Water Tank and Release Mechanism

The experiments were accomplished by utilizing a water tank (cross section of 50cm x 50cm) filled with demineralized water. A horizontal beam, that is adjustable in height supports the release mechanism. This allowed the release height to be varied from 15cm to 95cm. To ensure that two spheres could simultaneously be released at a defined lateral distance, a metallic plate with small carefully drilled holes is used. This plate is held in a horizontal position by an electromagnet. Upon deactivation of the electromagnet, a compressed spring forcefully and rapidly pushes down the plate, and the spheres descend towards the water surface. A priori tests demonstrated that the steel spheres do not experience any rotation and impact the water at the same time.

Spheres and Coating

Steel spheres with a diameter of 20mm and 12mm were used for the experiments. Without any coating, the spheres are hydrophilic (static contact angle of 85°), causing hardly any air entrainment upon water entry. Some of the spheres were coated with a superhydrophobic coating, which increased the static contact angle to over 165°. This causes a large air cavity to form behind the spheres upon water entry.

Water entry events were recorded using two different High Speed Cameras:

• Photron FASTCam-SA3 (2000 fps, 1024x1024 pixels) used for close-up recordings to capture intricacies of the air cavity development, the splash dome development, etc.
• Kron Technologies CHRONOS-2.1HD (1512 fps, 1280x1024 pixels) used for wide area recordings to capture the sphere trajectories.

Particle Image Velocimetry was applied to obtain two-dimensional velocity fields of the water in the vicinity of the spheres. Fluorescent tracer particles were added to the flow and illuminated by a 2.0 Watt Continuous Wave green laser and tracked by a High-Speed Camera. All tank walls, except for the front wall, were covered by black matte sheets. An inclined mirror was used to reflect the laser sheet back to the area of interest. With this setup, it was possible to reduce unwanted reflections and to obtain an adequately illuminated area around the spheres. Lens filters were also used to improve the quality of the recorded particle images.

On 24 May 2024 the 'Lange Nacht der Forschung' (Long Night of Research), an event dedicated to presenting science and research to the public, took place at approximately 270 exhibition venues and 2800 stations across Austria. We took this opportunity to demonstrate and explain the events associated with water entry and the underlying physical phenomena by performing live High-Speed Camera measurements. Visitors could choose between different objects, ranging from differently coated steel spheres to simplified bird models for performing these water entry tests. Differences in cavity entrainment, trajectory stability and splash development could be investigated on the spot and compared and discussed in detail. Thanks to the HSC recordings, we could replay the results for the audience if requested. The event drew a large number of visitors of all ages and backgrounds and was a huge success.