### How do microorganisms swim?

#### Overview

If you were a microorganism — say, a bacterium or a sperm cell — the world would feel very different to you. Not just because you are 1µm tall, but also because the physics you experience at this scale is quite different to what we experience as metre-sized humans.

As humans, we are familiar with the notion that everything moves according to Newton’s second law: $F = ma$. But as a microorganism, your mass is so small, that the acceleration term becomes neglible. Instead, you live in an inertialess world, where all forces just balance out, all the time.

Many microorganisms swim, and if you have taken a fluid mechanics course (although this is not required), you will know that fluids move according to the Navier–Stokes equation. A big part of what makes this equation hard is the acceleration term. Not a problem for a microorganism!

So really our governing equations become a lot simpler, and we are able to use techniques you will have learnt over the last three years to start looking at how these microorganisms really swim.

#### How we might structure the the project

At the start, we could have something like:

- Introduction to Stokes flow; derivation as a limit of the Navier–Stokes equations
- Solutions to Stokes flow using Green’s functions
- Looking up different methods of solving the Stokes equations on the shape of a microorganism
- Coding up the simplest model of a microorganism (if you like Python)

#### Prerequisites

This is an applied maths project and there will be an opportunity to do some computational work, as well as some analytical work, so you should have some familiarity with Python, but you do not have to be a numerical analysis expert!

There are no prerequisite models for this project. Possibly useful modules to have taken are Fluid Mechanics III and Mathematical Biology III for the Green’s functions. However, **these are not essential** because, together, we will derive the governing equations and learn an intuition for the physics which is completely new.

#### Resources

- Life at low Reynolds number, , 1976.
- Small particles in a viscous fluid, course notes, , 2014.
- A Physical Introduction to Suspension Dynamics, textbook, , 2012.
- Stokesian Dynamics online game, to get a feel for the physics, .