Enhanced diffusion due to motile bacteria

The effect of bacterial motion on the diffusion of a molecule of high molecular weight is studied by observing the mixing of two streams of fluid in a microfluidic flow cell. We show that the presence of motile E. coli bacteria in one of the streams results in a marked increase in the effective diffusion coefficient of Dextran, which rises linearly with the concentration of bacteria from a baseline value of 0.2×10−7 to 0.8×10−7 (cm2/s) at a concentration of 2.1×109/ml (approximately 0.5% by volume). Furthermore, we observe that the diffusion process is also observed to undergo a change from standard Fickian diffusion to a superdiffusive behavior in which the diffusion exponent rises from 0.5 to 0.55 as the concentration of bacteria rises from 0 to 2.1×109/ml.