Evolution of nanoliter size fluid droplet on micropatterned surface
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Sådhanå (2020)45:220 https://doi.org/10.1007/s12046-020-01461-8
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Evolution of nanoliter size fluid droplet on micropatterned surface SAIKAT RAY and RANJAN GANGULY* Department of Power Engineering, Jadavpur University, Kolkata 700098, India e-mail: [email protected] MS received 1 May 2018; revised 13 November 2018; accepted 21 November 2018 Abstract. Static shape of liquid droplets on textured surface draws significant attention from the standpoint of several engineering applications, ranging from heat transfer to bio-printing. This paper discusses the equilibrium behavior of a nanoliter size droplet dispensed on a micropatterned surface. For a given combination of intrinsic wettability of the surface, liquid surface tension and the geometric morphology of the textured surface, the liquid droplet prefers to be on Cassie-Baxter, Wenzel or an intermediate, hybrid state. Here we have carried out an energy-based simulation of nanoliter size droplets on micropatterned surface, using an open-source surface evolver fluid interface tool SE-FIT. A simplified periodic geometry of rectangular (straight or tapered) micropillars of specified dimensions is chosen for the micropatterned surface. For a given solid surface texture, we found that droplet prefers to transit from Wenzel to Cassie state beyond a threshold intrinsic sessile contact angle (which the liquid would have subtended on a microscopically smooth surface of the same solid material). This critical transition contact angle is plotted against the roughness parameter. Present study helps in designing the wettability-engineered surfaces for specific engineering applications. Keywords.
Wettability; Cassie Baxter state; Wenzel state; microtextured surface; surface evolver.
1. Introduction Behavior of liquid droplets dispensed on solid surfaces has strong engineering relevance for different reasons. For applications like self-cleaning surfaces [1] (e.g., on the top coating of a solar photo-voltaic cell), one would require a liquid droplet to roll-off very easily. On the other hand, for applications like printing of biomarkers on micropatterned surfaces [2], sticky surfaces are preferred where the dispensed liquid would wet the internal grooves and crevices of the micropatterned substrates. Sometimes, an intermediate behavior between the above two types is also preferred. For example, researchers have found that a hybrid or partial wetting condition offers the most favorable heat transfer coefficient during dropwise condensation [3]. The equilibrium behavior of liquid droplet on a solid surface is dictated by the Young-Laplace equation [4, 5], which relates the Laplace pressure p (the pressure difference across the liquid interface) with the mean curvature jm of the interface, the gravity (g) and the surface tension (c) as
This paper is a revised and expanded version of an article presented in ‘‘First International Conference on Mechanical Engineering’’ held at ‘Jadavpur University’, Kolkata, Ind
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