How should the contact angle of a noncircular wetting boundary be described?

TL;DR

This paper proposes a new approach to describe the contact angle of noncircular wetting boundaries by using an average contact angle that accounts for surface property differences, challenging traditional single-point models.

Contribution

It introduces an average contact angle model based on surface property differences and friction, replacing traditional single-point contact angle models for noncircular wetting boundaries.

Findings

Noncircular wetting boundaries result from surface property differences.

Average contact angle provides a better evaluation than single-point CA.

Traditional Cassie and Wenzel methods become obsolete with this new approach.

Abstract

For over 200 years, wettability has made significant contributions to understanding the properties of objects, advancing technological progress. Theoretical model of the contact angle (CA) for evaluating wettability has constantly been modified to address relevant emerging issues. However, these existing models disregard the difference in the CA along the contact line and use a single-point CA to evaluate the entire contact line. From this perspective, there is no reasonable explanation for noncircular wetting. Here, we reveal that noncircular wetting boundaries result from property differences in the surfaces along the boundary, and utilize friction as a comprehensive factor reflecting local wettability. Average CA is proposed to evaluate the contact line instead of the single-point CA, making the Cassie method and Wenzel method obsolete, which will take an average property of the whole surface as a weight coefficient of the single-point CA, ignoring the subordination between physical properties and roughness in systematics.