The effects of flow and external fields on dense microbial populations

Microorganisms often actively respond to multiple external stimuli to navigate towards their preferred niches. This talk will discuss experimental and theoretical work investigating the behaviour of dense suspensions of microorganisms subject to externals fields and fluid flows. We investigate the movement of magnetotactic bacteria, whose swimming direction is influenced by magnetic fields, as well as gravitactic microorganisms, which align themselves with gravity. We demonstrate that the interplay between hydrodynamic interactions among individuals - which are significant in the case of dense suspensions - and the application of external fields, gives rise to striking collective behaviour and rheology at the population level.