The Effects 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 displacing fluid forms fingers that vary in size and shape to form complex patterns. The interface created from these patterns affects mixing between the two fluids, and therefore understanding how these patterns evolve in time is essential in applications such as enhanced oil recovery, bioremediation, and microfluidics.

Although bacteria can be found virtually everywhere viscous fingering occurs, there are no studies on the effects of their presence on the displacement dynamics. Here, we seek to begin filling this knowledge gap by using as invading fluid an active suspension of fluorescent motile E. coli, and observing how bacteria motility affects the interface and mixing zone between the two fluids in a mesofluidic Hele-Shaw cell. We identify four distinct regions in the resultant fingering pattern unique to these experiments. Having characterized how these dense motile suspensions are still capable of collective swimming (and therefore effective viscosity reductions) for a range of solvent viscosities tested (1-17 mPa·s) in previous work, we propose possible mechanisms for their emergence. These unexpected observations are a first step towards understanding how the interplay between active suspensions of motile bacteria and fluid-mechanical instabilities affects overall mixing under these complex flow conditions, which are found in both natural and engineered environments.