Role and significance of wetting pressures during droplet impact on structured superhydrophobic surfaces

The impact dynamics and spreading behavior of droplets impinging on structured superhydrophobic surfaces are dependent on both the droplet initial conditions and the surface texture. The equivalence of wetting and dewetting pressures is classically known to be a critical factor in determining the state of a droplet during the contact and spreading phases. The present study extensively examines the underlying physics behind this pressure balance during the impact process and its direct role in determining the wetting process. Extensive three-dimensional simulations employing droplet impact on a structured superhydrophobic surface has been performed to reveal the intricacies of the interactivities of the fluid with the microstructure. Insight onto the acute role of wetting pressures and the implications of the same on determining the wetting dynamics, with internal fluidics of the droplet during the impact process, has been discussed. The phenomenon of state transition from the Cassie-Baxter to the Wenzel up on impact is also investigated and the intricate flow mechanics at play within the posts has been presented. Knowledge of pressure distribution and internal flow structures within the droplet during its interaction with the surface at different instances of time reveals the root mechanism behind the impalement of the droplet to a fully wetting state. Analysis of the internal pressure and flow distribution also presents necessary justification for the existence of a partially impaled state. The time evolution of spread for different scenarios is in agreement with experimental results and the article provides insight onto the role of wetting pressure in determining fluidic interactions on such surfaces.