The wettability of solid surfaces modified by vacuum deposition of stearic acid

Abstract

Wettability of a solid surface plays an important role in dropwise condensation. In this work, physical vapor deposition of stearic acid was used to modify the wettability of glass surfaces. Dynamic advancing and receding contact angles were measured to study the effect of fractional coverage on the surface wettability. The experimental results show that the growth of stearic acid on the glass tends to occur by island growth. The advancing and receding contact angles increase with increasing film thickness, and approach 110 and 90°, respectively, at a film thickness of 20 nm. For films with thickness >20 nm, the substrate surface is completely covered by the deposited stearic acid, and the contact angle hysteresis is smaller than on a heterogeneous surface. During the measurment of contact angles, the movement of the three phase contact lines causes the molecular layer to rearrange for a partially covered film; however, the films become stable as the glass surfaces are completely covered. The dynamic contact angle data show that the surface of a vacuum deposited film is uniform in wettability, and is not affected by the surface heterogeneity in microstructure. The results of XPS analysis and a simple condensation test for the deposited surfaces are also reported.