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Graduate Seminar

Aditya Khair
Department of Chemical Engineering, Carnegie Mellon University
Title: Phoretic particle transport [Joint seminar with SIAM student chapter]

Abstract: Electric fields, temperature gradients, and solute fluxes can be employed to transport colloidal particles in microscale environments, including microfluidic devices and porous media. Such "phoretic" particle motion arises due to the imposed (electrical, thermal, or concentration) field driving the nano-scale interfacial, or screening, layer that envelopes a colloid out of equilibrium. Morrison (1970) proved that the phoretic velocity of a colloid with a thin surrounding interfacial layer is independent of the particle size or shape, and even the colloid concentration. This remarkable result has been observed for concentrated charged dispersions and is the reason that DNA strands of different length (but uniform charge) move at equal electrophoretic velocities in free solution, requiring a gel matrix, for example, to affect separation. Here, we present three instances of phoretic transport that "break" Morrison's result. First, we demonstrate that non-Newtonian fluid rheology leads to size- and shape-dependent electrophoretic motion of charged colloids. Second, we show that the transient phoretic dynamics of an ion-permselective particle persist on a time scale orders of magnitude larger than that of a dielectric (inert) colloid. Third, we predict that solute or temperature advection causes relative diffusion-phoretic or thermo-phoretic drift, respectively, between colloidal particles.

Date: Wednesday, February 26, 2014
Time: 5:30 pm
Location: Wean Hall 8220
Submitted by:  Brian Kell