Effects of confinement and external fields on structure and transport in colloidal dispersions in reduced dimensionality

Abstract

In this work, we focus on low-dimensional colloidal model systems, via simulation studies and also some complementary experiments, in order to elucidate the interplay between phase behavior, geometric structures and transport properties. In particular, we try to investigate the (nonlinear!) response of these very soft colloidal systems to various perturbations: uniform and uniaxial pressure, laser fields, shear due to moving boundaries and randomly quenched disorder. We study ordering phenomena on surfaces or in monolayers by Monte Carlo computer simulations of binary hard-disk mixtures, the influence of a substrate being modeled by an external potential. Weak external fields allow a controlled tuning of the miscibility of the mixture. We discuss the laser induced de-mixing for the three different possible couplings to the external potential. The structural behavior of hard spheres interacting with repulsive screened Coulomb or dipolar interaction in 2D and 3D narrow constrictions is investigated using Brownian dynamics simulations. Due to misfits between multiples of the lattice parameter and the channel widths, a variety of ordered and disordered lattice structures have been observed. The resulting local lattice structures and defect probabilities are studied for various cross sections. The influence of a self-organized order within the system is reflected in the velocity of the particles and their diffusive behavior. Additionally, in an experimental system of dipolar colloidal particles confined by gravity on a solid substrate we investigate the effect of pinning on the dynamics of a two-dimensional colloidal liquid. This work contains sections reviewing previous work by the authors as well as new, unpublished results. Among the latter are detailed studies of the phase boundaries of the de-mixing regime in binary systems in external light fields, configurations for shear induced effects at structured walls, studies on the effect of confinement on the structures and defect densities in three-dimensional systems, the effect of confinement and barriers on two-dimensional flow and diffusion, and the effect of pinning sites on the diffusion.

Depinning and heterogeneous dynamics of colloidal crystal layers under shear flow

“…Further interesting aspects are the impact of oscillatory shear flow and of structured walls (which can influence the crystalline structure [34,52]) on the dynamics of the system. We also note that there is increasing interest concerning the interplay of shear flow and strong confinement in glasslike colloidal systems (see [7] for a corresponding molecular dynamics study with a much wider slit-pore width).…”

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

“…The Laser Induced Demixing regime of the phase diagram for an equimolar binary mixture with diameter ratio σ B /σA = 0.414 exposed to an external periodic potential, commensurate to the S 1(AB) square lattice for the case that only the smaller component of the mixture interacts with the external potential. After Wilms et al [38].…”

“…In Fig. 6 we present [3,4,38] the phase diagram and configuration snapshots for the Laser Induced Demixing regime for the case (I). Above and below the horizontal blue line no Laser Induced Demixing was observed.…”

Dynamical modes of sheared confined microscale matter

“…Further, the material response to shear is intimately connected to the nonequilibrium dynamics of the constituent elements, that have been the subject of recent research with non-Brownian particles, 13,14 polymer-, 15 active bacteria-, 16 and colloidal suspensions in amorphous, [17][18][19] fluid-, 20,21 as well as crystalline states. [22][23][24][25] Colloidal suspensions under external fields have proven to be a powerful test bed system, that is used to study the role of channel geometry 9,17,26 hydrodynamic interactions, 24,27 frictional interparticle contact and lubrication, 28,29 as well as plastic events, [30][31][32] to cite a few. Key advantages of using colloidal particles are the possibilities to directly visualize the particle dynamics via video microscopy, and to tune the pair interactions using external fields.…”