Droplet settling on solids coated with a soft layer

TL;DR

This paper develops a comprehensive model to understand how soft coatings on solids influence the settling behavior and shape of droplets during gravitational descent, revealing significant effects of soft layer properties.

Contribution

It introduces a novel three-phase lubrication model combining analytical and numerical methods to analyze droplet settling on soft-coated surfaces.

Findings

Soft layers delay air drainage during droplet settling.

Droplet shape is significantly affected by the softness of the coating.

Linear response of soft layers alters the droplet's settling dynamics.

Abstract

Gravitational settling of a droplet in air onto a soft substrate is a ubiquitous event relevant to many natural processes and engineering applications. We study this phenomenon by developing a three-phase lubrication model of droplet settling onto a solid substrate coated by a thin soft layer represented by a viscous film, an elastic compressible layer and an elastic sheet supported by a viscous film. By combining scaling analysis, analytical methods, and numerical simulations we elucidate how the resulting droplet dynamics is affected by the nature of the soft layer. We show that these soft layers can significantly affect the droplet shape during gravitational settling. When there is a linear response of the deformations of the soft layer, the air layer takes longer to drain as compared to the case of a droplet settling onto a rigid substrate. Our results provide new insight into the coupled interactions between droplets and solids coated by a thin film of a soft material.