FROM DROPLET GROWTH TO FILM GROWTH ON A HETEROGENEOUS SURFACE - CONDENSATION ASSOCIATED WITH A WETTABILITY GRADIENT

Condensation experiments on a cold solid substrate with a gradient of contact angle (theta) are reported. The gradient is sufficiently small so that the driving force due to the imbalance of the interfacial tensions at the contact line is negligible throughout most stages of the experiment. The condensation proceeds by forming spherical droplets near the hydrophobic side (theta similar or equal to 100 degrees) and by forming quasi-films near the hydrophilic side (theta similar or equal to 0 degrees). In the middle part is the crossover region, where the condensation proceeds by forming islands of nonspherical shapes. It is found that the hysteresis of contact angle due to the pinning of contact lines by surface heterogeneity plays an important role in the growth of the condensation pattern. The effects of the wettability gradient and the contact-angle hysteresis on the nucleation rate, the growth rate of islands, the dynamics of coalescence, and the growth morphology are studied and discussed. An important result is that the growth remains self-similar with the same growth law as for perfect hemispherical drops (with negligible pinning force), although the droplets exhibit complicated shapes.