High efficient fog-water harvesting via spontaneous swallowing mechanism

Inverse wettability gradient movement triggered by water droplets’ surface energy is conductive to many fields, such as droplet transport, water supply and water nanogenerator. Here, a novel phenomenon of spontaneous inverse wettability gradient movement of droplet (i.e., the “swallowing”) is revealed (with an energy transformation ratio at 64.1%) and successfully applied to a new fog-water harvester (i.e., unique gradient wire: UGW). In wetting area on UGW, the largest droplet hanging on more hydrophilic end acts as a gluttonous droplet moving towards inverse wettability to continuously coalesce other droplets and to gain kinetic energy from released surface energy of droplets, resulting in an accelerative reciprocating action like a windshield wiper. As for a given fog flow of 150 g s⁻¹ m⁻², the maximum kinetic energy and velocity of the gluttonous droplet are 1.83 μJ and 3.8 cm s⁻¹, respectively. This process needs no extra energy consumption and realizes a high water harvest efficiency (1.83 μL cm⁻¹ s⁻¹). This “Swallowing” action on such UGW allows droplet to accelerate gradually, which is beneficial to power generation from surface energy, heat transfer, and fog-water harvesting, etc.