Nonequilibrium Phases of Sheared Colloidal Suspensions

We have constructed a shear cell which can be loaded onto a confocal microscope thus allowing us to observe the 3-D micro-structure of sheared colloidal suspension. The plates move in the  X direction.

Under slow shear, Brownian diffusion plays a significant role in the relaxation processes which take place over the timescale of an oscillation. In this regime, the suspension can display liquid, crystalline, and glass-like thermodynamic phases. However, under fast shear, the system is driven out of equilibrium and forced to adopt new micro-structures. Therefore, the physics of the shearing experiment changes with shear rate. Confinement can also play a crucial role in the suspension rheology. For example, we find that under high shear, and when the gap between the shearing plates is less than 12 particle diameters, the suspension forms a beautiful buckled pattern that is not observed in bulk. The pictures below show four slices equally spaced in the Z direction through a sheared suspension in a gap that holds about three and a half layers.

Below we show a schematic diagram of the structures formed by the suspension. The particles form buckled layers that flow over one another in the X direction. This pattern is very similar to the densest packing of spheres in 2D.

Below we show a movie of a buckled pattern that is formed when a dense colloidal suspension is confined to a gap that can hold less than six layers of particles and is subjected to high shear. The movie starts with the focal plane at the bottom layer. As the movie progresses, the focal plane moves up to the top layer and then back down to the bottom layer. The movie was taken by Itai Cohen. The music for this movie was composed by Vinay Prabhakar.

Download a copy of our paper on this novel buckled phase here.