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Research Papers

Effects of Surface Topography and Colloid Particles on the Evaporation Kinetics of Sessile Droplets on Superhydrophobic Surfaces

[+] Author and Article Information
Wei Xu

 Department of Mechanical Engineering, Stevens Institute of Technology, Hoboken, NJ 07030Chang-Hwan.Choi@stevens.edu

Chang-Hwan Choi1

 Department of Mechanical Engineering, Stevens Institute of Technology, Hoboken, NJ 07030Chang-Hwan.Choi@stevens.edu

1

Corresponding author.

J. Heat Transfer 134(5), 051022 (Apr 13, 2012) (7 pages) doi:10.1115/1.4005715 History: Received September 24, 2010; Revised July 21, 2011; Published April 11, 2012; Online April 13, 2012

In this paper, the evaporation kinetics of microliter-sized sessile droplets of gold colloids (∼250 nm in particle diameters) was experimentally studied on micropatterned superhydrophobic surfaces, compared with those of pure water on a planar hydrophobic surface. The structural microtopography of superhydrophobic surfaces was designed to have a constant air fraction (∼0.8) but varying array patterns including pillars, lines, and wells. During evaporation in a room condition, the superhydrophobic surfaces exhibited a stronger pinning effect than a planar surface, especially in the initial evaporation stage, with significant variations by the surface topographies. Compared to a pure water droplet, colloids exhibited further promoted pinning effects, mainly in the later stage of evaporation. While the well-known evaporative mass transport law of sessile droplets (i.e., linear law of “V2/3t”) was generally applicable to the superhydrophobic surfaces, much smaller evaporation rate constants were measured on the patterned superhydrophobic surfaces than on a planar hydrophobic surface. A colloidal droplet further showed lower evaporation rate constants than a pure water droplet as the concentration of particles in the droplets increased over the evaporation. Such transition was more dramatic on a planar surface than on the micropatterned surfaces. Whereas there was no clear correlation between evaporation mode and the evaporation rate observed on the superhydrophobic surfaces, the prominent decrease of the evaporation rate on the planar hydrophobic surface was accompanied with the onset of a second pinning mode.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

SEM images (45 deg tilted views) of pillar (a), line (b), and well (c) patterns. The insets show the top views of the patterns with the denotation of the structural dimension parameters (D, X, Y). In case of the line pattern (b), two directions (P: parallel, T: transverse) were distinguished for experiments

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Figure 2

Optical graphs of evaporating droplets on a smooth hydrophobic surface (a) and a superhydrophobic surface (b, well pattern). Evaporation kinetics is measured by analyzing the change of droplet profiles (contact angle, base diameter, height, and volume) with time

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Figure 3

Evolution of the contact angle (CA, triangle in symbol) and base diameter (BD, circle in symbol) of water (CAw and BDw , blue in color) and colloids (CAc and BDc , red in color) droplets on a smooth hydrophobic surface (a) and microstructured superhydrophobic surfaces (b: Pillar, c: LineT , d: LineP , e: Well). Each data point represents the measurement value for every 40 s. Evaporation phases are divided by blue dotted lines for water and red dashed lines for colloids, respectively. Each mode is also represented with numbers (I: initial pinning mode, II: receding mode, III: mixed mode, and IV: second pinning mode) for water (w in subscript) and colloidal (c in subscript) droplets. The wetting transition from a Cassie–Baxter state to a Wenzel state of a water droplet on LineT is marked by a blue arrow.

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Figure 4

SEM images of the deposition of Au nanoparticles on a smooth hydrophobic surface (a) and micropatterned superhydrophobic surfaces (b: pillar, c: line, d: well) after the complete evaporation of colloidal droplets

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Figure 5

Evaporative mass transport rate (regression of droplet volume2/3 with time) of water (blue) and colloidal (red) droplets for varying surface samples (a: smooth, b: pillar, c: line, d: well). In case of a line pattern (c), an arithmetic average of the volumes measured from transverse (LineT ) and parallel (LineP ) directions is used for the data. Each data point represents the measurement value for every forty seconds. Evaporation phases are divided by blue dotted lines for water and red dashed lines for colloids, respectively. Each mode is also represented with numbers (I: initial pinning mode, II: receding mode, III: mixed mode, and IV: second pinning mode) for water (w in subscript) and colloidal (c in subscript) droplets. K is the evaporation rate constant.

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