Capillary-Induced Motion of Particles Bridging Interfaces of a Free-Standing Thin Liquid Film.

We demonstrate a new form of capillary force experienced by neutrally buoyant spherical particles adsorbed simultaneously at both interfaces of a thin liquid film of spatially varying thickness. The force is proportional to the slope of the interface and the difference between the local contact angle and the equilibrium value, and exists even when the two bounding interfaces have zero curvature. We derive the expression for the force, which when balanced against the hydrodynamic drag gives the trajectory of the particle. The measured trajectories for spherical particles of varying diameters in thin films compare well with predictions.