Hanging droplets from liquid surfaces

Nature uses surface tension to support dense objects on liquid surfaces, from water striders to insect larvae for mating and survival, which has inspired fabrication of man-made robotic systems for transport across water. These systems present hydrophobic surface to prevent sinking. Here, we show that a droplet of a denser aqueous solution, containing a polyelectrolyte can hang on the surface of a less dense aqueous solution containing an oppositely charged polyelectrolyte. The magnitude of the interfacial forces acting on the droplet and the shapes of the hanging droplets can be controlled by releasing heights of the droplet and the polyelectrolyte concentrations. Coacervate sacs with homogeneous and heterogeneous surfaces can be produced that hang from the surface and, by capillary forces, form well-ordered arrays. Controlled locomotion and rotation can be achieved by functionalizing the hanging droplets with magnetic microparticles. The suspended droplets are in direct contact with air enabling in situ manipulation of the droplets and using the encapsulated aqueous phases for compartmentalized cascading chemical reactions with selective transport. These hanging droplets have potential applications in functional micro-reactors, micro-motors and biomimetic micro-robots.