Dancing Drops

Active matter describes systems that operate far from thermodynamic equilibrium: active-matter constituents continuously consume energy, converting it into directed motion or mechanical work. Swarms of bacteria and flocks of birds are classic natural examples, with emergence of long-range order from local interactions. Artificial active-matter systems typically require elaborate setups, with constituents engineered to convert light or chemical energy into motion. Here, we explore an active-matter system powered instead by condensation.

By coating a surface with a thin oil film and cooling it below the dew point, water droplets nucleate and grow. Remarkably, the largest droplets begin to “dance,” tracing self-avoiding, sinuous paths across the surface like a serpent, before twirling in place like whirling dervishes. This spontaneous collective motion, fuelled entirely by condensation, is a new class of active matter that can be used for applications such as harvesting water from the atmosphere.