Emergent behavior driven by self-generated gradients: from active droplets to bacterial suspension

The out-of-equilibrium dynamics of chemotactic active matter—be it animate or inanimate—is coupled to the environment which is a chemical landscape constantly modified by secretions from the motile agents, nutrient consumption, or respiration. This can give rise to complex collective dynamics, altering the migration strategies or pattern formation by the active agents.

In this talk, I will first discuss self-propelling droplets as a model for chemically active particles that modify their environment by leaving chemical footprints, which act as chemorepulsive signals to other droplets. I will present our analysis of this communication mechanism quantitatively both on the scale of individual agent–trail collisions as well as on the collective scale where droplets actively remodel their environment while adapting their dynamics to that evolving chemical landscape. We show in experiment and simulation how these interactions cause a transient dynamical arrest in active emulsions where swimmers are caged between each other’s trails of secreted chemicals. These findings yield principles for predicting how negative autochemotaxis shapes the navigation strategy of chemically active particles.

Next, I will discuss the dynamics of aerobic bacterial suspensions that are confined in tight spots where oxygen availability is spatially variable. In such systems, the cellular motility is coupled to the local oxygen concentration. We show that bacteria collectively consume oxygen, generate an oxygen gradient, and then respond to it by separating into two distinct aerobic and anaerobic phases.