<p>A rigid sphere rebounding from the impact with a water droplet resting on a hydrophobic surface creates a liquid bridge between the surface and the sphere, eventually leaving a portion of the droplet on both. Yet, when the rigid sphere is substituted by a deformable one, the dynamics are not well understood. In this video, captured at a millimetric level using a Phantom v2511 high-speed camera, we present different regimes originating from hydrogel spheres (diameter appx. 16 mm) impacting a resting water droplet (volume between 0.15-0.45 mL). The most intriguing of these regimes is ‘Lift-Off’, happening at relatively low sphere impact heights (30-125 mm), in which the droplet bounces to a significant height, leaving no trace of water on the surface unlike what is commonly found in rigid sphere impact cases. The large deformation of the impacting hydrogel sphere pushes the pinned droplet contact line from its static position leading to droplet lift-off. In cases of higher impact heights (greater than 150 mm), the droplet spreads radially as a water sheet just above the surface. The thin liquid sheet then breaks up from the opposing forces of inertia and capillarity into multiple droplets. The unique acoustic characteristics of these regimes are also captured through the synchronization of an ultrasonic microphone and a high-speed camera. The beautiful imagery reveals rich and complex physical phenomena with possible applications in surface cleaning, plant disease spreading, non-stick surface coatings, and mixing in chemical processes.</p>

Deformable Sphere Impact on Resting Droplets

Paper Proceedings: https://asmedigitalcollection.asme.org/FEDSM/FEDSM2021/volume/85284

ASME 2021 Fluids Engineering Division Summer Meeting (FEDSM2021)

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