Autonomous droplet transport on a chemically homogenous superhydrophilic surface

The autonomous and directional transport of microdroplets is critical to the development of self-powered droplet-based microfluidic devices. So far the studies on the self-transport of droplets have employed chemically inhomogeneous surfaces. Remarkably, we observed the self-propulsion of aqueous droplets on a chemically homogeneous superhydrophilic surface. The self-transport phenomenon is attributed to the existence of a precursor layer between droplet and surface that minimizes contact line pinning and droplet is driven by a net force owing to the asymmetry in the precursor layer width across droplet. We provide a theoretical model considering the electrostatic, capillary, and evaporation induced forces to quantitatively predict transport velocity which is in good agreement with experimental observations. The effect of droplet volume, surface roughness, relative humidity, and composition on the droplet velocity is studied. We demonstrated the directional self-transport and coalescence of droplets on a superhydrophilic track in a superhydrophobic background and the self-transport of droplets containing biological cells depicting biological cargos.