The very long range nature of capillary interactions in liquid films

TL;DR

This paper demonstrates how optical micro-manipulation can directly measure long-range capillary forces between micron-sized particles in liquid films, revealing a force that decays as an inverse power law with a small exponent.

Contribution

It introduces a method to directly measure capillary interactions between particles in liquid films and characterizes the force's decay behavior with particle size.

Findings

Force decays as an inverse power law with separation

Exponent of decay is close to but less than one

Capillary interactions are very long ranged in the mesoscopic scale

Abstract

Micron-sized objects confined in thin liquid films interact through forces mediated by the deformed liquid-air interface. This capillary interactions provide a powerful driving mechanism for the self-assembly of ordered structures such as photonic materials or protein crystals. Direct probing of capillary interactions requires a controlled force field to independently manipulate small objects while avoiding any physical contact with the interface. We demonstrate how optical micro-manipulation allows the direct measurement of capillary interactions between two micron sized spheres in a free standing liquid film. The force falls off as an inverse power law in particles separation. We derive and validate an explicit expression for this exponent whose magnitude is mainly governed by particles size. For micron-sized objects we found an exponent close to, but smaller than one, making capillary interactions a unique example of strong and very long ranged forces in the mesoscopic world.