Battering Bacteria Against a Barrier

The introduction of active matter into a colloidal system leads to ill-defined parameters in the behavior of formed colloidal solids. In order to better understand the influence of active matter in industrial and medical applications we form flow-stabilized colloidal solids (FSS) with both active and passive particles. We hypothesize that these FSS will illuminate the influence of particle shape and motility of active matter on the dynamics. Three strains of bacteria, motile and non-motile Escherichia coli and Staphylococcus epidermidis, were chosen to isolate the influence of activity from that of particle shape. In order to form FSS, a bacterial suspension flows through a quasi-2D microchannel. The bacteria encounters a barrier perpendicular to the direction of fluid flow where shear and compressive forces cause pile formation. These bacteria are suspended in a nutrient broth to sustain the bacteria with an added polymer, polyvinylpyrrolidone, which reduces adhesion to the barrier, the walls of the channel, and other bacteria. It has been previously shown in colloidal experiments that above a critical flow rate triangular, solid-like piles form on the barrier. When using non-motile E. coli we observe formation of piles. We have observed that the motile bacteria avoid the barrier thus preventing pile formation; illustrating the influence of motility on pile formation.