Sliding dynamics of a particle in a soap film

TL;DR

This study explores how millimeter-sized particles slide in soap films, revealing the dominant role of gravitational deformation and the combined effects of air and film friction, supported by experimental and theoretical analysis.

Contribution

It introduces a model linking particle dynamics to gravitational film deformation and quantifies the friction contributions from air and the film.

Findings

Particle moves toward film center in damped oscillations.

Gravitational deformation dictates particle motion, not catenoid shape.

Friction from air and film are nearly equal, with surface viscosity estimated.

Abstract

We investigate the sliding dynamics of a millimeter-sized particle trapped in a horizontal soap film. Once released, the particle moves toward the center of the film in damped oscillations. We study experimentally and model the forces acting on the particle, and evidence the key role of the mass of the film on its shape and particle dynamics. Not only is the gravitational distortion of the film measurable, it completely determines the force responsible for the motion of the particle - the catenoid-like deformation induced by the particle has negligible effect on the dynamics. Surprisingly, this is expected for all film sizes as long as the particle radius remains much smaller than the film width. We also measure the friction force, and show that ambient air and the film contribute almost equally to the friction. The theoretical model that we propose predicts exactly the friction coefficient as long as inertial effects can be neglected in air (for the smallest and slowest particles). The fit between theory and experiments sets an upper boundary of 0.01 {\mu}Pa s m for the surface viscosity, in excellent agreement with recent interfacial microrheology measurements.