Impact of hexadecane and water droplets on non-wetting surfaces

It is well known that a superhydrophobic surface may not be able to repel impacting droplets due to the so-called Cassie-to-Wenzel transition (CWT). Unlike other work, no rebound is observed when low surface tension liquids (σ = 27.5 mN/m at 25°C) are concerned, even for very low impact velocity and if very high values of θR and low hysteresis are available. A CWT occurs after the impact due to the easier penetration of low surface tension fluids in the surface structure. This suggests that, even if it is possible to produce surfaces with an enhanced static repellence against oils and organics, generally the realization of synthetic materials with self-cleaning and anti-sticking abilities in dynamic phenomena still remains an unsolved task. Moreover, it is demonstrated that also the surface chemistry and the physico-chemical interactions with the liquid drops and/or the wettability gradient of the surface asperities play an important role in determining the CWT threshold. Therefore, the available classical numerical simulations of drop impacts onto dry surfaces are not able to capture the final outcomes of the impact for all the possible fluids