Contact Line Shape on Ultrahydrophobic Post Surfaces

Abstract

In this work, we investigate the configuration of the contact line of a water drop lying on an ultrahydrophobic post surface using the numerical algorithm Surface Evolver. For the special situation of Cassie wetting, we propose a modified definition of the contact line as the line in space where the meniscus starts to curve upward out of the plane of the composite surface. In our simulations, it is found that the contact line is very strongly distorted, indicating a strong tendency of the drop to "ball up" in those areas where it is not in contact with the solid surface. The distortion of the contact line corresponds to a pronounced deformation of the liquid-air interface around the base of the drop. We discuss the consequences of this distortion for the definition and practical measurement of the contact angle on ultrahydrophobic surfaces.

Micro to nano: Surface size scale and superhydrophobicity

“…In two recent publications, we have reported on the characterization of silicon post surfaces that, once coated with a thin layer of a hydrophobic fluoropolymer, were wetted in the composite mode [14,23]. For such hydrophobicized post-type materials, the solid fraction is easily derived from the top area of the posts because, under normal circumstances (i.e., without the application of an additional volume force), the liquid menscus is unable to penetrate the post structure.…”

“…II) It is probable that the distortion of the contact line also plays a role [23,28–30]: On a surface with a smaller size scale, the contact line is probably more strongly distorted than on a surface with larger surface features. This distortion of the contact line will provide an additional force that facilitates drop movement.…”

Constitutive modeling of contact angle hysteresis

“…Schwartz [23] proposed a lubrication model of the Navier-Stokes equations. Several researchers [24][25][26][27] have used the Surface Evolver program for minimization of surface energy of the drop to obtain realistic drop shapes on heterogeneous surfaces. Porcheron and Monson [28] studied the Wenzel and Cassie states using a lattice gas model.…”

Modeling contact angle hysteresis of a liquid droplet sitting on a cosine wave-like pattern surface

“…In this study, a small droplet sitting on an ideal homogeneous rough surface is simulated to observe the contact line distortion clearly so that the influence of the contact line distortion on the contact angle can be obviously confirmed. The corrugation of contact line due to structure of the roughness is not pronounced if the drop size is sufficient large, but the drop still has an irregular shape as found in experiments [35][36][37]39]. In this study, the shape of the three-phase contact line is unusual and non-circular so the contact angle hysteresis cannot be simply addressed by a single value as shown in Fig.…”