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.

We describe the response of poly(methyl methacrylate) surfaces to localized mechanical stimulation by the tip of an atomic force microscope (AFM) in water, methanol, ethanol, and aqueous alcohol solutions. Simply pressing the AFM tip into the surface with no horizontal motion fails to produce visible features in subsequent low contact force images. A single small-area (40 × 40 nm 2 ) high contact force scan has little effect in air but in water or in alcohol-water mixtures produces soft bumps (local volume increase) adjacent to the scanned area. These bumps typically have lateral dimensions of ∼100 nm and rise tens of nanometers above the surrounding surface. Larger, micron-scale scans produce approximately parallel, raised ridges 50-150 nm apart. These structures are stable over time periods of hours or more in air and in solvent. We present evidence that these modifications are due primarily to stress-enhanced solvent uptake in material surrounding the area of tip-polymer contact.