Droplet lift-off from hydrophobic surfaces from impact with soft-hydrogel spheres

Abstract Droplet impacts on superhydrophobic surfaces may result in complete bouncing, with the absence of contact hysteresis and viscous dissipation leading the droplet to fully rebound off the surface. This rebound usually happens in the retraction phase, when the droplet retracts back after reaching a maximum spread diameter. Here, we present experimental evidence of a novel bouncing phenomenon where a sessile droplet on a hydrophobic surface bounces off the surface in its spreading phase when a soft deformable hydrogel sphere axisymmetrically impacts the droplet. We term this as 'Lift-Off' and propose a simple force balance based on the deformation characteristics of the hydrogel sphere to explain the out-of-plane jump of the droplet during spreading. We observe three different impact regimes, and propose their dependency on a modified elastic 'Mach' number (Ma*) with (Ma*)=0.2 corresponding to the onset of lift-off. We also report on the unique acoustic signatures of lift-off cases, associated with the capture of air-bubbles through the air-borne retracting droplet rim. Concerning the novel lift-off results, this work may have potential applications for drainage and surface cleaning, non-stick surface coating, industrial mixing and plant disease spreading.