Continuous, autonomous sub-surface cargo shuttling

Water-walking insects can harness capillary forces by changing body posture to climb or descend the meniscus between the surface of water and a solid object, necessary for predation, escape and survival. Inspired by these systems found in nature, we demonstrate autonomous, aqueous-based synthetic systems that overcome the meniscus barrier and shuttle underwater cargo beneath the water surface to and from a landing site and a targeted drop-off site. We manipulate the sign of the contact angle of an aqueous droplet contained within a coacervate sac or of a hydrogel droplet by controlling the vertical force on the droplet. The cyclic buoyancy change occurs continuously, as long as the supply of reactants diffusing into the sac or droplet from the semi-infinite surrounding aqueous phase is not exhausted. These findings are directly applicable to autonomously driven reaction or delivery systems, sensors, and micro-/milli-robotics.