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Hanging droplets from liquid surfaces

Significance Heavier objects usually sink in a less dense fluid. Water-walking anthropods and biomimetic water-walking robots harness surface tension in order to overcome this tendency, floating on top of liquids. By hanging a coacervate-encased droplet of a denser aqueous dextran solution from the surface of an aqueous solution of poly(ethylene glycol) (PEG), we harness the binding of dense droplets to interfaces by surface tension to build two-dimensional ensembles of structurally complex droplets. Applications ranging from reaction vessels with selective transport to motors and robotics are enabled by our findings. Natural and man-made robotic systems use the interfacial tension between two fluids to support dense objects on liquid surfaces. Here, we show that coacervate-encased droplets of an aqueous polymer solution can be hung from the surface of a less dense aqueous polymer solution using surface tension. The forces acting on and the shapes of the hanging droplets can be controlled. Sacs with homogeneous and heterogeneous surfaces are hung from the surface and, by capillary forces, form well-ordered arrays. Locomotion and rotation can be achieved by embedding magnetic microparticles within the assemblies. Direct contact of the droplet with air enables in situ manipulation and compartmentalized cascading chemical reactions with selective transport. Applications including functional microreactors, motors, and biomimetic robots are evident.