Nonequilibrium dynamics of a confined colloidal bilayer in a planar shear flow

Abstract

Using Brownian dynamics (BD) simulations we investigate the impact of shear flow on structural and dynamical properties of a system of charged colloids confined to a narrow slit pore. Our model consists of spherical microions interacting through a Derjaguin-Landau-Verwey-Overbeek (DLVO) and a soft-sphere potential. The DLVO parameters were chosen according to a system of moderately charged silica particles (with valence Z~35) in a solvent of low ionic strength. At the confinement conditions considered, the colloids form two well-pronounced layers. In the present study we investigate shear-induced transitions of the translational order and dynamics in the layers, including a discussion of the translational diffusion. In particular, we show that diffusion in the shear-melted state can be described by an analytical model involving a single shear-driven particle in a harmonic trap. We also explore the emergence of zigzag motion characterized by spatiotemporal oscillations of the particles in the layers in the vorticity direction. Similar behavior has been recently observed in experiments of much thicker colloidal films.

Shear-induced laning transition in a confined colloidal film

“…2). We can thus conclude that while the limiting behavior of the trilayer at low and high shear rates, respectively, is identical to that in bilayer systems [27], the breakdown of ordering at intermediate shear rates shows pronounced differences.…”

“…In the shear-molten state the long-time behavior of the MSD become diffusive (i.e., linear in time) at long times, as expected in a liquid-like state in the absence of any center-of-mass motion. The intermediate dynamics can be further analyzed in terms of an effective single-particle model describing a particle in a harmonic potential (mimicking the spatial confinement in z-direction) [27]. Here we rather focus on specific features of the laned state.…”

“…Following previous studies [27,28] we consider a model colloidal suspension where two spherical particles (diameter d) with distance r ij interact via a combination of a repulsive Yukawa potential, u Yuk (r ij ) = W exp [−κr ij ] /r ij and a repulsive soft-sphere potential u SS (r ij ) = 4 (d/r ij ) 12 . The interaction parameters are set to W/(k B T d) ≈ 123 (where k B T is the thermal energy) and the inverse Debye screening length is set to κ d ≈ 3.2 (for details, see Ref.…”

“…(3), N L describes the instantaneous number of particles and N b i the number of neighbors of particle i in the considered layer (the position of each layer can be extracted from the density profile, see below). The value of N b i was calculated from the intralayer pair correlation function by determining the radius related to the first minimum of the intralayer pair correlation function [27]. Perfect square or hexagonal order is characterized by Ψ 4/6 = 1, respectively.…”

“…Spatial confinement is modeled by two plane parallel, smooth, uncharged surfaces separated by a distance L z along the z direction and of infinite extent in the xy− plane. We employ a purely repulsive fluid-wall potential [27] …”