Numerical analysis of anisotropic wetting on chemical stripe-patterned surfaces

Abstract

There is a considerable need and scope for improved prediction and approximation of wetting behavior on chemically patterned surfaces for designing such surfaces with tunable wettability. In this study, anisotropic wetting on chemical stripe-patterned surface with alternating hydrophobic/hydrophilic stripes is investigated. It focuses on the effects of wettability contrast by introducing stripes of wettability of different ranges. The numerical analysis has been performed using the open access software ‘Surface Evolver’. Analysis of the shape and movement of the three-phase contact line, and examination of droplet energetics and the stability of liquid droplet with a range of wettability contrast of the chemical stripe-patterned surfaces has been carried out. To investigate the stability and energetics of droplet, normalized energy for a range of droplet volume has been calculated to seek the most stable state of a droplet. It is found that the lowest value of normalized energy indicates the equilibrium state that we consider to be the most stable. Additionally, the volume of droplet which exhibits the minimum normalized energy increases with the number of chemical stripes. Thus, the system becomes more stable when droplet is of small volume and resides on fewer number of stripes. Moreover, a narrow- hydrophilic stripe is found to be more preferable compared to the hydrophobic stripe for attaining the global energy minima. Wider hydrophilic stripe causes both parallel and perpendicular contact angles to decrease for a preferred spreading in the parallel direction. Furthermore, droplet residing on more stripes displays a decrease in the degree of anisotropy. Because of a larger energy barrier in the perpendicular direction, droplet prefers to spread toward the parallel grooves/stripes. Liquid droplet is observed to distort, where the droplet distortion is defined as the ratio of the length of the elongated droplet in the major to minor. Distortion increases with an increment in the volume of the droplet and the wider hydrophilic stripe causes a more elongated droplet shape resulting in large distortion. The shape of a droplet evolves from an oval/elliptical to spherical geometry in our study when droplet volume is increased. At the stable state, it becomes spherical and with further increment in the volume, the oval/elliptical shape reappears.

The findings of the present study can provide a reliable guideline for the prediction of wetting behavior on chemical-stripe patterned surfaces to facilitate the design of such surfaces with controlled wettability.