How and When the Cassie-Baxter Droplet Starts to Slide on the Textured Surfaces

TL;DR

This paper investigates the conditions under which Cassie-Baxter droplets begin to slide on textured surfaces, combining theoretical modeling with simulations to clarify the sliding initiation mechanism and effects of condensation-evaporation cycles.

Contribution

It provides a theoretical framework for predicting droplet stability and sliding onset on textured surfaces, validated by simulations, and addresses key debates in the field.

Findings

Sliding initiates with front end slip.

Advancing and receding contact angles differ at sliding onset.

Condensation-evaporation cycles can promote droplet translation.

Abstract

The Cassie-Baxter state droplet has many local energy minima on the textured surface, while the amount of the energy barrier between them can be affected by the gravity. When the droplet cannot find any local energy minimum point on the surface, the droplet starts to slide. Based on the Laplace pressure equation, the shape of a two-dimensional Cassie-Baxter droplet on a textured surface is predicted. Then the stability of the droplet is examined by considering the interference between the liquid and the surface microstructure as well as analyzing the free energy change upon the de-pinning. Afterward, the theoretical analysis is validated against the line-tension based front tracking method simulation (LTM), that seamlessly captures the attachment and detachment between the liquid and the substrate. We answer to the open debates on the sliding research field: (i) Whether the sliding initiates with the front end slip or the rear end slip, and (ii) whether the advancing and receding contact angles measured on the horizontal surface are comparable with the front and rear contact angle of the droplet at the onset of sliding. Additionally, a new droplet translation mechanism promoted by cycle of condensation and evaporation is suggested.