Fig. 3. Contact angle of condensing microdroplets on nanostructures.
Advancing contact angle θa measured by ESEM imaging as a function of the droplet radius r for materials A (nanocylinders, red dots), T4 (truncated nanocones, green dots) and E4 (nanocones, blue dots). In all cases, θa increases and saturates with r, but angles are systematically higher by about 20° and 30° on T4 and E4 than on A. Angles on A increase from 110° ± 5° to 140° ± 3° as r varies from 0.6 to 6 µm, on T4 from 150° ± 2° to 163° ± 2° as r varies from 2 to 7 µm, and on E4 from 140° ± 7° to 171° ± 3° as r varies from 0.3 to 7 µm. The solid line is the model for nanocones described in the text and in the Methods section (Eqs. 1 and 2). We also report with dotted lines the contact angles obtained for millimetric water drops on A, T4 and E4, θa = 167° ± 3°, 160° ± 2° and 164° ± 3°, respectively. Interestingly, these angles are much larger for A and slightly smaller for T4 and E4 than the saturation value at microscale. Error bars represent standard deviation.