Self-assembly and control of water drops at an interface

From the beautiful organization of swarming birds to the growth of molecular crystals, self-assembly occurs all around us at a wide range of length scales. In this work, we reveal the self-assembly and pattern formation of triboelectrically charged water droplets that are floating at an oil-air interface. We show that the assembly occurs because of capillary attraction, more commonly known as the Cheerios effect, and the opposing electrostatic repulsion between the drops. Using experiments and theory, we show that the depth of the oil bath plays a significant role in the distance between the drops assembled at the interface. We highlight the relevance of the type of the boundary containing the entire system by showing that even drops with a net zero electric charge can self-assemble under certain conditions. Furthermore, we present ways to control the motion and the assembly of the drops at an interface.