Acoustic waves for active reduction of droplet impact contact time

Minimising droplet impact contact time is critical for applications such as self-cleaning, anti-erosion or anti-icing. Recent studies have used texturing of surfaces to split droplets during impact or inducing asymmetric spreading, but these require specifically designed substrates which cannot be easily reconfigured. A key challenge is to realise an effective reduction in contact time during droplet impingement on a smooth surface without texturing but with an active and programmable control. Our experimental results show that surface acoustic waves (SAWs), generated at a location distant from a point of droplet impact, can be used to minimise contact time by as much as 35% without requiring a textured surface. Besides, the ability to switch on and off the SAWs means that reduction in droplet impact contact time on a surface can be controlled in a programmable manner. Moreover, our results show that by applying acoustic waves, the impact regime of the droplet on the solid surface can be changed from deposition or partial rebound to complete rebound. To study the dynamics of the droplet impact, we developed a numerical model for the multi-phase flow and simulated different droplet impingement scenarios. Numerical results revealed that the acoustic waves could be used to modify and control the internal velocity fields inside the droplet. By breaking the symmetry of the internal recirculation patterns inside the droplet, the kinetic energy recovered from interfacial energy during the retraction process is increased, and the droplet can be fully separated from the surface with a much shorter contact time. Our work opens up opportunities to use SAW devices to minimise the contact time, change the droplet impact regime and program/control the droplet’s rebounding on smooth/planar and curved surfaces as well as rough/textured surfaces.