Place: EUROTHERM - Seminar Nr. 84, Namur, Belgium
Liquid droplets on rough hydrophobic substrates are typically observed either in the Cassie/Baxter (or “fakir”) state, i.e. on top of the roughness structures , or in the Wenzel state, where the droplet completely wets the substrate . While virtually all studies hitherto have been considering droplets much larger than the roughness scale of the substrate , we, for the ﬁrst time, investigate the case where the size of the droplet is comparable to the surface sperities. This is a very natural situation that occurs whenever a liquid drop condenses or evaporates on such a substrate. We ﬁnd a new stable state where the droplet is partially impaled, not yet contacting the bottom surface. The penetration depth decreases upon evaporation, leading to a qualitatively new self-cleaning mechanism. We demonstrate the generic character of this ﬁnding and emphasize its scale invariance, both being issues of considerable importance for practical applications.