Controlling colloidal sedimentation using time-dependent shear

M. Krüger^1a and J. M. Brader^2b

¹ Massachusetts Institute of Technology, Department of Physics - Cambridge, MA 02139, USA
² Department of Physics, University of Fribourg - CH-1700 Fribourg, Switzerland

^a kruegerm@mit.edu
^b joseph.brader@unifr.ch

Received: 10 September 2011
Accepted: 8 November 2011

Abstract

Employing a recently developed dynamical density functional theory we consider the time-dependent response of a colloidal sediment above a wall to an externally applied shear flow, demonstrating the time-dependent changes of the density distribution and its center of mass after switching steady shear either on or off and under oscillatory shear. Following the onset of steady shear we identify two dynamical mechanisms, clearly distinguished by their characteristic timescales. Shortly after the onset of shear we observe a transient enhancement of the local packing structure at the wall, reflecting the self-organization of colloids into lanes. On a much longer timescale these effects are transmitted out to the bulk, leading to collective migration of particles away from the wall and an increase in the center of mass. Under oscillatory shear flow the center of mass enters a stationary state, reminiscent of a driven damped oscillator.