Capillary bridging and long-range attractive forces in a mean-field approach

TL;DR

This paper models capillary bridging in confined mixtures, revealing how metastable condensates form bridges that cause long-range attractive forces, with phase transitions and hysteresis, applicable across various systems.

Contribution

It introduces a mean-field approach to predict force-distance behavior and phase diagrams for capillary bridges in confined mixtures, highlighting a discontinuous transition and hysteresis.

Findings

Discontinuous transition between bridged and unbridged states.

Linear force-distance curves with hysteresis.

Applicability to liquid crystals and polymer mixtures.

Abstract

When a mixture is confined, one of the phases can condense out. This condensate, which is otherwise metastable in the bulk, is stabilized by the presence of surfaces. In a sphere-plane geometry, routinely used in atomic force microscope (AFM) and surface force apparatus (SFA), it can form a bridge connecting the surfaces. The pressure drop in the bridge gives rise to additional long-range attractive forces between them. Minimizing the free energy of a binary mixture we obtain the force-distance curves as well as the structural phase diagram of the configuration with the bridge. Numerical results predict a discontinuous transition between the states with and without the bridge and linear force-distance curves with hysteresis. We also show that similar phenomenon can be observed in a number of different systems, e.g. liquid crystals and polymer mixtures.