Colloidal suspensions flowing through microchannels were studied for the effects of both shear flow and the proximity of walls on the particles’ self-diffusion. Use of hydrostatic pressure to pump micron-sized fluorescent silica spheres dispersed in water-glycerol through poly(DiMethylSiloxane) channels with a cross section of 30x26 micron, allowed to vary the Péclet number (Pe) from 0.01 to 50. To obtain the diffusion coefficients, image-time series from a Confocal Scanning Laser Microscope were analysed with a method that, after finding the particle trajectories, subtracts the instantaneous convective displacements and then measures the slopes of the Mean Squared Displacements in the flow (x) and shear (y) directions. The thus obtained Dx and Dy, which should be equal to the free diffusion coefficient in the dilute limit regardless of shear, both increase strongly with Peclet number (for Pe&gt;10) in a concentrated suspension. This effect of the shear-induced collisions is counteracted by the contribution of the wall, which causes a strong local reduction in Dx and Dy.

Effects of shear and walls on the diffusion of colloids in microchannels

249th National Meeting (2015)

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