Volume 110, Issue 3, 15 January 1999
Index of content:
- SURFACES, INTERFACES, AND MATERIALS
Instability and dewetting of evaporating thin water films on partially and completely wettable substrates110(1999); http://dx.doi.org/10.1063/1.477810View Description Hide Description
Stability, dynamics and dewetting of thin (<100 nm) evaporating water films on partially and completely wettable substrates are studied based on numerical solutions of the nonlinear thin film equation, as well as by simplified semianalytical approaches. The instability and rupture of aqueous films are engendered by the hydrophobic attraction, whereas the net van der Waals force is repulsive for aqueous films on most substrates. An evaporating aqueous film on a partially wettable surface thins uniformly to a critical thickness, and then spontaneously dewets the substrate by the formation of growing holes. Complete nonlinear simulations as well as the linear analyses are used to predict the most important, experimentally accessible characteristics of the instability such as the length scale and time scale of the instability and the mean film thickness at the instant of rupture. Curiously, in contrast to nonthinning films, the number density of holes decreases slightly with increased strength and range of hydrophobic attraction, and also with decreased strength of LW repulsion, even though both of these factors promote the macroscopic nonwettability. The rate of evaporation has the most significant influence on the length scale, where the exponent, q lies in a narrow range from −0.17 to about −0.26, depending on the rate of evaporation and the critical thickness. Thin aqueous films on completely wettable (free energy per unit area is positive) surfaces are also unstable when the free energy does not decrease monotonically with the film thickness. Simulations show that instability in such cases leads to the formation of quasiequilibrium microscopic “islands” or “pancakes” made up of largely flat thin and thick films.
Interaction of cyclobutane with the Ru(001) surface: Low-temperature molecular adsorption and dissociative chemisorption at elevated surface temperature110(1999); http://dx.doi.org/10.1063/1.477811View Description Hide Description
We have studied the interaction of cyclobutane with the hexagonally close-packed Ru(001) surface. High-resolution electron energy loss spectroscopy (HREELS) has been used to identify the vibrational modes of both and adsorbed at 90 K as a function of cyclobutane exposure. We have observed a vibrational mode not observed in the gas phase at 2600 cm−1 (2140 cm−1) which is attributed to the strong interaction of the cyclobutane C–H (C–D) bonds with the rutheniumsurface. Two different adsorption geometries for cyclobutane on Ru(001) have been proposed based on the dipolar activity of this softened C–H mode. We have also measured the trapping-mediated dissociative chemisorption of both and at surface temperatures between 190 and 1200 K. The measured activation energies with respect to the bottom of the physically adsorbed well for and are 10 090±180 and 10 180±190 cal/mol, respectively. The trapping-mediated chemisorption of cyclobutane is believed to occur via C–C bond cleavage, as judged by the absence of a kinetic isotope effect. The measured ratios of the preexponential factors for desorption relative to reaction of 21±2 and 47±4 for and respectively, are in the expected range considering the greater entropy gain associated with the transition state for desorption relative to the transition state for C–C bond cleavage.
110(1999); http://dx.doi.org/10.1063/1.477812View Description Hide Description
We describe a new, biased Monte Carlo scheme to determine the crystal structures of zeolites from powder diffraction data. We test the method on all publicly known zeolite materials, with success in all cases. We show that the method of parallel tempering is a powerful supplement to the biased Monte Carlo.
Correlation and dimerization effects on the physical behavior of the charge transfer salts: A density matrix renormalization group study of the quarter-filling t–J model110(1999); http://dx.doi.org/10.1063/1.477884View Description Hide Description
The present work studies the quasi-one-dimensional -based compounds within a correlated model. More specifically, we focus our attention on the composed influence of the electronic dimerization-factor and the repulsion, on the transport properties and the localization of the electronic density in the ground-state. Those properties are studied through the computation of the charge gaps (difference between the ionization potential and the electro-affinity, IP-EA) and the long- and short-bond orders of an infinite quarter-filled chain within a model. The comparison between the computed gaps and the experimental activation energy of the semiconductor allows us to estimate the on-site electronic repulsion of the molecule to 1.16 eV.