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 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* ≈ 0.1 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. These results may have potential applications for drainage and surface cleaning, non-stick surface coating, industrial mixing and plant disease spreading.
|Original language||English (US)|
|State||Published - Dec 22 2022|
Bibliographical noteKAUST Repository Item: Exported on 2023-01-10
Acknowledgements: The authors would like to thank Dr. Silvana Martini of the USU food science department for her gracious permission to let the authors use the AR G2 rheometer in her laboratory for the rheology measurement. The authors would also like to thank Dr. Randy Hurd from Weber State University, Utah, US and Dr. Zhao Pan from University of Waterloo, Canada for valuable inputs and discussions. J.A. acknowledges partial support from University of Hawaii at Manoa and the ONR Redhill project #N00014-20-1-2651. The authors would also like to thank Dr. Sandip Dighe from King Abdullah University of Science & Technology, Thuwal, KSA for the Supplementary experiments.