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

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 Bparticles. 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 perpetuous 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.

Vacancy diffusion in colloidal crystals as determined by dynamical density-functional theory and the phase-field-crystal model

“…Future work should address the dynamics of dipolar mixtures of colloidal particles with different dipole moments [49], where an equilibrium density functional for a binary mixture [50] is needed. These mixtures show more complex possibilities of mixed crystals as a function of the asymmetry in their dipole moments [51].…”

Two-dimensional colloidal mixtures in magnetic and gravitational fields

“…Second, the binary magnetic mixture was examined in the nonequilibrium situation of oscillatory gravity, which is a simple model of colloidal shaking. Thereby, Brownian dynamics simulations and dynamic density functional theory were used to study the dynamic response of the system [43]. For other parameters, see Ref.…”

“…Separation line between the occurrence of the colloidal brazil-nut effect and the absence of this effect in the parameter space of dipolar ratio m and mass ratio M . Monte Carlo simulation data (contoured white squares), density functional data (full black circles) and the transition line implied by the depletion bubble picture (contoured circles) are shown.For other parameters, see Ref [43]…”

How does a thermal binary crystal break under shear?

“…Thereby, shear deformation constitutes an elementary source of strain. Colloids pose an ideal model system for studying distortions on the particle scale [6] and allow to access the microscopic processes governing structural changes via experiment [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] and computer simulation [23][24][25][26][27][28][29][30][31][32]. For one-component systems, the plastic deformation of a strained solid is well-explored and a connection between mesoscopic deformation and atomistic rearrangements has been established [33][34][35][36][37][38].…”