Size-dependent contact angle, and wetting and drying transition of a droplet adsorbed on to a spherical substrate: Line tension effect

TL;DR

This study investigates how droplet size influences contact angle and morphological transitions on spherical substrates, highlighting the role of line tension in wetting and drying behaviors.

Contribution

It introduces a size-dependent analysis of contact angles and morphological transitions considering line tension effects on spherical substrates.

Findings

Drying transition occurs on hydrophobic substrates with small droplets.

Wetting transition occurs on hydrophilic substrates with large droplets.

Contact angle varies with droplet size and line tension sign.

Abstract

The size-dependent contact angle and the drying and wetting morphological transition are studied with respect to the volume change for a spherical cap-shaped droplet placed on a spherical substrate. The line-tension effect is included using the rigorous formula for the Helmholtz free energy in the droplet capillary model. A morphological drying transition from a cap-shaped to a spherical droplet occurs when the substrate is hydrophobic and the droplet volume is small, similar to the transition predicted on a flat substrate. In addition, a morphological wetting transition from a cap-shaped to a wrapped spherical droplet occurs for a hydrophilic substrate and a large droplet volume. The contact angle depends on the droplet size: it decreases as the droplet volume increases when the line tension is positive, whereas it increases when the line tension is negative. The spherical droplets and wrapped droplets are stable when the line tension is positive and large.