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Controlling Octagon-to-Square Wetting Interface Transition of Evaporating Sessile Droplet through Surfactant on Microtextured Surface Xin Zhong, Junheng Ren, Karen Siew-Ling Chong, Kian Soo Ong, and Fei Duan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02509 • Publication Date (Web): 27 Mar 2018 Downloaded from http://pubs.acs.org on March 29, 2018
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ACS Applied Materials & Interfaces
Controlling Octagon-to-Square Wetting Interface Transition of Evaporating Sessile Droplet through Surfactant on Microtextured Surface Xin Zhong,†,¶ Junheng Ren,†,¶ 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 ¶Equal first contribution to this work. E-mail:
[email protected] Abstract Producing and maintaining specific liquid patterns during evaporation holds great potential for techniques of printing and coating. Here we report the control over the evolution of surfactant solution droplets on the micropyramid substrates during evaporation. The polygonal droplet shape is achieved during the drying rather than solely at the beginning.
As the initial surfactant concentration is 0.04 mM, the
droplet maintains its initial octagonal shape throughout the lifetime. Interestingly, the initial octagonal shape transforms into a square during the evaporation as the initial surfactant concentration reaches 0.8 mM. These findings can shed light on wetting pattern control for complex solutions required in various applications.
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Keywords: wetting transition; surfactant adsorption; sessile droplet; micropyramid surface; evaporation. Achieving a desired liquid wetting pattern is an essential step in a large number of applications such as bio-arrays of DNA molecules, 1–3 ink-jet printing, 4,5 coating, 6 etc. The desired polygonal profiles of droplets have been accomplished by engineering the surfaces with physical and/or chemical topographic patterns. 7–17 Raj et al., for example, succeeded in depositing the initial circular, square, hexagonal and octagonal droplets by tailoring the surface nanostructures. 7 However, it is notable that the fluids previously employed for displaying the predetermined wetting profiles are single-component, like pure water or organic liquids. It bears a limitation in many practical applications in which multicomponent solutions are normally adopted.
The drying processes of multi-component
solution droplets, exemplified by the Whisky solution 18 and the Ouzo droplet, 19 are normally investigated on flat surfaces. Due to the higher volatility of ethanol than that of the rest components, the aqueous droplets containing alcohols were observed to exhibit the wetting transitions during the evaporation. 18,19 However, on patterned surfaces the evolutions of complex solution droplets are by far less examined, and in most previous studies the desired droplet morphologies were observed just after the droplets were formed on the substrates. How such a droplet shape evolves, particularly whether the shape can be prevented from wrecking in the evaporation process, still remains unexplored. Such a question is crucial to many applications, such as ink-jet printing which requires the solution to exhibit and keep a predetermined pattern until it is completely dried. 20,21 Probing the development of droplet profiles during drying on textured surfaces is of vital importance, especially to maintain a constant droplet profile to the end of evaporation. To meet this challenge, we investigate the behavior of evaporating aqueous droplets containing surfactant on the patterned surfaces. In a surfactant solution droplet, the surfactant concentration would increase with the continuous water evaporation, thus the associated liquid-vapor and solid-liquid surface tensions would decrease. Although the decreasing dynamic contact angle contributes to the depinning force
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ACS Applied Materials & Interfaces
acting at the three-phase line, the decreasing liquid-vapor and solid-liquid surface tensions act against it and could assist the droplet to maintain its initial wetting area. Based on this envisioning, we evaluate the effect of surfactant concentration on droplet shape evolution on a micropyramid surface throughout the droplet lifetime, and we manage to keep the polygonal wetting shapes till the end of droplet evaporation. Interestingly, it is found that the initial wetting shapes of the droplets transform from an octagon to a square at high surfactant concentrations. Our study demonstrates a novel attempt in developing and stabilizing intended wetting configurations by utilizing the evaporation-induced change in the solutions. The droplet surfactant solutions were prepared by dissolving cetrimonium bromide (CTAB, Sigma Aldrich, >99%, critical micelle concentration, cmc = 0.92 mM) in the nanofiltered water with resistivity at 18.2 MΩ-cm, and the surfactant concentrations, Csurf , of the solutions were 0 mM, 0.04 mM, 0.08 mM, 0.8 mM, 1.0 mM and 1.5 mM. The droplets with the same initial volume controlled at 0.3 µL were dispensed by a micropipette (Thermo Fisher Scientific). The droplets were drying on the poly (methyl methacrylate) substrates engineered with micropyramids, as shown in Figure 1 (a). The poly (methyl methacrylate) polymer surface was engineered by nanoimprint lithography such that it was imprinted with a nickel shim mold at the temperature of 140 ◦ C and the 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 and forming the micropyramid structures. The demolding was done at 50 ◦ C. The structure of the micropyramid was visualized by a confocal microscope (DCM8, Leica Microsystems) and presented in Figure 1 (b). Each substrate was cleaned thoroughly to remove contaminants by rinsing with the deionized water (Milli-Q), and drying with the compressed nitrogen gas. The geometric parameters are illustrated in Figure 1 (b). The geometry of the micropyramid is characterized by its central height, h = 11 µm, side length, d = 31.2 µm, and diagonal length, l = 24.7 µm. The substrate is chemically isotropic, reflected by the isotropic wetting of both the pure water and surfactant solution droplets.
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The droplets were evaporating in the open conditions with the surrounding temperature and humidity controlled at 22±1 ◦ C and 51.8±4.4%. When the sessile droplet had the first contact with the micropyramid surface, the droplet spread very rapidly until it reached the maximum wetting area. The time of the first contact is denoted as t0 , and the moment when the rapid spreading was completed can be indicated as the beginning of evaporation, denoted as t˜ =
t tf
= 0 where t is the instantaneous evaporation
time and tf is the droplet lifetime. Such fast spreading, lasting shorter than 0.3 s, was captured by an inverted Nikon Eclipse Ti high-speed camera at 1000 frames per second (fps). Once the initial rapid spreading was finished, the droplet formed an octagonal wetting shape on the micropyramid surface. It took about five to ten minutes for the droplets at various surfactant concentrations to dry out. The lifetime drying of the droplet was simultaneously visualized by an optical microscope (Nikon Eclipse LV100ND) at 1 fps from a top view, and two HiSpec-2 high-speed cameras at 1 fps along the two directions as demonstrated in Figure 1 (a,b): one is parallel to the side and the other is parallel to the diagonal line of a micropyramid bottom, which are respectively indicated as line-of-sight ”k” and line-of-sight ”6 ”. The corresponding side profiles of the droplet captured along the two lines-of-sight are shown in Figure 1 (a). The octagonal wetting interface of the droplet is comprised of four sides parallel to the sides of a micropyramid bottom, and the other four sides parallel to the diagonal lines of the micropyramid bottom, which are referred to as ”k” side and ”6 ” side, and the corresponding lengths are denoted as Lk and L6 , respectively. The ratio of Lk and L6 is indicated as the side ratio, α =
Lk L6
, for characterizing the inequilateral degree of the
wetting octagonal profile. The evolution of the droplet interface was dependent on the surfactant concentration in the evaporation process. As the droplet is free of surfactant, as shown by Figure 2 (a), the initially generated octagonal solid-liquid interface shrinks to a circle once the droplet enters the depinning stage. Different from it, with a low initial surfactant concentration, Csurf , of 0.04 mM (Figure 2 (b)), the droplet remains its octagonal interface throughout
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(a)
Microscope
Droplet
Camera
“
”
1
Line-of-sight “
