Investigation of wettability and droplet energetics on micro-scale wavy and v-grooved surfaces

Abstract

In order to develop micro-structured surfaces with tunable wetting properties, it is critical to understand the relationship between the surface roughness and wetting state. Basic research on developing hydrophobic surfaces with regulated wettability has piqued people’s curiosity in the recent years. Surfaces having directional wetting properties and inducing anisotropic wetting behavior that encourage liquid drainage are beneficial in a broad variety of applications.

In this study, the wetting behavior on surfaces that can exhibit directional wetting, such as V-grooves and periodic wavy surfaces, are investigated using a numerical technique with a focus on the droplet stability and anisotropic wetting conditions. Surface Evolver, an open access software, is employed to develop 3D models of liquid droplets and to assess the shape and spread of liquid droplets for a broad range of parametric space. Along with a detailed examination of the stability and energetics of liquid droplets, the apparent contact angles on micro wavy and V-grooved surfaces are numerically quantified for a range of droplet volume. The effect of surface roughness parameters of micro V-grooved surfaces such as groove height, groove width, pillar width, groove angle and so on is examined and then compared to the same for the dimensional variation of pitch, amplitude and depth of the asperities of micro wavy surfaces. The findings are found to be consistent with the previously reported experimental studies and analytical models.

To distinguish the stabilities of liquid droplets on both micro V-grooved and micro wavy substrates, a dimensionless normalized version of interfacial energy is employed. For different anisotropic configurations, stable and metastable droplets with increasing droplet size on these surfaces is analyzed. It is found that multiple metastable wetting states can be obtained for constant droplet volume and larger number of pillars beneath the droplet is required for bigger droplet size to be stable. However, orthogonal contact angles are always larger than the parallel ones due to the free energy barrier caused by pinning, though this effect is less pronounced for micro wavy surfaces.

The essential characteristics that have a dominating influence on the anisotropy of droplets is explored using variation of the geometric properties of the surface roughness. Due to the larger dispersion of the liquid droplet along the groove direction, the Wenzel state of wetting showed a higher degree of anisotropy. The droplet height of Cassie droplets increased as the droplet volume increased, but the height of Wenzel droplets remained roughly constant regardless of the droplet size. The Wenzel state of wetting has a higher droplet expansion ratio than the Cassie state. Water droplets fall down the pillars and spread along the grooves on surfaces with a smaller groove depth and wider grooves, resulting in a lower apparent contact angle, whereas higher groove height and shorter groove width have the opposite effect due to the higher aspect ratio and larger energy barrier. The wetting properties of groove angles on V-grooved surfaces were likewise comparable.

Moreover, the chemical alteration of these rough micro-structured surfaces is also studied. With a higher intrinsic contact angle of the surface material, the apparent contact angle in both the parallel and orthogonal directions of the grooves increased, but with a faster increment rate of parallel contact angle than orthogonal contact angle, resulting in a lower degree of wetting anisotropy and a shift toward isotropic wetting.

In general, the developed numerical model provides reasonable predictions of anisotropic wetting behavior. This can be a useful tool in improving the design of micro-structured rough surfaces with directional wetting by means of optimizing the geometric parameters such as groove size, shape, spacing, as well as the chemical nature of the surface