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Dropwise condensation of water vapor from a naturally cooling, hot water reservoir onto a hydrophobic polymer film and a silanized glass slide was studied by direct observation and simulations. The observed drop growth kinetics suggests that smallest drops grow principally by the diffusion of water adsorbed on the substrate to the drop perimeter, while drops larger than about 50 microm in diameter grow principally by direct deposition from the vapor onto the drop surface. Drop coalescence plays a critical role in determining the drop-size distribution and stimulates the nucleation of new, small drops on the substrates. Simulations of drop growth incorporating these growth mechanisms provide a good description of the observed drop-size distribution. Because of the large role played by coalescence, details of individual drop growth make little difference to the final drop-size distribution. The rate of condensation per unit substrate area is especially high for the smallest drops and may help account for the high heat transfer rates associated with dropwise condensation relative to filmwise condensation in heat exchange applications.

Nanometer scale single asperity tribochemical wear of silicon nitride was examined by measuring the wear of atomic force microscope tips translated against a variety of substrates in aqueous solutions. We show that the chemical nature of the substrate plays an important role: significant wear was observed only when the substrate surface is populated with appropriate metal-hydroxide bonds. Mica and calcite substrates, whose water-exposed cleavage surfaces lack these bonds, produced little if any tip wear. As a function of contact force F N and scan duration t, the length of the tips in this work decreases approximately as (F N t) 0.5. We propose that pressure-induced intermediate states involving hydroxyl groups form on both the tip and the substrate; chemical reactions subsequently form transient bridging chemical bonds that are responsible for tip wear.

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