Bottom-up modelling of dense bacterial suspensions

Dense active suspensions, such as those formed by swarming bacteria (L3,L4), constitute a type of active matter that is particularly hard to model: cells are in frequent direct contact with each other and the underlying substrate so that they cannot be described as swimmers interacting solely via hydrodynamic interactions. Neither can these systems be modelled as dry active matter given the ample evidence that the surrounding fluid plays a crucial role in the collective dynamics. Recently, we demonstrated that some of the fascinating collective dynamics of dense bacterial suspensions can be quantitatively and faithfully accounted for by particle-in-fluid models [1,2] (T1). Such hybrid models are in fact simple enough to be the starting point of coarse-graining procedures leading to continuous theories, a much-desired goal to make progress toward a real understanding of these systems, beyond mere numerical simulations.

The PhD topic will be centred on the derivation and study, via kinetic-theory like methods, analysis, and numerical simulations (T9,T10), of continuous theories for dense bacterial suspensions starting from the above-mentioned particle-in-fluid models (T3,T4). This will be done both at mean-field level (deterministic PDEs) and at fluctuating hydrodynamic level, using, e.g., Dean-Kawasaki-like equations.

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[1] Li, H, et al. (2018) Proc Natl Acad Sci 116:777 [2] Li, H, et al. (2024) Phys Rev X 14:041006