This is a collaborative work with Prof. D. Beysens at PMMH-ESPCI and CEA-ESEME, France. Condensation patterns (or breath figures) appear when a vapour saturated atmosphere is in contact with a colder substrate. This phenomenon is called heterogeneous condensation, because the wetting properties of the substrate lowers the energy barrier that opposes to the condensation. This makes possible that dew forms at higher temperatures than vapor condensates in the atmosphere, and that the dew does not appear as a homogenous layer but forming a pattern [D. Beysens et al. Phys. Rev. Lett. 57 (1986) 1433] [D. Beysens C. R. Physique 7 (2006) 1082].

We studied the effect of the contact angle (or more generally, of the wetting properties of a substrate) on the growth law for the condensation pattern. We also focus on the types of drops formed (Wenzel, Cassie-Baxter, and mixed Cassie-Baxter-Wenzel) and the mechanisms involved in their transformation.

We obtained (among other interesting things [R.D. Narhe et al. Appl. Surf. Sci. 256 (2010) 4930]) that the growth laws are essentially the same: all compatible with a first regime without coalescence (power law with exponent 1/3) and with a second regime, self-similar with many coalescence events (power law with exponent 1). This exponents have been observed previously in smooth planar surfaces. In our experiments, the surface texture scale is much bigger than the one at which the nucleation events occur. Consequently, the surface chemistry is much more relevant during condensation and leads to similar results as on a smooth surface. Also, we showed that the energy balance during coalescence is enough for inducing a transition from Cassie-Baxter or Cassie-Baxter-Wenzel to the most stable Wenzel state (because in our experimental conditions the critical angle is bigger than the equilibrium contact angle). Also, the coalescence of Cassie-Baxter and Cassie-Baxter-Wenzel drops with Wenzel drops results always into the Wenzel type.

• • R.D. Narhe et al. Appl. Surf. Sci. 256 (2010) 4930.

We acknowledge Professor Jordana for taking SEM images of the substrate. This work was partly supported by the Spanish MEC (ref. FIS2008-01126) and by Departamento de Educación (Gobierno de Navarra). R.D. Narhe acknowledges the support of Marie Curie International Incoming Fellowship (MCIIF) within the 7^th European Community Framework Program.

(a) SEM images of zinc surface (one day deposition) showing micro-flowers-like structure (size, a ~ 12 μm; separation, b ~ 25 μm; height c ~ 10 μm). (b) Area inside the circle in (a) at larger magnification. It shows nano sheets with average thickness of 300 nm. (c) Contact angle measurement of water drop on the surface:

Types of drops observed during condensation: Wenzel, Cassie-Baxter, and mixed Cassie-Baxter-Wenzel (they appear due to the mechanisms depicted in the highlights page):