Impact of Different Liquid Drops on Micro/Nano-Structured Superhydrophobic Surfaces

One of the most fascinating natural phenomena is the ability of some surfaces to repel water drops. Since the discovery of lotus leaf surface structure, several attempts of artificial, biomimetic superhydrophobic surfaces (SHS) have been made. Nowadays, we know that superhydrophobicity arises from a combination of surface morphology and chemical composition. However, different surfaces can display analogous wetting properties in static conditions (e.g. similar contact angles) but radically divergent drop impact output. Thus, drop impact studies provide enhanced insight on surface wetting properties in dynamic conditions. We fabricated SHS with different morphology and/or chemical composition. Namely, surface S had a flower-like alumina structure with nanoscale cavities (as observed with FESEM), chemically modified with fluorosilanes; surfaces LAU and FAS had a terrace-like alumina structure with micro-cavities, modified with lauric acid or fluorosilanes, respectively. Then, we assessed their quasi-static wetting properties (e.g. advancing contact angle θA, receding contact angle θR and contact angle hysteresis Δθ) and drop impact behavior in a range of Weber number 1<We<650 with two different liquids, namely water and hexadecane, to study the effect of surface tension σ on drop impact output. All surfaces were superhydrophobic (e.g. θR>135°, Δθ<10°), but while S and FAS had θR>120° with hexadecane, LAU was oleophilic (θR≈0). In water drop impacts, S surfaces always produced a rebound, indicating a stable Cassie-Baxter wetting state. Meanwhile, for LAU and FAS a Cassie-to-Wenzel transition (CWT) was observed at high We, with partial rebound as an output. Such behavior is consistent with results in literature: nano-cavities on S surface cause high capillary pressure PC against wetting, while micro-cavities on LAU and FAS were penetrated by drops when wetting pressures (i.e. effective water hammer pressure PEWH and gas layer pressure PGL) exceeded PC. Significantly, CWT occurred at higher We for LAU than for FAS, notwithstanding their identical surface structure and water contact angles. This result hints at a role of surface chemistry in drop impact behavior, a phenomenon that has never been reported before and certainly deserves further studies. On the other hand, hexadecane drops never rebounded, even on S and FAS surfaces. Antonini et al. (Langmuir 2013) defined θR>100° as a criterion for water drop rebound. However, this does not hold for hexadecane: PC is lower when s is smaller, thus causing CWT even if the surface is oleophobic in static conditions. In conclusion, the results highlight that it is not possible to easily correlate contact angles and drop impact dynamics of low- and high-surface-tension liquids on different surfaces, as CWT can be observed even on statically repellent surfaces. Thus, an accurate design of surface properties must be pursued in the future research towards dynamically amphiphobic, biomimetic surfaces.