Determining timescales for directed assembly of particles into bands by shear flow and electric fields

Suspended colloidal particles assemble into “bands” when subject to shear flow and a DC electric field over a range of conditions in combined Poiseuille and electroosmotic counterflow through microchannels. The initial stage in this process appears to be an accumulation stage, where particles are attracted to and concentrated near the channel wall. Spatially averaged grayscales and their standard deviation were calculated from evanescent-wave visualization images of band assembly to estimate the near-wall particle concentration and its local variations, respectively. The timescale for the accumulation stage was estimated from the standard deviation and used to scale physical time. The average grayscale appears to grow exponentially during the accumulation stage and exhibits self-similar exponential growth when normalized by its maximum value over a range of experimental parameters.