We the droplets: A constitutional approach to active and self-propelled emulsions

The field of active matter, and particularly active emulsions, is growing rapidly, with significant progress made recently on both theoretical and experimental fronts. Here, we summarize experimental research progress related to active droplets. The constitution of active droplets, in particular the chemical compositions and structure of interfaces, is critical. We discuss how emulsion properties such as mechanism of motion, speed, trajectory, interaction strength, and lifetime are related to the droplet composition. We consider not only traditional single emulsions but also more complex variants, such as Janus droplets, Pickering emulsions, and multiple emulsions. Active behavior of isolated droplets as well as pairwise and multibody interactions between droplets is described. The influence of physical barriers that shape the local chemical gradients and fluid flow is also highlighted. This review provides perspective on the past, current, and promising future experimental research directions in active droplet research.