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Octagonal Wetting Interface Evolution of Evaporating Saline Droplets on Micropyramid Patterned Surface Xin Zhong, Junheng Ren, Mingfeng Lin, Karen Siew-Ling Chong, Kian Soo Ong, and Fei Duan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07533 • Publication Date (Web): 01 Aug 2017 Downloaded from http://pubs.acs.org on August 5, 2017
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Octagonal Wetting Interface Evolution of Evaporating Saline Droplets on Micropyramid Patterned Surface Xin Zhong,† Junheng Ren,† Mingfeng Lin,† 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, and Institute of Materials Research and Engineering, A∗Star, 2 Fusionopolis Way, Innovis, Level 9, Singapore 138634, Singapore E-mail:
[email protected]
∗ To
whom correspondence should be addressed Technological University ‡ A*Star † Nanyang
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Abstract Textured surfaces have been extensively employed to investigate the dynamics, wetting phenomena and shape of liquid droplets. Droplet shape can be controlled via the manipulation of topographic or chemical heterogeneity of a solid surface by anchoring the three-phase line at specific sites. In this study, we demonstrate that droplet shape on topographically patterned surface can be modified by varying the concentration of salt potassium chloride (KCl) in the droplet solution. It is found that at the beginning of evaporation, the shape of solid-liquid interface is closer to a rectangle upon increasing the salt concentration. Such a variation in the solid-liquid interface versus the salt concentration is explained by the analysis of free energy difference. It indicates that the increases in solid-liquid and liquid-vapor surface tensions by raising the salt concentration result in a favored extension of the three-phase line intersecting the micropyramid bottom sides than the counterpart intersecting the micropyramid diagonal edges. The saline droplets experience a pinning stage at first and a depinning one afterward. The onset of depinning is delayed, and at which the instantaneous contact angle is larger upon raising the salt concentration. The three-phase line which intersects the micropyramid diagonal edges recedes ahead of the one along the micropyramid bottom sides, making the octagonal wetting interface evolve toward a circle. A close view at the droplet edge indicates that the three-phase line repeats "slow slip-raid slip" across row by row of micropyramids during the depinning stage.
Keywords: droplet shape control, octagonal wetting interface; saline droplet; micropyramid surface; evaporation.
Introduction Superficial structures on a solid surface are of vital importance to determine the surface wettability. In nature, purely smooth surfaces are rare, but they exhibit different degrees of roughness which have great impacts on solid-liquid interactions. Such interactions can be exemplified by "lotus effect", or certain insects such as microvelia which can skate at water surface due to the air 2 ACS Paragon Plus Environment
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trapping between the legs of the insects and water. 1 Artificial patterning on solid surfaces in scientific and industrial fields has been extensively explored, and is normally realized through creating topographic or chemical heterogeneities. Surface engineering, mainly through micropatterning, nanoprinting, coating, etching, etc., is commonly employed to study the wetting phenomenon, to control the solid-liquid interface, and to even induce self-driven motion of a liquid drop in contact with a textured surface. Xu, et al. improved the understanding on how a water droplet receding is affected by the roughness of regular patterned substrates. The pinning-depinning transition and the bottom area at Cassie-Wenzel transition are sensitive to the surface roughness. 2 Cassie state indicates that the droplet sits above the patterned surface where air remains trapped below the droplet, and thus “air-pockets" are formed. As it transits to Wenzel state, the trapped “air-pockets" are replaced by liquid, so the droplet is fully in contact with the solid surface. Evaporative dynamics of multicomponent droplets have been also investigated on the patterned surfaces, particularly the micro-scale stepwise motion of the threephase line across island rows. 3 Besides, by examining the dynamic contact line on textured surface at micro-scale, Paxson, et al. found that droplet adhesion is governed by capillary bridges at the receding three-phase line. 4 Adhesion/release of droplets to/from a solid surface can be controlled by employing an anisotropic surface engineered with arrays of poly (p-xylylene) nanostructures. The retention force difference reached as high as 80 µ N in the pinning and releasing directions. 5 The micropatterned surface is capable of manipulating droplet shape as well. It was found that the droplet solid-liquid area is able to exhibit various polygons from squares to dodecagons on different substrates treated either topographically or chemically. 6,7 In addition, spontaneous motion of a droplet could be manipulated on specific-designed patterned surfaces. 8–13 Bliznyuk, et al. devised a lithographically treated pattern featuring stripes of alternating high and low wettability. Such an anisotropic feature made the liquid propagate parallel, and transported perpendicular to the stripes. 16 Chu, et al. created a uni-directional spread of a sessile droplet on an asymmetric nanostructured surface. Such an automatic spreading highly depended on the asymmetry of the nanostructure. 17 The other motions in a variety including droplet
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dynamic expansion/contraction, 13,14 spontaneous separation of droplet from a solid surface, 15 actuation of streams of droplet, 10 selective transport of droplet 11 correlated with surface anisotropy can be found in Ref. [ 18]. On the other hand, droplet wettability and its shape on patterned surfaces can be controlled by simply modifying the droplet solution composition, although the related studies are far rarer than the ones devoted to solid surface intervention. Courbin, et al. initiated the control of droplet shape via using different liquid mixtures characterized by various equilibrium three-phase angles on the same textured surface. The droplet imbibition on the patterned surface has a square profile for a binary solution consists of 25% ethanol and 75 vol% isopropanol, an octagonal shape for pure isopropanol, a rounded octagon for hexane liquid, and a circle for silicon oil. 19 Droplet composition was also investigated by varying the water-ethanol ratio to control the shape of solid-liquid interface on substrate with micropyramid cavities. The bottom area of the binary droplet evolves from an octagon to a rectangle upon increasing the ethanol component. 20 These approaches utilized liquid mixtures consist of components with low surface tensions like alcohol or silicon oil. Droplets could barely form but evolve toward to films at high concentrations of the low surface tension liquids. In our study, we employed solutions by dissolving potassium chloride (KCl) in pure water with different concentrations to obtain various high liquid-vapor surface tensions of the saline droplets. At first, the influence of KCl is analyzed on the wettability and shape of saline droplets at the beginning of evaporation in the subsection "Droplet Initial Wettability". Afterward, the lifetime evaporation of droplets on various salt concentrations is discussed in the subsection "Evaporation Dynamics across Droplet Lifetime". Herein droplet depinning, an interesting phenomenon occurring in the middle of evaporation, is emphasized in respect to its dependence on the salt concentration, and droplet shape evolution during the depinning from an entire view of the droplet. During the depinning phase, contact line exhibits a unique behavior as across the micropyramids island from a close view, which is emphasized at last in the subsection "Close View of Contact Line Depinning". These findings improve our understanding of the influence of droplet composition on evaporative dynamics on textured surfaces from a different angle through adopting
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liquid mixtures of high surface tensions.
Materials and Methods Surfaces and Solution Samples The poly (methyl methacrylate) substrates were patterned with micropyramid islands using nanoimprint lithography. The poly (methyl methacrylate) polymer was imprinted with a nickel shim mold at a temperature of 140 ◦ C and a pressure of 20 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. Each substrate was cleaned prior to tests to remove contaminants by rinsing under deionized water (Milli-Q), and drying by compressed nitrogen gas. Micropyramid geometries are summarized in Table 1, including the central height h, side length d, diagonal length l and roughness r denoting the ratio of the top surface area of a micropyramid to its bottom surface area. The geometric parameters are indicated in Figure 1 (c). The structure of the fabricated micropyramid was visualized by a confocal microscope (DCM8, Leica Microsystems) and presented in Figure 1 (b). Table 1: Geometric parameters of the PMMA substrate engineered with micropyramids. h (µ m) d (µ m) l (µ m) r 11 30 23.89 1.24
The homogeneous saline solution samples were prepared by dissolving potassium chloride powders (KCl, Sigma Aldrich, >99%) in nano-filtered water with resistivity at 18.2 MΩ-cm. The initial concentration CKCl of the KCl solutions were 0%, 5%, 10%, and 20%, which were lower than the saturation concentration at roughly Csat =23.7%. 21 During droplet evaporation, the loss of water led by evaporation resulted in an increase in salt concentration and the droplet could reach a state of supersaturation, under which the formation of crystalline could be initiated. The moment when crystalline starts to appear is defined as precipitation time tp . At late stage, the crystalline 5 ACS Paragon Plus Environment
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can grow to a degree that it defects the liquid-vapor interface or the three-phase line. But still, we report contact angle and baseline length both prior to and after the deformation of droplet spherical profile for the sake of providing full-spectrum information.
Figure 1: (a) Schematics of the experimental configuration. Droplet is simultaneously visualized by the microscope from top view and the fast cameras from side views along the two line-of-sights. (b) The structure of micropyramids at a PMMA substrate. (c) Schematics of the micropyramids geometry characterized by the central height h, bottom side length d and diagonal edge length l. The two line-of-sights are along the side and the diagonal line of micropyramid bottom area respectively.
Droplet Characterization The experimental setup is shown in Figure 1 (a). The optical microscope (Nikon Eclipse LV100ND) with the brightfield schema was employed to record droplet evaporation process from top view. Droplet images were captured every 1 s over the full-spectrum of evaporation. Simultaneously, two HiSpec-2 high-speed cameras were used to capture droplet side profile every 1 s from two side views. One view is parallel to the micropyramids bottom side, and the other is along the diagonal line of the bottom surface. The “parallel" and “diagonal" axes of viewing are therefore with a 45 ◦ interval as demonstrated in Figure 1 (a, c), and are indicated as line-of-sight "∥" and line-of-sight "̸ ", respectively. The droplets with the same initial volume controlled at 0.4 µ l were dispensed by a micropipette 6 ACS Paragon Plus Environment
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(Thermo Fisher Scientific). The base diameters along the two line-of-sights, and the wetting interface, namely the solid-liquid interface, were analyzed by the software of NIS Elements. Snapshots of droplet profile from side views were post-processed by the program, ImageJ, to extract information of contact angle. 22 To ensure experimental reliability, three tests were done for every CKCl . In the following figures of this article, the error bars of the parameters relevant to contact angle, depinning time and precipitation time indicate standard deviations from the three repeated results along each line-of-sight. The error bars of the parameters relevant to baseline length along each line-of-sight indicate standard deviations from six results since along each line-of-sight there are two baseline length values for one droplet. The droplets were evaporating in an open condition with the surrounding temperature and humidity maintained at 22±1 ◦ C and 55±2%.
Results and Discussion Droplet Initial Wettability The influence of dissolved KCl salt on droplet wettability on the textured surface can be reflected by the wetting interface, contact angle and baseline length of the droplet at the initial moment of evaporation. The initial contact angle and the initial baseline length, obtained at time 0 s of droplet evaporation, are denoted as θ0 and L0 , respectively. The wetting interface of the pure water and saline droplets on the micropyramid surface exhibits an octagonal shape, despite that it is partially blocked by the upper droplet body, as demonstrated in Figure 2 (a). The snapshots taken from top view in Figure 2 (a) also show that the octagonal wetting interface has two baselines with each one along line-of-sight "∥" and line-of-sight "̸ ", respectively. The side profiles of the droplets in Figure 2 (a) show that the initial contact angle along line-of-sight "∥" is larger than the counterpart along line-of-sight "̸ ". This is in consistent with the larger initial baseline length along lineof-sight "̸ " than that along line-of-sight "∥". The initial baseline length, L0 , normalized to the averaged initial baseline line length of the pure water droplet, Lw0 , along each line-of-sight, as demonstrated in Figure 2 (b), is found to decrease with CKCl , revealing that the wetting interface 7 ACS Paragon Plus Environment
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appears to be smaller at a higher salt concentration. Since the droplets were produced with the same initial volume, a smaller initial baseline corresponds to a larger initial contact angle. It can be seen from the side profiles in Figure 2 (a) that the droplet is more balled up at a higher CKCl . The initial contact angle, θ0 , normalized to the averaged initial contact angle of the pure water droplet,
θw0 , along each line-of-sight is shown in Figure 2 (c). θ0 /θw0 taken from both the line-of-sights are enlarged at a higher CKCl . The normalized initial baseline length L0 /Lw0 , and the normalized contact angle θ0 /θw0 exhibit linear variations with CKCl , and such relations are fitted by the best fitting lines. The corresponding fitting equations are indicated in Figure 2 (b, c). (a)
KCl 0%
KCl 5% 1.1
(b) y=-2.03E-3x+1.01
500 m
500 m
L0/Lw0
1.0 0.9
=
<
<
KCl 10%
0.8
KCl 20%
Line-of-sight “ ∂ CKCl ∂ CKCl
(7)
(8)
L∥ would always be more extended than L̸ with an increase of CKCl . By introducing the value of d and h, it can be acquired that τ would be increased when
∂ γSL ∂ γLV > 0.42 ∂ CKCl ∂ CKCl
(9)
The degree that the wetting interface varies towards a rectangular profile with raising CKCl , although not as significant, manifests in the case of water-ethanol binary droplets on the PMMA substrate engineered with the microcavities as well. 20 The side ratio was reported to increase from less than 1.5 for the pure water droplet to 3.5 for the one with 30 vol % ethanol. Different from salt KCl, ethanol introduced in water reduces both the solid-liquid and liquid-vapor surface tensions. 13 ACS Paragon Plus Environment
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Although the manner of contact line intersecting a microcavity is not provided, the variation in the free energy difference Fpill , resulting from the decreases in both the solid-liquid and liquidvapor surface tensions, are presumably to make the extension of the parallel sides favored upon increasing the ethanol concentration. The unexpected conclusions can be drawn that ethanol and salt KCl, with opposite effects on liquid-vapor and solid-liquid surface tensions, lead to same varying tendencies of the wetting interface profile.
Evaporation Dynamics across Droplet Lifetime Dissolved KCL in droplet also influences its lifetime evaporation dynamics, reflected by the evolutions of the contact angle and baseline length at various salt concentrations. The contact angle and baseline length from each line-of-sight, normalized to its respective initial value obtained at 0 s of evaporation, are plotted in Figure 5. It can be seen that for the saline droplets with intermediate CKCl , the contact angle reduces and the baseline keeps nearly constant at first, suggesting that the droplets experienced the constant contact radius mode (CCR) which features the decreasing contact angle and constant radius of a droplet during its evaporation. Afterward the contact angle remains unchanged, while the baseline is reduced, indicating the droplets underwent the constant contact angle mode (CCA) which is characterized by the constant contact angle and decreasing radius of a droplet in the proceeding of evaporation. Different from the saline droplets, the pure water droplet experienced the CCR, CCA and at last the mixed mode characterized by simultaneously decreasing contact angle and baseline. During the initial CCR stage, the contact angle decreases while the baseline maintains nearly unchanged. By taking the best fitting line of the contact angle curve during the CCR stage, the averaged reducing rates of contact angles at various CKCl are obtained and plotted in the inset of Figure 5 (a2). The contact angle decreases more slowly at a higher CKCl , suggesting the suppression on evaporation by salt. As for the latter CCA stage, the baseline is reduced and contact angle remains roughly constant at various CKCl . The reducing rate of baseline during the CCA stage seems to be attenuated upon increasing the CKCl as well. To compare the evaporation dynamics of saline droplets to those of water-ethanol binary droplet14 ACS Paragon Plus Environment
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1.0
1.0
0% 5% 10% 20%
0.8
0% 5% 10% 20%
0.8
0.6 /
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0.6 0.4
0.4 0.2 0.0
0.2 (b1) Line-of-sight “ ”
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(a1) Line-of-sight “
