Impact of Motile Bacteria on Viscous Fingering

Viscous fingering is a hydrodynamic instability that occurs when a less viscous fluid displaces a more viscous one. Instead of progressing as a uniform front, the less viscous fluid forms fingers to create complex patterns. Understanding how these patterns and their associated gradients evolve over time is of critical importance in characterizing the mixing of two fluids. Here, we investigate the impact of replacing the less viscous fluid with an active suspension of motile bacteria, relevant to applications such as microbial enhanced oil recovery, bioremediation, and microfluidics.

In this series of experiments, a suspension of motile E. coli capable of collective swimming is injected into a microfluidic Hele-­Shaw cell under viscous fingering conditions. Using fluorescent microscopy, we observe changes within the mixing zone of the two fluids compared to inactive fluids. Most notably, we observe a "rafting" phenomenon where some of the bacteria group together and form a patterned interface between the two fluids. We quantify the impact these active suspensions have on the formation of viscous fingering patterns and mixing efficiency between the two fluids, and conversely, ­­­report details of the collective swimming behavior in the presence of a viscous­-gradient front.