Capillary force restoration of droplet on superhydrophobic ribbed nano-needles arrays

Previous experiments have observed squeezing and relaxing droplets between two identical superhydrophobic surfaces decorated by the ribbed nano-needles arrays (RNNA) leading to a fully reversible dynamic process. This reversibility presents a capillary force restoration accompanying deformation of the water droplet, which mainly depends on the surface microstructures. Based on the quasi-static equilibrium of droplet compression, we have deduced and numerically computed the capillary force utilizing a classical mechanics approach. The simulated forces agree well with the experimental results. The restoration mechanism is further explored and revealed by the calculated microscopic contact angle and the liquid-air interface around the nanoneedles.