Shear Melting of a Colloidal Glass
Entry: Chia Wei Hsu, AP 225, Fall 2010
Christoph Eisenmann, Chanjoong Kim, Johan Mattsson, and David Weitz, Phys. Rev. Lett. 104, 035502 (2010)
Colloidal suspensions serve as a model system for glass transition. A colloidal glass exhibit cooperativity and dynamic heterogeneity that are typically seen in glass transitions, as well as its own unique properties such as shear melting. Because of the large particle size, colloids form soft solids and can be fluidized through shear. In order to understand the relationship of shear-induced melting to others (such as melting by increasing temperature or reducing volume fraction), we need to understand the microscopic behavior of shear-melting.
The authors study sterically stabilized poly(methylmethacrylate) particles (average radius <math>R=0.6\mu m</math>) suspended in a mixture of cis-decalin and cycloheptylbromide (which matches the particle density and index of refraction) at a volume fraction of <math>\phi=0.61\pm0.03</math>. The colloid particles are fluorescently labeled, and the suspension is contained between two parallel glass plates <math>40 \mu m</math> apart in a shear cell.
A symmetrical triangular time-dependent strain is applied in the <math>y</math> direction with strain amplitude up to <math>\gamma \approx 0.5</math> and strain periods between 25 and 100 <math>s</math>. At such shear rate, particle motions are dominated by the imposed shear (as opposed to diffusion). The particle positions <math>x(t)</math> and <math> y(t)</math> are tracked with confocal microscopy during shear, with data collected 2 frames per second. Then the mean displacement due to shear is subtracted.
The authors first calculate an effective mean-squared-displacement (MSD), <math>\langle\Delta x^2(\Delta t)\rangle_0=\langle ( x ( t_0 + \Delta t)-x(t_0))^2\rangle</math>, where<math>t_0</math> is the initial time of a half strain-cycle. MSD is compared for different values of <math>\gamma</math> (fig 1a, b). The MSD crosses over from subdiffusive to a steady state diffusive behavior at a transition at <math>\gamma \approx 0.08</math> is observed. At the transition, the distribution of particle displacements <math>P(\Delta x (\Delta t))</math> becomes non-Gaussian, reflecting dynamic heterogeneities. This is measured by the non-Gaussian parameter <math>\alpha_2</math> (fig 1c, d).