Using confocal microscopy, we directly observe that simple shear flow induces transient crystallization of colloids by wall-normal propagation of crystallization fronts from each shearing surface. The initial rate of the front propagation was $1.75\pm 0.07$ colloidal layers per unit of applied strain. The rate slowed to $0.29\pm 0.04$ colloidal layers per unit of applied strain as the two fronts approached each other at the midplane. The retardation of the front propagation is caused by self-concentration of shear strain in the growing bands of the lower-viscosity crystal, an effect that leads to a progressive reduction of the shear rate in the remaining amorphous material. These findings differ significantly from previous hypotheses for flow-induced colloidal crystallization by homogeneous mechanisms.

• Received 18 July 2010

DOI:https://doi.org/10.1103/PhysRevLett.105.228302