Shear melting of a colloidal glass

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[Under construction -- Nick Schade (fall 2009)]

The authors use confocal microscopy to investigate shear melting of colloidal glasses at the microscopic level. A phase transition occurs at a strain of about 8%, as this is when shear melting begins. For larger shear strain than this, cooperative motions of groups of particles can be found, where the average group size is about 3 particles. It is also evident that diffusive behavior of the particles is driven by shear for large strains. In particular, it is found that the effective diffusion coefficient is linearly proportional to shear rate, and this can be explained mathematically with a modified form of the Stokes-Einstein model of diffusion in which thermal energy is replaced by shear energy.

General Information

Keywords: colloid, glass, shear melting, phase transition

Authors: Christoph Eisenmann, Chanjoong Kim, Johan Mattsson, and David Weitz.

Date: February 4, 2009.

Departments of Physics and HSEAS, Harvard University, Cambridge, Massachusetts 02138, USA

Preprint (2009). [1]

Summary

A colloidal glass of poly(methyl methacrylate) particles with an average diameter of 1.2 microns is used to study the effects of shear melting. The particles are sandwiched between two parallel glass plates in a specially designed air-tight shear cell. The particles are fluorescently labeled so that their positions can be tracked using a confocal microscope when shear is applied. The position of each particle is tracked in two dimensions and its position is determined after subtracting the mean displacement due to the shear applied.

For strains <math>\gamma < 0.08</math>, subdiffusive behavior is observed, consistent with the normal behavior of a colloidal glass. In this regime, the particle motion is highly heterogeneous. The system transitions to the diffusive regime for larger shears, however, suggesting that for strains larger than 0.08, shear melting occurs, and so this quantity corresponds to a glass transition for the colloid system. The particle motion is more homogeneous for these larger strains. The fact that the non-Gaussian parameter <math>\alpha_2</math> is maximum at <math>\gamma \approx 0.08</math> provides further evidence of a phase transition at this quantity of shear.

Connection to soft matter