Metastable droplets on shallow-grooved hydrophobic surfaces.

The equilibrium shapes of water droplets on shallow-grooved hydrophobic surfaces are studied experimentally. The dependence of the two final states, notably metastable Cassie-Baxter and Wenzel, on the underlying geometric pattern is analyzed and discussed. Surprisingly, in contrast to theoretical expectations, a significant portion of the droplets are in the Cassie-Baxter state. The anisotropy of the patterns, defined by the relative groove and ridge widths, allows studying the influence of different mechanisms of spreading in orthogonal directions on the final shape of the droplets. The validity of the Cassie-Baxter and Wenzel models in the case of anisotropic surfaces is investigated, comparing the experimental data with theoretical predictions in the two respective regimes. The influence of varying ridge widths for fixed groove widths on the final state adopted by the droplets, i.e., Cassie-Baxter or Wenzel, is discussed. ©2011 American Physical Society