Shear-melting colloidal glasses

Colloidal glasses

If a suspension of hard sphere-like colloidal particles is quenched into a non-equilibrium state at volume fractions above roughly 58%, it forms a repulsive glass. Since particles are so closely packed, they may wiggle around, but they cannot escape the so-called cages built by their nearest neighbors within experimental time scales: they are dynamically arrested. Such a glass, although very similar in structure to a liquid at slightly smaller volume fraction, can resist much more stress than the latter. However, it is known from bulk rheology experiments that a glass yields and starts to flow if exposed to a shear strain of about 10%. We are interested in how this yielding happens on a microscopic scale and have therefore constructed a shear-cell which can be mounted on top of a confocal microscope. This enables us to apply a well-controlled shear strain to our system while monitoring the movements of single colloids.

Our System

We use sterically stabilized, NBD-dyed PMMA spheres, which are 1.2 microns in diameter. The particles are suspended in a mixture of CHB and cis-Decalin in order to match the buoyancy as well as the index of refraction at the same time. The slight electrostatic repulsion, which is due to charging of the particles, is screened by adding tetrabutyl ammonium chloride. In order to apply a shear strain to the glass, we have constructed a parallel plate-like shear cell. A coating of strongly polydisperse particles on top and bottom plate provides enough friction and prevents crystallization. The distance between the two plates can be varied and a piezo-electric device can drive the top plate horizontally up to 80 microns.

Yielding

Applying an oscillatory shear strain to the colloidal glass leads to yielding as can be seen in the mean squared displacement, which evolves from a plateau (glass) to a linear behavior (liquid). Yielding does not depend on the strain rate but on the strain amplitude with a strain of roughly 10% being sufficient to break the cage structure. We find that at yielding particles move much more cooperatively than in the unsheared glass.

Objectives

We hope to understand how yielding takes place on a microscopic scale. We are especially interested in the question if applying a shear strain to a jammed system is equivalent to increasing the temperature (or rather decreasing the volume fraction in a hard sphere system). Furthermore we would like to compare our results with attractive glasses.

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Shear-melting colloidal glasses
Last Updated October 5, 2006
Web Page by Christoph Eisenmann (ce@deas.harvard.edu)