For very tiny organisms like bacteria, when they swim through water, the size of the difference forces they encounter mean they experience the water in the way we would experience a very viscous fluid like honey. The types of flow patterns that emerge in a fluid depend on the viscosity and the size and speed of objects within the fluid and can be characterised by the Reynolds number.
“Low Reynolds number” flow (often called Stokes flow after physicist Sir George Stokes) is smooth and mathematically predictable. Many of its properties are described in the following lecture due to the celebrated fluid dynamicist Sir G. I. Taylor:
In this low Reynolds number world where bacteria live, various non-intuitive phenomena are at work. For example, flapping a tail like a fish does not actually propel a microorganism and symmetry in the swimming stroke must be broken to achieve propulsion. Some of Dr Bennett’s research involves theoretical modelling of bacteria swimming close to a surface and understanding how the bacteria interact with the surface via the fluid. The goal of this project is to gain a better understanding of the initial stages of biofilm formation and design strategies to prevent formation of biofilms that lead to infections.
I have used this modelling to show how physical features of the bacteria, such as the body shape or friction created by hair-like structures called pili, affects the way bacteria move close to a flat surface. Currently, I am investigating whether the shape of a surface can be used to modify the interactions between the bacteria and the fluid, with the goal of using the flow created by a swimming bacterium to push it away from the surface.