Capillary interactions between soft capsules protruding through thin fluid films

TL;DR

This study investigates how the softness of particles affects capillary interactions during the drying of thin fluid films, revealing that softer particles experience weaker interactions and cluster formation over time.

Contribution

It introduces a coupled numerical method to analyze the drying of films with soft particles, highlighting the influence of particle softness on capillary forces and clustering behavior.

Findings

Softer particles exhibit smaller meniscus deformations.

Weaker lateral capillary forces are observed with increasing particle softness.

Particle clustering occurs during late-stage drying, influenced by softness.

Abstract

When a suspension dries, the suspending fluid evaporates, leaving behind a dry film composed of the suspended particles. During the final stages of drying, the height of the fluid film on the substrate drops below the particle size, inducing local interface deformations that lead to strong capillary interactions among the particles. Although capillary interactions between rigid particles are well studied, much is still to be understood about the behaviour of soft particles and the role of their softness during the final stages of film drying. Here, we use our recently-introduced numerical method that couples a fluid described using the lattice Boltzmann approach to a finite element description of deformable objects to investigate the drying process of a film with suspended soft particles. Our measured menisci deformations and lateral capillary forces, which agree well with previous theoretical and experimental works in case of rigid particles, show the deformations become smaller with increasing particles softness, resulting in weaker lateral interaction forces. At large interparticle distances, the force approaches that of rigid particles. Finally, we investigate the time dependent formation of particle clusters at the late stages of the film drying.