Photoinduced formation of zinc nanoparticles by UV laser irradiation of ZnO.

Simple exposure of single-crystal ZnO to 193 nm excimer laser radiation at room temperature results in unexpected coloration. The gray to nearly black colored material, seen principally in the irradiated laser spot, is superficial. We present unambiguous evidence that this coloration is due to high densities of metallic Zn nanoparticles growing on the exposed surface of the crystal. Higher fluence laser exposure generates accumulated surface metal just outside of the irradiated spot. We suggest that the near surface bulk is photodecomposing; thermally driven diffusion leads to surface Zn metal aggregation.

Motion and dissolution of drops of sparingly soluble alcohols on water.

The dissolution of liquids with low mutual solubility is typically slow. However, drops of sparingly soluble, low-density, low-surface-tension liquids often dissolve rapidly on water due to surface tension instabilities and gradients. We report observations of the motion and dissolution of drops of aliphatic alcohols of a wide range of alkyl chain lengths as they dissolve in water. The alcohol drops are rendered visible by adding small amounts of iodine or other dyes. These drops display dewetting instabilities, fragmentation, fingering, and oscillation. As the length of the alcohol carbon chain increases from n = 4 to n = 9, dissolution slows dramatically. The roles of alcohol solubility and water surface area in promoting rapid dissolution are discussed.

Dropwise condensation: experiments and simulations of nucleation and growth of water drops in a cooling system.

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.

Scanning-induced growth on single crystal calcite with an atomic force microscope.

We report observations of localized growth on the (1014) surface of single-crystal CaCO3 in supersaturated solutions while scanning with the tip of an atomic force microscope (AFM). At low contact forces, AFM scanning strongly enhances deposition along preexisting steps. This enhancement increases rapidly with increasing solution supersaturation, and is capable of filling in multilayer etch pits to produce defect-free surfaces at the resolution of the AFM. Attempts to achieve similar deposition rates in the absence of scanning require high supersaturations that produce three-dimensional crystal nuclei, which are important defects. Localized deposition produced by drawing the AFM tip back and forth across step edges can produce monolayer deposits extending well over a micron from the scanned area. These tip-induced deposits provide convincing evidence for the importance of ledge diffusion in calcite crystal growth.

Influence of molecular weight on nanoscale modification of poly(methyl methacrylate) due to simultaneous mechanical and chemical stimulation.

We report observations of poly(methyl methacrylate) films modified by the synergistic effect of solvent exposure and mechanical stress applied by the tip of an atomic force microscope (AFM). We show that these modifications are sensitive to polymer molecular weight as well as solvent strength and the force applied by the tip. Small-area scanning often produces localized patches of raised material as well as depressed areas. The volume change associated with the depressed areas generally increases with increasing solvent strength, increasing applied normal force, and decreasing polymer molecular weight. In contrast, the volume change associated with the raised patches is greatest for 25-145K Mw films in 60 and 100% ethanol solutions. In each case, the normal force applied by the AFM tip must exceed a threshold to significantly modify the surface; this threshold is associated with an increase in lateral force applied by the AFM tip during small-area scanning. We attribute the raised patches to mechanically enhanced swelling due to diffusion of solvent into near-surface material. Permanent net volume loss, when observed, is attributed to localized polymer dissolution.