Effects of boundaries on structure formation in low-dimensional colloid model systems near the liquid-solid-transition in equilibrium and in external fields and under shear

Abstract

A brief review focusing on low-dimensional colloidal model systems is given describing both simulation studies and complementary experiments, elucidating the interplay between phase behavior, geometric structures, and transport phenomena. These studies address the response of these very soft colloidal systems to perturbations such as uniform or uniaxial compression, laser fields, randomly quenched disorder, and shear deformation caused by moving boundaries.Binary hard-disk mixtures are studied by Monte Carlo simulation, to investigate ordering on surfaces or in monolayers, modeling the effect of a substrate by an external potential. By weak external laser fields the miscibility of the mixture can be controlled, and the underlying mechanism (laser-induced demixing) is clarified. The stability of various space-filling structures is discussed only for the case where no laser fields are present.Hard spheres interacting with repulsive screened Coulomb or dipolar interaction confined in 2D and 3D narrow constrictions are investigated by Brownian Dynamics simulation. With respect to the structural behavior, it is found that layers or planes throughout the microchannel are formed. The arrangement of the particles is disturbed by diffusion, and can also be modified by an external driving force causing a density gradient along the channel. Then the number of layers or planes gets reduced, adjusting to the density gradient, and this self-organized change of order also shows up in the particle velocities.The experimental work that is reviewed here addresses dipolar colloidal particles confined by gravity on a solid substrate on which a set of pinning sites has been randomly distributed. The dynamics of the system is studied by tracking the trajectories of individual particles, and it is found that the mean square displacements of particles that are nearest neighbors of pinned particles are strongly affected by these

Density functional theory of freezing for binary mixtures of 2D superparamagnetic colloids

“…The latter induces dipole moments in the particles according to their magnetic susceptibility. These superparamagnetic suspensions are very good realizations of a classical 2D repulsive dipole-dipole system [9][10][11][12][13][14]. Recently, for a one-component system, a density functional approach was put forward to calculate the freezing transition [15,16].…”

Extruding the vortex lattice: Two reacting populations of dislocations

“…Figure 2(b) shows indeed that the sleeve-system is closer than the channel to the continuum model. This is then reminiscent to the description [18,19] of bacterial cell wall growth and provides a physical mechanism for the observations in colloidal dynamics as seen in Deutschländer et al [20].…”

How does a thermal binary crystal break under shear?

“…This structure is very persistent in two dimensional mixtures and was found for granulates [76] and ionic crystals, as well [77,78]. In previous experiments, twodimensional suspensions were studied by confining superparamagnetic colloidal particles to the air-water interface of a hanging water droplet [10,71,79,80] or to a planar glass substrate [21,81,82]. Parallely aligned dipole moments are induced by applying an external magnetic field H perpendicular to the plane of confinement.…”

“…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].…”