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Competition between Particle Migration and Chaotic Advection in Microchannels

James F. Gilchrist, Lehigh University, Department of Chemical Engineering, Bethlehem, PA 18015 and Changbao Gao, Department of Chemical Engineering, Lehigh University, 111 Research Dr., Bethlehem, PA 18015.

Shear-driven particle migration is investigated in 1D, 2D, and 3D steady pressure-driven microchannel flows. In simple pressure-driven flow, self-organization of 1 micron charge-stabilized colloidal microspheres occurs due to particle migration, typically driving particles away from the walls toward the center of the channel despite Brownian and collisional diffusion. In 3D flows generated by staggered herringbone mixers (Stroock et al, Science, 2002), mixing is significantly enhanced through chaotic advection. Dispersion and self-organization are in direct competition, resulting in nontrivial concentration patterns. In channels whose geometry induces flow in the transverse direction to the pressure gradient, competition between particle self-organization and mixing due to advection results in concentration profiles, where the underlying 3D flow templates pattern formation. Using high-speed confocal laser scanning microscopy, we directly image the microspheres to measure the averaged 3D spatial concentration profile and locate individual particles to determine the local particle positions to confirm local concentrations and to determine particle interactions, local 3D velocity, and suspension structure as a function of the bulk concentration, Reynolds and Péclet number, and the underlying flow topology.