Living near interfaces can be beneficial to microbes. Liquid-air interfaces may be the source of oxygen and sunlight. Solid surfaces accumulate sediments, which can include valuable nutrients. They can also offer anchoring points for the formation of protective biofilm. Films of liquid over a solid wall have both of these types of beneficial interfaces. However, an unavoidable consequence of being near surfaces is a shearing when the film flows. These flows can have a large impact on the swimming dynamics of microbes.

In a series of papers with Arnold Mathijssen and Amin Doostmohammadi, we investigated the swimming dynamics of motile microbes in flowing films. We began by building an analytical model for the flows produced when microbes swim in a film. We were able to reproduce many of the different flows generated by swimmers. These include flagellated bacteria that swim by propelling fluid back and then pushing fluid forward (called “pusher-type swimmers”), and puller-type swimmers that pull fluid toward their head and drag their body forward. We also reproduced ciliated swimmers with their many hair-like flagella.

We then put our swimmer models in a flowing film and looked at the dynamics. We found there is a sweet spot for rinsing bacteria away. When the film isn’t flowing, the microbes are attracted most strongly to the solid surface and they stay near the floor until the flow is strong enough to peel microbial swimmers off the surface. That is to say, at moderate flows bacteria in a film are washed away but that’s not the end of it. If much stronger flows are applied, the flow turns the microbes back down towards the surface, where they are more likely to form biofilms.

Run-tumble Dynamics vs. Hydrodynamic Interactions in Films

Swimmers like E. coli have “run-tumble” dynamics, meaning they swim forward for a time (run stage) then rapidly and randomly change to new direction (tumble stage). While they swim in the run stage, they perturb the surrounding fluid and interact with their surroundings through the flow they generate. This causes swimmers to be attracted to walls.

We asked “When does the hydrodynamic attraction win over the randomizing run-tumble dynamics in a film?” We found if we modelled the dynamics as just an effective constant noise then the swimming strategy of the microbes makes quite a difference. However, if the swimmers have a proper run stage followed by a brief and random tumble stage then the swimming strategy is not nearly as important.

We believe this will be of interest to other scientists in our field because it means that when they make simulations of swimmers they should prioritize getting the run-tumble dynamics correct before attempting to reproduce the more complicated but less important hydrodynamics of microbial swimming.