Acoustics of Drop Impact on Hydrophobic Surfaces and Liquid Pools

Abstract

Drop impact on surfaces has been studied comprehensively as it has a wide range of fundamental and practical implications. The splash, spreading, and rebound have been investigated with respect to substrate interfaces with hydrophobic surfaces being of particular interest. However, impact on wetted substrates is less understood for the hydrophobic surfaces. Also the associated acoustics of the drop impact has not be explored in terms of air entrainment and substrate vibrations. We investigate the impact of water drops (∼2–10 mm diameter) from heights 100–400 cm upon hydrophilic– and hydrophobic–coated solid surfaces as well as free floating liquid pools (0.15–0.45 ml). A contact microphone coated with the Glaxco water repellant provides a hydrophobic surface upon which impact vibrations can be quantified. An air microphone is synchronized to a high-speed Phantom camera for sound recording and optical visualization of the impact and rebound phases. Cross-correlation of the two microphones reveals distinct differences between the two solid substrates. The initial rebound and subsequent jet break-up are found in an analysis of the signal’s amplitude and phase. These results are compared to those from gel spheres and discussed for applications of non-contact coating monitoring.