Collective Nutrient Search by Chemotactic Active Agents in Two Dimensional Flows

Microorganisms interact and communicate through physical and chemical processes to regulate growth, movement and biochemical activities. Chemotaxis refers to the directed movement of microorganisms by sensing ambient chemical gradients. We consider a minimal model to describe a collection of chemotactic, active and interacting Brownian particles in two-dimensional model cellular and turbulent flows. We expect these traits, in general, to be useful for the microorganisms. The activity and chemotaxis together are beneficial in nutrient search and escaping the nutrient-scarce areas. However, the interplay of the underlying fluid flow, activity, and chemotactic sensing can give rise to emergent dynamical regimes that can have consequences for nutrient uptake and limited access to the nutrient source area. We find that the collective dynamics exhibits several interesting dynamical regimes as the non-dimensional parameter s based on the self-propulsion speed is varied: (i) A fluid -like phase for small s; (ii) a mixed phase at intermediate values of $s$, wherein the fluid contribution, activity and chemotaxis compete with each other; (iii) a chemotaxis dominated phase. The boundaries separating these dynamical regimes depend on the chemotactic strength. Moreover, the characterisation of transport properties suggests the presence of dynamical regimes corresponding to ballistic, diffusive, emergent trapping/clustering and super-diffusive phases. Our results are relevant to the complex collective nutrient search by microorganisms, such as bacteria, in a fluid environment.