Electrically Induced Colloidal Clusters for Generating Shear Mixing and Visualizing Flow in Microchannels

When aqueous suspensions of 1 mu m, negatively charged polystyrene particles are subject to a I kHz alternating electric field of strength greater than 7 kV(rms) m(-1), dynamic elliptical dusters of particles spontaneously form. With potential applications in microchannel fluidics in mind, we characterize how cluster formation and particle circulation, driven by induced dipole dipole interactions, is critically dependent on time, field strength, electrolyte concentration, and cell thickness. Logarithmic growth of duster size is observed, and particle velocity within the clusters is found to be proportional to cluster length. Increasing cell thickness from 10 to 60 mu m increases the projected cluster area but decreases cluster aspect ratio as the result of changing particle dispersal rates. Clusters are shown to generate significant fluid shear suitable for microchannel mixing applications. These clusters are observed to distort under transverse fluid flow and, above a critical flow rate, to undergo a transition to form regularly spaced particle streams, which may be suitable for two-dimensional visualization of fluid flow.