Stability of superhydrophobicity at the nanoscale
Alberto GIACOMELLO (Sapienza Università di Roma)
Superhydrophobicity emerges from the combination of surface roughness and hydrophobicity, which promotes the trapping of gas pockets at solid surfaces. Superhydrophobicity is of extreme technological relevance, as it renders surfaces self-cleaning, ultra liquid-repellent, and drag-reducing and can be realized with relatively inexpensive coatings. However, the trapped gas pockets can collapse due to increased pressure in the liquid.
In order to realize stable superhydrophobicity a common strategy is to reduce the roughness size towards the nanoscale in order to exploit capillary forces. We analyze here recent experiments of submerged nanotextured surfaces in which the quantity of gas entrapped in the nanostructures is measured via small-angle X-ray scattering as a function of the applied pressure. The related intrusion/extrusion cycles are characterized by large hysteresis loops.
In order to explain this phenomenon, we apply the Continuum Rare Events Method (CREaM) developed in order to compute the free energy profiles connected with the wetting process of patterned surfaces mimicking the experimental ones. Our analysis suggests that the hysteretical behaviour encountered at the nanoscale originates from the metastable trapping in the Cassie (suspended) and in the Wenzel (fully wet) states.
The calculated free energy barriers connected with the wetting process are – even at the nanoscale – much larger than the thermal energy, accounting for the strong metastabilities. The effect of the geometry of surface roughness and of contact angle hysteresis on the thermodynamics, on the kinetics, and on the reversibility of the intrusion process are discussed.
