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.

Poly͑methyl methacrylate͒ ͑PMMA͒ is highly resistant to laser ablation at 308 nm. Either very high fluences or absorbing dopants must be used to ablate PMMA efficiently at this wavelength. We investigate two dopants, pyrene and a common solvent, chlorobenzene, using time-of-flight mass spectroscopy. Both compounds improve the ablation characteristics of PMMA. For both dopants, the first step in ablation is an incubation process, in which absorption at 308 nm increases due to the production of CvC bonds along the polymer backbone. Incubation at 308 nm is similar to that observed for shorter ultraviolet wavelengths in previous studies. The principal ablation products and their corresponding temperatures are consistent with a photothermal ablation mechanism.

Scanning force microscopy ͑SFM͒ is employed to study nm-scale wear of single-crystal calcite in an aqueous solution. When the SFM tip is drawn back and forth in a linear fashion across the edge of a preexisting single atomic layer etch pit, dissolution is strongly enhanced at the point where the tip crosses the step. The wear rate as a function of contact force is consistent with a thermally activated wear process, where the activation energy is locally reduced in the strain field of the SFM tip. The activation volume for the strain dependence is on the order of the average volume per ion in the CaCO 3 lattice. This study provides further support for strain enhanced nucleation of double kinks along preexisting steps.

Laser-induced Mg production from magnesium oxide using Si-based agents and Si-based agents recycling