The wettability of solid surfaces modified by vacuum deposition of stearicacid

Wettability of a solid surface plays an important role in dropwise condensa tion. In this work, physical vapor deposition of stearic acid was used to m odify the wettability of glass surfaces. Dynamic advancing and receding con tact angles were measured to study the effect of fractional coverage on the surface wettability. The experimental results show that the growth of stea ric acid on the glass tends to occur by island growth. The advancing and re ceding contact angles increase with increasing film thickness, and approach 110 and 90 degrees, 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 move ment of the three phase contact lines causes the molecular layer to rearran ge for a partially covered film; however, the films become stable as the gl ass surfaces are completely covered. The dynamic contact angle data show th at the surface of a vacuum deposited film is uniform in wettability, and is not affected by the surface heterogeneity in microstructure. The results o f XPS analysis and a simple condensation test for the deposited surfaces ar e also reported. (C) 1999 Elsevier Science B.V. All rights reserved.