Capillary migration of microdisks on curved interfaces

TL;DR

This study investigates how microdisks with pinned contact lines migrate on curved fluid interfaces, revealing that slight deviations from perfect circularity induce migration driven by capillary energy variations.

Contribution

The paper provides experimental and theoretical analysis of capillary-driven migration of microdisks with pinned contact lines on curved interfaces, highlighting the role of contact line deviations.

Findings

Microdisks migrate toward regions of higher curvature.

Perfectly smooth disks do not migrate.

Slight contact line deviations cause migration proportional to deviatoric curvature.

Abstract

The capillary energy landscape for particles on curved fluid interfaces is strongly influenced by the particle wetting conditions. Contact line pinning has now been widely reported for colloidal particles, but its implications in capillary interactions have not been addressed. Here, we present experiment and analysis for disks with pinned contact lines on curved fluid interfaces. In experiment, we study microdisk migration on a host interface with zero mean curvature; the microdisks have contact lines pinned at their sharp edges and are sufficiently small that gravitational effects are negligible. The disks migrate away from planar regions toward regions of steep curvature with capillary energies inferred from the dissipation along particle trajectories which are linear in the deviatoric curvature. We derive the curvature capillary energy for an interface with arbitrary curvature, and discuss each contribution to the expression. By adsorbing to a curved interface, a particle eliminates a patch of fluid interface and perturbs the surrounding interface shape. Analysis predicts that perfectly smooth, circular disks do not migrate, and that nanometric deviations from a planar circular, contact line, like those around a weakly roughened planar disk, will drive migration with linear dependence on deviatoric curvature, in agreement with experiment.