Gravitational effect on the advancing and receding angle of a 2D Cassie-Baxter droplet on a textured surface

TL;DR

This study combines theoretical and numerical modeling to reveal how gravity influences the shape and pinning behavior of 2D Cassie-Baxter droplets on textured surfaces, despite not affecting the apparent contact angles.

Contribution

It introduces a comprehensive analysis of gravitational effects on droplet shape and movement, extending understanding beyond traditional small droplet assumptions.

Findings

Gravity alters droplet shape and pinning point positions.

Apparent contact angles remain unaffected by gravity.

Gravity significantly influences droplet tip movement during volume changes.

Abstract

Advancing and receding angles are physical quantities frequently measured to characterize the wetting properties of a rough surface. Thermodynamically, the advancing and receding angles are often interpreted as the maximum and minimum contact angles that can be formed by a droplet without losing its stability. Despite intensive research on wetting of rough surfaces, the gravitational effect on these angles has been overlooked because most studies have considered droplets smaller than the capillary length. In this study, however, by combining theoretical and numerical modeling, we show that the shape of a droplet smaller than the capillary length can be substantially modified by gravity under advancing and receding conditions. First, based on the Laplace pressure equation, we predict the shape of a two-dimensional Cassie-Baxter droplet on a textured surface with gravity at each pinning point. Then the stability of the droplet is tested by examining the interference between the liquid surface and neighboring pillars as well as analyzing the free energy change upon depinning. Interestingly, it turns out that the apparent contact angles under advancing and receding conditions are not affected by gravity, while the overall shape of a droplet and the position of the pinning point is affected by gravity. In addition, the advancing and receding of the droplet with continuously increasing or decreasing volume is analyzed, and it is showed that the gravitational effect plays a key role on the movement of the droplet tip. Finally, the theoretical predictions were validated against line-tension based front tracking modeling that seamlessly captures the attachment and detachment between the liquid surface and the solid substrate.