Emergence Arising from the Interplay between Active and Passive Particles

Active matter systems are capable of taking energy from their environment and converting it into directed self-propulsion. This drives them out of equilibrium, enabling them to showcase a range of dynamic behaviours not possible otherwise. From swimming bacteria forming biofilms at surfaces, to swarming fish evading predators, to the flow and clogging of people, wading through a railway station, many complex phenomena arise when active matter interacts with its environment. As such, harnessing more information about these ubiquitous dynamics through model colloidal systems has garnered ever increasing interest. In this study we focus on how active particles influence their environment; as they shape it, and it shapes them. We find that the passive environment, formed of Brownian microparticles, if sufficiently dense, is capable of guiding active particles to each other as they burrow through it, eventually leading to active-active clustering interactions. We also probe if opposite is also true, can active particles cluster and assemble their environment? This was shown to be possible through the use of thermal gradients, as active units mobilise through heating, their heat gradients pull in their passive environment, creating self-assembled structures around the active units. This active-passive interplay is further exploited by turning these structures into a device in the form of a random micro-laser. In random lasing, passive particles act as scatterers, inside a gain medium, which enhance the feedback of light, leading to amplification and eventually lasing. Usually, low concentration areas of passive particles lack enough light feedback for lasing, however we use an active particle to collect a dense population of passive particles into a pump area, creating a self-assembling laser