Microorganisms live in a very different world than us — a world commonly submerged in water. But, at such microscopic sizes, swimming in water seems far more like swimming through tar. Understanding the dynamics of how microorganisms travel through liquids can help us control their spread.

Since the microbial world is a liquid one, flows play an important role in setting microbe motion. Bacteria, for instance, commonly have “run-tumble” dynamics: They swim forward for a time then briefly tumble in a new, random direction. Walls and obstacles can impact these dynamics, not only by simply getting in the way but also by changing the flows caused by the bacteria. In this way, even distant objects interact with the swimmers through flow.

These hydrodynamic interactions are challenging to predict. We’ve worked with Joost de Graaf (University of Edinburgh), who has developed versatile models for computationally simulating the dynamics of self-propelled particles (such as living bacteria) within fluids. These simulations build swimmers out of beads, which appear raspberry-like in shape. The resulting models reproduce well the hydrodynamics of swimming cells, like E. coli or sperm, which means that in the future these models may be able to accurately predict swimming dynamics in complex geometries and flows.