For many microswimmers, like bacteria or spermatozoa, swimming through common fluids is like moving through mud. Unless they can produce enough thrust to overcome a fluid’s yield-stress, they are effectively stuck in a solid. A recent study breaks down exactly what a microswimmer has to manage, assuming they use a helical, corkscrew-like tail for propulsion.
The first barrier is creating enough force to be able to rotate in the fluid, but that alone is not enough to ensure forward motion. Once rotating, the swimmer’s thrust has to be large enough to deform the fluid around it. Without that, the swimmer is stuck. And, finally, once they’re moving, the swimmer’s tail pitch determines how fast they can move and whether the fluid’s characteristics slow it down.
The researchers hope their work can shed light on propulsion for bacteria in the body, as well as larger creatures like burrowing earthworms and fruit-invading parasites. (Image credit: SwedishStockPhotos; research credit: F. Nazari et al.; via APS Physics)