Soft repulsive mixtures under gravity: brazil-nut effect, depletion bubbles, boundary layering, nonequilibrium shaking

TL;DR

This study uses computer simulations and theory to explore how long-range repulsive interactions in a binary mixture under gravity lead to phenomena like the brazil-nut effect, boundary layering, and persistent boundary effects under time-dependent gravity.

Contribution

It introduces the depletion bubble mechanism explaining particle segregation and boundary layering in repulsive mixtures, supported by simulations and density functional theory.

Findings

Heavier particles float atop lighter ones due to depletion bubbles.

Boundary layering of particles occurs near container walls.

Persistent layering persists even under periodic gravity inversion.

Abstract

A binary mixture of particles interacting via long-ranged repulsive forces is studied in gravity by computer simulation and theory. The more repulsive A-particles create a depletion zone of less repulsive B-particles around them reminiscent to a bubble. Applying Archimedes' principle effectively to this bubble, an A-particle can be lifted in a fluid background of B-particles. This "depletion bubble" mechanism explains and predicts a brazil-nut effect where the heavier A-particles float on top of the lighter B-particles. It also implies an effective attraction of an A-particle towards a hard container bottom wall which leads to boundary layering of A-particles. Additionally, we have studied a periodic inversion of gravity causing perpetual mutual penetration of the mixture in a slit geometry. In this nonequilibrium case of time-dependent gravity, the boundary layering persists. Our results are based on computer simulations and density functional theory of a two-dimensional binary mixture of colloidal repulsive dipoles. The predicted effects also occur for other long-ranged repulsive interactions and in three spatial dimensions. They are therefore verifiable in settling experiments on dipolar or charged colloidal mixtures as well as in charged granulates and dusty plasmas.