Quorum sensing in emulsion droplet swarms driven by a surfactant competition system.

Quorum sensing enables unicellular organisms to probe their population density and perform behaviour that exclusively occurs above a critical density. We establish quorum sensing in emulsion droplet swarms that float at a water surface and cluster above a critical density. Our design involves competition between 1) a surface tension gradient that is generated upon release of a surfactant from the oil droplets, and thereby drives their mutual repulsion, and 2) release of a surfactant precursor from the droplets, that forms a strong imine surfactant which suppresses the surface tension gradient and thereby causes droplet clustering upon capillary attraction. The production of the imine-surfactant depends on the population density of the droplets releasing the precursor, so that the clustering only occurs above a critical population density. We exploit the pH-dependence of the imine-surfactant formation to trigger quorum sensing upon a base stimulus, establish dynamic droplet swarms that cluster and spread upon spatiotemporally varying acid and base conditions, and couple the clustering of two droplet subpopulations to a chemical reaction that generates of a fluorescent signal. We foresee that quorum sensing enables control mechanisms in droplet-based systems that display collective responses in contexts of e.g. sensing, optics or dynamic controlled droplet-reactors.