Quenching active matter using light: Light mediated motility in swarming Serratia marcescens*

Understanding the connection between external stimuli such as light and emergent response manifested in multicellular, collective and long-range motility of cells and cell-like active systems remains a challenge. Swarming, a mode of bacterial surface migration is characterized by emergent flows, persistent structures and collaborative collective motility. Here we present the effects of light on the collective motility in a dense and far from equilibrium active system – swarms of the bacteria Serratia marcescens. Using a experimental light-based technique, we induce and generate localized condensed domains differing in motility. We then study immobilization and quenching of flow inside these domains and map the response to light in terms of the light intensity and duration of exposure. Together these parameters determine the reversibility of the response, domain size and extent of the mobility impaired region as well as its temporal evolution. Complementing our experimental results, we propose and analyze a minimal Brownian dynamics model to study the escape of bacteria from the exposed region before they are completely immobilized and trapped.