Title: The physics of the colloidal glass
Speaker: Eric Weeks
Speaker Info: Emory University
Brief Description:
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Abstract:
We study concentrated colloidal suspensions, a model system which has a glass transition. By using a confocal microscope to follow the three-dimensional motion of colloidal particles, we can directly observe the microscopic behavior responsible for the macroscopic viscosity divergence of glasses. We approach the glass transition by increasing the packing fraction of the particles. We find that particle dynamics are heterogeneous in both space and time: particle motion is characterized by a dynamic correlation length scale which grows as the glass transition is approached. Our most recent experiments use small magnetic particles to locally "poke" on the colloidal samples. By using an external magnet, we can pull a magnetic particle at constant force; oscillate a magnetic particle back and forth; and rotate clusters of magnetic particles. The response to these perturbations changes significantly near the glass transition. For example, we study the motion of the colloidal particles surrounding a moving magnetic particle. We find the magnitudes of displacements of the surrounding particles decay exponentially in space, in contrast to the flow expected for a Newtonian fluid. We study how this decay length varies with geometry (translation or rotation), packing fraction, and dimensionless Peclet number (forcing speed).Date: Friday, March 14, 2003