids or mixtures are used in practical applications. To contribute to the understanding of the wetting and evaporation behaviors of mixture droplets on micropatterned substrates, we experimentally examine the evaporation dynamics of droplets with the water−ethanol binary mixture on the substrates with micropyramid cavities Received: November 22, 2016 Revised: January 16, 2017 Published: January 17, 2017 1147
DOI: 10.1021/acs.langmuir.6b04195 Langmuir 2017, 33, 1147−1154
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microscope (LV100D-U, Nikon) and from two sides by high-speed cameras (HiSpec 2G Mono, Fastec Imaging) as schematized in Figure 1. The optical microscope with a bright-field schema was adjusted to focus on the solid−liquid interface to capture the evolution of droplet shape. Information about the droplet baseline and wetting area was obtained from the top view using the NIS Elements software of the microscope (Nikon). Meanwhile, the high-speed cameras were set to capture the droplet from two side views (Figure 1). The locations of the two cameras were determined by the two axes of the wetting areas of droplets that were nonequilateral octagons. The two side-view axes are at an angle at 45°, named as line-of-sight 0° and line-of-sight 45°, which are indicated in Figure 1. The contact angle was computed through the low-bond axisymmetric droplet shape analysis.38 Each case was conducted at least three times to ensure the experimental repeatability. The error bars in the following figures were the standard deviations obtained from those repeated experiments. The experiments were conducted under ambient conditions at a temperature of 23 ± 1 °C and a relative humidity of 45 ± 5%.
over the lifetime of droplet evaporation, i.e., from the moment that a droplet is positioned on the substrate to the moment that the droplet is totally evaporated, by employing a microscope and a high-speed camera simultaneously. The initial wetting area of droplets is found to evolve from an octagon to a square with increasing ethanol concentration if we consider the octagon to be a square with its four corners cut. The ethanol concentration also affects the pinning−depinning transition during droplet evaporation. This study can pave a path for droplet shape control by solely manipulating the droplet composition, which provides a simpler and more cost-effective method in the applications compared to the complex substrate patterning.20
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EXPERIMENTAL METHODOLOGY
Preparation of Test Samples. The water−ethanol binary solutions for droplets were prepared by mixing nanofiltered water with a resistivity of 18.2 MΩ·cm generated by a Milli-Q gradient A10 water purification system (Millipore Corporation) and ethanol (ACS reagent, purity ≥99.5%, Sigma-Aldrich). The droplets were positioned on the micropyramid substrate with an initial volume of 0.5 μL by a micropipette (Thermo Fisher Scientific). The droplet baselines are around 1 mm in the present study. The poly(methyl methacrylate) substrates were patterned with micropyramid cavities using nanoimprint lithography.37 The poly(methyl methacrylate) polymer was imprinted with a nickel shim mold at a temperature of 140 °C and a pressure of 5 bar for 10 min using an Eitre 6 nanoimprinter (Obducat) so that the polymer could fill up the cavity of the nickel shim mold. The demolding was done at 50 °C. The structure of the fabricated micropyramid cavities was visualized by a confocal microscope (DCM8, Leica Microsystems) and presented as Inset-2 of Figure 1. As demonstrated by Inset-2 and Inset-3 of Figure 1, the top surface of the micropyramid cavity is a square with a side length of l = 30 μm, and the depth from the top surface to the peak point is h = 11 μm. Measurements of the Baseline and Contact Angle. Droplet evaporation was simultaneously visualized from the top with an optical
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RESULTS AND DISCUSSION Droplet Initial Wettability. Unlike the isotropic wetting on plain substrates, sessile droplets form octagonal initial wetting areas on the micropyramid substrates regardless of the ethanol concentration (Figure 2a). The octagon has two axes of symmetry, which define the positions of the two side-view cameras described as line-of-sight 0° and line-of-sight 45°, respectively. The droplets present different initial baselines and different initial contact angles along the two lines of sight. The initial baseline along line-of-sight 0° is shorter than that along line-of-sight 45°, and the initial contact angle along line-of-sight 0° is larger than the one along line-of-sight 45°. Besides, the initial contact angles along both lines of sight depend on the ethanol concentrations. As the ethanol concentration increases from 0 to 30%, the initial contact angles along line-of-sight 0° and line-of-sight 45° decrease by 15 and 21°, respectively. The initial contact angle on the plain substrate also decreases as the ethanol concentration increases (Figure 2b). The scale bars in Figure 2a slightly decrease, but all represent 200 μm with increasing ethanol concentration, which reveals that the initial baselines increase with an increase in the ethanol concentration. This trend is clearly demonstrated by the linear fitting shown in Figure 2c. Furthermore, a higher ethanol concentration also leads to a larger difference between the initial contact angles and baselines along the two lines of sight (Figure 2b,c). The dependence of the initial contact angles and the initial baselines on the ethanol concentration can be interpreted by Young’s equation, γSV = γLV cos θ + γSL, which describes an equilibrium state of the three-phase line of a sessile droplet. In the equation, γSV, γLV, and γSL are the solid−vapor, liquid− vapor, and solid−liquid surface tensions, respectively, and θ is the contact angle. The presence of ethanol at the liquid−vapor interface of a binary liquid droplet leads to a decrease in the overall γLV, so the droplet exhibits a smaller initial contact angle.39 Because the initial droplet volume is almost the same, a smaller initial contact angle is accompanied by a larger initial baseline. The varying initial contact angles and baselines with increasing ethanol concentration result in a difference for the wetting areas as presented by the top view in Figure 2a. To describe the variation of the wetting area with the ethanol concentration quantitatively, we hereby introduce a geometric parameter, initial wetting side ratio (b/a), where a and b are the side lengths of the octagon along line-of-sight 45° and line-ofsight 0°, respectively. Figure 3 displays that b/a increases as an exponential function of the ethanol concentration. The value of
Figure 1. Experimental setup. Side-view cameras 1 and 2 are placed at the same height level at an angle of 45°. The substrate is patterned with micropyramid cavities using nanoimprint lithography. Inset-1 and Inset-2 show the SEM image and the 3D view of the micropyramid cavities captured by field-emission scanning electron microscopy (FESEM, JSM-7600F, JEOL, Ltd., Japan) and the confocal microscope (DCM8, Leica Microsystems), respectively. Inset-3: Cross-sectional curve of the micropyramid cavity obtained from Inset-2 by the confocal microscope. As demonstrated in Inset-2 and Inset-3, the top surface of the micropyramid cavity on the substrate is a square with a side length of l = 30 μm. The depth of the pyramid cavity from the top surface to the peak point is h = 11 μm. Line-of-sight 0° and line-ofsight 45°, indicated by the red dotted−dashed line and the orange dashed line, define the two side-view cameras in all of the figures unless otherwise mentioned. 1148
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Figure 2. Droplet initial wettability on the substrate with micropyramid cavities. (a) Initial top view and side views of droplets at different mass concentrations of ethanol c0. Variations of (b) the initial contact angle and (c) the initial baseline of droplets with the mass concentration of ethanol c0. Subscripts 0 and 45 indicate the measurement along line-of-sight 0° and line-of-sight 45°, respectively, and the subscript ps represents the initial wetting data on the plain poly(methyl methacrylate) substrate. The scale bars in (a) are of different lengths, but all indicate 200 μm.
Surface energy analysis reveals that the circular contact areas are thermodynamically favored, whereas the straight contact lines are locally preferred for the micropillared surfaces.40 Moreover, the contact lines parallel to the pillar array are preferred to diagonal lines when Fpill = γSLB − γLVdl < 0, where Fpill is the free-energy difference between a straight air/water interface and a contact line intersecting a single cylindrical pillar, B is the length of the pillar’s surface in contact with a droplet, and dl is the length of the chord linking the three-phase points on the pillar’s surface.40 Although the substrates applied in the present experimental study are patterned with the micropyramid cavities rather than pillars, the underlying nature is same. As shown in Figure 3, the contact lines are either parallel or diagonal to the micropyramid cavity array, i.e., along line-of-sight 0° and line-of-sight 45°, respectively. The contact lines parallel to the micropyramid array (b) are longer than the diagonal lines (a) regardless of the ethanol concentrations. It suggests that the parallel contact lines are preferred, which is consistent with ref 40. When the droplet consists only of water, the relatively high γLV causes the liquid droplet to ball up to reach a small free surface. Confined by the patterns at the substrate, the liquid phase forms in an octagon, which resembles a circle, but a square does not. With an increase in the ethanol concentration, however, the corresponding γLV is reduced; accordingly, the binary solution could reach a larger wetting area. The parallel line (b) increases, while the diagonal line (a) decreases. Therefore, it can be inferred that increasing the ethanol concentration enhances the preference of parallel contact lines. The wetting area evolves toward a squarelike area that approaches a film-like state. For the same reason, we can observe that pure ethanol failed to form a droplet anymore but spread as a film in the experiments instead.
Figure 3. Variation of the initial wetting side ratio b/a with the mass concentrations of ethanol c0, in which b and a represent the side lengths of the octagonal wetting area long line-of-sight 0° and line-ofsight 45°, respectively. R2 is the goodness of the exponential fitting. Inset: Top-view photographs right before depinning, i.e., right before the moment that the sessile droplets start to recede. The scale bar indicates 200 μm.
b/a of less than 1.5 for the pure water droplets jumps to 3.5 for the droplets of 30% ethanol concentration. Such a variation is also manifest in the inset of Figure 3. The droplet wetting area gradually changes from a nearly regular octagon to an area akin to a square but with its four corners slightly cut off for the droplets with 30% ethanol concentration. It is inferred that the droplet wetting area is evolving from an octagon to a square with increasing ethanol concentration if we consider the octagon to be a square with four corners cut. It enlightens us that by varying the droplet composition we would be able to control the droplet wetting area on a textured surface. 1149
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Figure 4. Droplet evaporation on the substrate with micropyramid cavities as a function of the normalized evaporation time t′. The mass concentration of ethanol is c0 = (a) 0, (b) 10, and (c) 20%. The scale bars in (a−c) are of different lengths, but all indicate 200 μm.
Figure 5. Variation of the baseline L and the contact angle θ with the normalized evaporation time t′ at the mass concentration of ethanol c0 = (a) 0, (b) 5, (c) 10, (d) 15, (e) 20, and (f) 30%.
Droplet Evaporation. Figure 4 shows the snapshots of three evaporating droplets with c0 = 0, 10, and 20% at different times on the substrate with micropyramid cavities. The time is normalized with the evaporation lifetime of each droplet. As defined previously, the evaporation lifetime ranges from the moment that a droplet is positioned on the substrate to the
moment that the droplet is totally evaporated. Figures 2 and 3 indicate the consistent behaviors. The droplets initially form octagonal wetting areas on the substrate. As the ethanol concentration increases, the initial contact angles decrease; however, the initial baselines and side ratios increase. Figure 4 shows that the droplets, regardless of the ethanol concen1150
DOI: 10.1021/acs.langmuir.6b04195 Langmuir 2017, 33, 1147−1154
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Langmuir tration, experience a pinning−depinning transition during evaporation. At first, the contact angles keep decreasing, reflected by the droplet side views and the sharpening contact lines from the top view. The baselines and the wetting areas remain almost constant during the same period. As more time elapses, the droplets reach their receding contact angles and start to shrink because of evaporation. The wetting areas are no longer in octagon shapes but have irregular round forms until the end of drying. Droplets would undergo a transition from the Cassie state to the Wenzel state when they evaporate on the rough surfaces.28 In the present experiments, our droplets remain in the Cassie state in the major lifetime of evaporation. The Wenzel state occurs right before the complete drying of droplets, and hence such a wetting-state transition of droplets is not discussed in this study. For the detailed process of droplet evaporation, one may refer to Videos in the Supporting Information, which were captured as the top view and side views along line-of-sight 0° and line-of-sight 45° at an ethanol concentration of 20%. To quantitatively analyze the evaporation dynamics of droplets, we plot the evolutions of contact angles and baselines versus time along the two lines of sight in Figure 5. It is showed that the entire evaporation period can be divided into four stages, i.e., the constant contact line (CCL) stage, the first mixed stage, the constant contact angle (CCA) stage, and the final mixed stage, which are denoted as Stage-I, Stage-II, StageIII, and Stage-IV, respectively, in Figure 5. Regardless of the ethanol concentration, Stage-I always occupies a large portion of the entire droplet evaporation lifetime, during which the wetting area maintains an octagon shape as shown in Figure 4. As seen in Figure 5a, Stage-II does not appears for the pure water droplets. Once the pure water droplets depin, they enter Stage-III directly, i.e., the CCA stage, shrinking with the constant contact angles and decreasing the baselines along both lines of sight. In Stage-IV, the droplets recede with simultaneously decreasing contact angles and baselines. When the ethanol concentration increases to 5%, the first mixed stage (Stage-II) appears between the CCL stage (StageI) and the CCA stage (Stage-III). In Stage-II, the contact angles along both lines of sight decrease, and the baseline along lineof-sight 0° remains constant but that along line-of-sight 45° decreases (Figure 5b). As the ethanol concentration further increases, the relative time period of Stage-I decreases whereas that of Stage-II increases (Figure 5c−f). The last stage (StageIV) generally occupies the same time length regardless of the ethanol concentration. The appearance of Stage-II is due to the different depinning times along the two lines of sight (Figure 6). As the ethanol concentration increases, the depinning times along both lines of sight decrease linearly as demonstrated by the linear fittings in Figure 6. The depinning time along line-of-sight 0° decreases more slowly than that along line-of-sight 45°. The wetting area of the pure water droplet is nearly a regular octagon (Figure 2a), which results in similar depinning times along the two lines of sight (Figure 6). The pure water droplets experience three stages during evaporation (Figure 5). As the ethanol concentration increases, the difference between the two depinning times increases as a result of the increasing anisotropic wetting along the two lines of sight (Figures 2 and 3). Therefore, a larger percentage of Stage-II is observed during the evaporation of a droplet with a higher ethanol concentration, as shown in Figure 5.
Figure 6. Variation of (a) the normalized depinning time td′ and (b) the depinning time td as a function of the ethanol concentration c0.
The local forces at the three-phase contact line can be used to explain the droplet pinning−depinning transition.41,42 The depinning force is defined as Fd = 2LγLV(cos θ − cos θc), where L is the baseline of the droplet and θc is the apparent contact angle on the patterned surface. We introduce θ, the dynamic contact angle during evaporation, for elucidation. The pinning force on the pillared surfaces can be expressed as Fp = 2LγLV[(cos θro − cos θao)ϕ + Hr],43,44 where θro and θao are receding and advancing contact angles on the plain surface, ϕ is the solid fraction, and Hr is the adhesive force due to surface roughness and heterogeneity. The driving ratio Fd/Fp can be used to interpret such a transition on pillared surfaces.43,44 At the beginning of evaporation, Fd/Fp is smaller than unity. As the time elapses, the dynamic contact angle θ decreases. Thus, the depinning force Fd increases and Fd/Fp increases. When Fd/Fp becomes larger than unity, the droplet depins and enters the CCA stage. In the present study, the droplets have different initial contact angles (Figure 2b) but similar depinning contact angles of around 35° as shown in Figure 7 under different ethanol concentrations and different lines of sight. The similar
Figure 7. Variation of the depinning contact angle θ with the ethanol concentration c0. Subscripts 0 and 45 indicate the quantities along lines of sight 0° and 45°, respectively. 1151
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single value. This results from the fact that the droplet wetting area is octagonal at the CCL stage but spherical at the CCA and mixed stages (Figures 4 and 5). The spherical wetting area results in the same circumscribed and inscribed circles. A higher ethanol concentration leads to a shorter CCL stage (Figure 5). Therefore, the volume range becomes a single value earlier at a higher ethanol concentration. Moreover, ethanol enhances the droplet evaporation. The droplet evaporates faster at a higher ethanol concentration. It has been reported that the free spherical droplets evaporate following the d2 law, where d is the droplet diameter. Thus, V2/3 decreases linearly with time.49,50 For a sessile droplet, the drying process becomes more complex becauase of the wetting and evaporation dynamics. Liu et al.51 reported that V2/3 was linearly proportional to time for the pure water sessile droplets evaporating on plain surfaces but became nonlinear for the water−ethanol mixture droplets. Figures 8b and S2b show that V2/3 decreases nonlinearly with both time and normalized time as the ethanol concentration ranges from 0 to 30%. To draw a clear picture of this nonlinearity, the variations in V2/3 slope with time at 0 and 30% of ethanol concentration are plotted at the insets of Figures 8b and S2b. More detailed information can be found in Figure S3. Pittoni et al.52 demonstrated that V2/3 was linearly proportional to time in water droplet evaporation on the smooth poly(methyl methacrylate) substrate. Therefore, the nonlinear relationship of V2/3 with time in the evaporation of pure water droplets of the present experiment is attributed to the microstructures on the poly(methyl methacrylate) substrate due to nonuniform wetting and depinning behaviors. Besides the surface-patterned structures, droplet evaporation is also influenced by many other factors.3 Figure 9 shows the
depinning contact angle at different initial ethanol concentrations results from the fact that droplets mainly contain water in the latter stage of evaporation because ethanol evaporates faster than water in the water−ethanol droplets.45 Thus, the different initial contact angles at different ethanol concentrations lead Fd/Fp to the critical value at different times. The different initial contact angles along the two lines of sight result in anisotropic wetting and the evaporation dynamics of droplets. Additionally, the highest evaporation flux normally occurs in the vicinity of the droplet three-phase line,46,47 and ethanol tends to concentrate near the droplet three-phase line as well.48 The volatile ethanol could intensify the evaporation there, and such a rapid loss of solution is very likely to initiate droplet depinning. As the ethanol concentration increases, the difference between the two initial contact angles grows larger (Figure 2b). Thus, the two depinning times differ further with an increase in ethanol concentration. The octagonal wetting area leads to an irregular droplet shape. To analyze the variation in droplet volume, we have calculated the volumes of spherical caps using the circumscribed and inscribed circles of the octagonal wetting area as the bottom surfaces of the caps. The actual droplet volume should fall in the range of these two values (details in Supporting Information). The variation of the normalized volume range with time is presented in Figure 8a, where droplet volume V is normalized by the average value of the initial volumes of these two spherical caps. Correspondingly, the variation of the range of the volume to the power of two-thirds, V2/3, with time is shown in Figure 8b. As time elapses, the droplet dries gradually, and the volume range decreases and eventually becomes a
Figure 8. Variation of the normalized (a) droplet volume V range and (b) V2/3 range with time. The upper and lower limits of the floating columns in (a) are the volumes of the spherical caps whose bottom surfaces are the circumscribed and inscribed circles of the octagonal wetting area, respectively. The illustration can be seen in Figure S1 of the Supporting Information. Inset: Variation of the slope of V2/3 at 0 and 30% of ethanol concentrations. The slope of V2/3 is calculated using the average V2/3 of the two spherical caps in this figure and Figure 9.
Figure 9. Variation of the normalized V2/3 of (a) pure water droplets and (b) water−alcohol mixture droplets with normalized evaporation time. Inset: Variation of the slope of V2/3 with normalized evaporation time. The experimental conditions of the references are listed in Table S1. The normalized time of refs 52−55 in (a) is scaled by the presented evaporation time because refs 52−55 do not show the whole evaporation process for the pure water droplets, whereas that of the other cases is scaled by the evaporation lifetime as defined previously. 1152
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Langmuir evaporation of pure water and the water−alcohol mixture droplets on various substrates. The variation of the V2/3 slope with normalized time can be found in Figure S4. The experimental conditions of the references in the figure are listed in Table S1. Figure 9a indicates that the pure water droplets evaporate on nonpatterned substrate surfaces with V2/3 linearly decreasing as the time elapses, whereas our experimental results indicate that a nonlinear relationship results from the microstructures on the substrate surfaces. The curve slopes of refs 52−55 differ because of many factors as shown in Table S1. One important factor is the substrate materials. A detailed analysis can be referred from Pittoni et al.52 Figure 9b shows that V2/3 decreases nonlinearly with time in the evaporation of water−alcohol solution droplets regardless of the roughness of the substrates. In our experiments, we think that both the microstructures and ethanol concentration contribute to this nonlinearity. The four factors in Table S1 do not lead to nonlinearity in the pure water droplet evaporation (Figure 9a). However, the situation changes for the water−alcohol droplets. Liu et al.51 reported that the variation trend of V2/3 with time changes as the relative humidity varies for the water−ethanol droplets of same ethanol concentration. Furthermore, as the ethanol concentration changes, the curve slope varies.51
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Huicheng Feng: 0000-0002-2047-2406 Fei Duan: 0000-0002-7469-7184 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful for financial support from Nanyang Technological University. REFERENCES
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CONCLUSIONS We conducted an experimental study on the evaporation dynamics and shape manipulation of water−ethanol binary solution droplets on the substrates with micropyramid cavities by varying the ethanol concentration. The droplets formed octagonal contact areas on the micropyramid surface. As the ethanol concentration increased, the droplet wetting area varied from an octagon to a square. The initial wetting side ratio, defined as the ratio of lengths of the long side and short side of an octagon, increased from 1.5 at 0% ethanol concentration to 3.5 at 30% ethanol concentration. Additionally, the pure water droplets on the patterned substrate exhibited three stages over the lifetime, i.e., the constant contact line (CCL) stage, the constant contact angle (CCA) stage, and the final mixed stage. The water−ethanol binary droplets experienced one more mixed stage between the CCL and CCA stages as a result of the anisotropic depinning times along the two lines of sight. Droplets depinned earlier at a higher ethanol concentration. The micropyramid cavities on the substrate contributed to the nonlinear behavior of the volume to the power of two-thirds (V2/3) as a function of evaporation time. The reduced liquid− vapor surface tension owing to the presence of ethanol, together with the patterned surface, accounted for the alteration in droplet shape and evaporation regimes. The present study might provide us the possibility to control the droplet wetting area and evaporation on the textured surfaces by simply varying the droplet composition.
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Side view along line-of-sight 0° of droplet evaporation at 20% ethanol concentration (AVI) Side view along line-of-sight 45° of droplet evaporation at 20% ethanol concentration (AVI)
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b04195. Figures S1−S4 and Table S1 (PDF) Top view of droplet evaporation at 20% ethanol concentration (AVI) 1153
DOI: 10.1021/acs.langmuir.6b04195 Langmuir 2017, 33, 1147−1154
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DOI: 10.1021/acs.langmuir.6b04195 Langmuir 2017, 33, 1147−1154
