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.

Oxidation experiments using a particular grade of highly oriented pyrolytic graphite have allowed observation of large numbers of both monolayer and multilayer etch pits on the same samples, formed under identical conditions. Scanning tunneling microscopy was used to measure pits produced after various etch times, temperatures, and O2 pressures. From these data pit growth rates, activation energies, and reaction orders were derived. Although multilayer pits were observed to grow over 3 times faster than monolayer pits in air, both types of pits had the same activation energy. Multilayer etch pits were sometimes observed to form at screw dislocations in the graphite but were also seen in the absence of such defects. The experimentally determined reaction rates and activation energies were not consistent with a direct reaction of edge-carbon atoms with atmospheric oxygen, but instead suggest a chain reaction or preequilibrium process. A mechanism for oxidation of multilayer pits involving reaction of partially oxidized sites on adjacent graphite layers is suggested.

The interactions between biotin and avidin or streptavidin, a prototypical example of a specific biological ligand−receptor system, were studied by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (TOF-SIMS). Although this and other ligand−receptor systems have been studied by several techniques, including AFM, in this paper, a statistical analysis method which makes use of the properties of the Poisson distribution was applied, and the rupture strength of an individual interaction was obtained from the total adhesion forces measured by the AFM. Tip- and surface-modification chemistries were investigated by XPS and TOF-SIMS. The magnitudes of the interactions between biotin−avidin and biotin-streptavidin pairs, as determined by the Poisson method, were found to be 173 ± 19 and 326 ± 33 pN, respectively, for loading rates between 2 × 105 and 8 × 105 pN·s-1. These values are comparable to the values reported by other groups for the same systems. The statistical method used in this work has several advantages. It requires no assumptions about the surface energies or contact area between the AFM tip and the substrate, it is not limited by the force resolution of the instrument, and the number of measurements required to extract the individual unbinding force is significantly lower than that required by other methods.

Pasteur's brilliant observation, using optical microscopy, that crystals of a salt of "racemic acid" actually existed in two enantiomorphous forms ['] not only uncovered the existence of enantiomers, but indeed stimulated the development of modern structural theory. The direct (without resorting to diffraction techniques) observation of supermolecular chirality and the relationship between supermolecular chirality and molecular structure have fascinated chemists ever since. Starting with early studies of the morphology of chiral crystals by optical microscopy, the scale for the observation of chiral supermolecular assemblies has been approaching that of the molecules themselves. McConneli and Weis first recorded the formation of chiral crystalline monolayer domains in Langmuir films of enantiomerically pure lipids by epifluorescence optical microscopy.[zl More recently, chiral monolayers have been observed at molecular resolution by scanning probe microscopy. Thus, chiral symmetry breaking from achiralc31 and racemicf4I molecules has been detected in Langmuir films by atomic force microscopy (AFM), and enantiomorphous monolayer domains from achiral liquid crystal (LC) molecules on graphite have been observed by scanning tunneling microscopy (STM) .Is1 In related work, amino acids have been spontaneously resolved by crystallization in two dimensions at the air-water interface.16]We report here on the molecular resolution STM observation of enantiomorphous images of crystal monolayers grown from pure enantiomers. Furthermore, a racemic mixture of the same molecules produced images of co-existing enantiomorphic domains indistinguishable from those obtained from the enantiomerically pure materials, providing strong evidence for chiral symmetry breaking to give a two-dimensional (2-D) conglomerate. These results suggest a complete 2-D analog of Pasteur's famous system.The experiments were performed on enantiomerically enriched and racemic biphenylbenzoates 1. These have one tetra-1 hedral stereogenic center (indicated by an asterisk). Both enantiomers were prepared in approximately 99 YO ee; the racemate was also prepared."] All three compounds were liquid crystalline with a smectic A phase at room temperature.Each STM sample was prepared by placing a small amount (< 1 mg) of the liquid crystal on a freshly cleaved surface of highly oriented pyrolytic graphite. Scanning tips were prepared from mechanically cut 0.025 cm platinum/iridium (80: 20) wire. Images were recorded in air at room temperature using a commercially available STM (Nanoscope 11, Digital Instruments, Inc.) with tip positive bias.

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.