Wenzel–Cassie Wetting Transition of Droplet on Rough Metal Substrates

A droplet evaporation experiment of Wenzel-Cassie wetting transition on metal substrates is presented. The evolution of two types of solution droplets on surfaces with different roughness values is measured using a video contact angle analyzer from a composite to a wetted state. The hydrophobic property increases with surface roughness, whereas the contact angle decreases over time. The contact angle with alcohol-water solution droplet is smaller than that with water droplet, but the former spreads faster than the latter. The alcohol-water solution can seep into the rough surface easier and more quickly than the water solution. The state completely changes into the Wenzel state. Compared with water droplet, the alcohol-water solution state (Wenzel state) is more sensitive to the change in surface roughness. The free energy of the metastable state Gwater is larger than that of Galcohol-water, and the wettability of the water droplet is greater than that of the alcohol-water one. The actual contact angle is closer to the Cassie model, and the system's free energy of the Cassie state is larger than that of the Wenzel state. The unstable Cassie state can translate into the relatively stable Wenzel wetting state, which is irreversible.