Condensation And Mobility Studies Of Fluid Interfaces

Condensation is of central importance in a broad range of areas in nature and industry. Aerosol-cloud interactions, a currently a significant open question in climate modeling, and water harvesting mechanisms on organisms such as cacti, beetles, and spiders, are natural processes that are rely on condensation. Condensation is an effective method for transferring heat due to the latent heat required for a fluid to change phase from a gas to a liquid. Improvements in condensation processes would have an impact in a variety of industrial areas such as thermal management, environmental control, microelectronics, desalination, and power generation. Dropwise condensation is preferable over filmwise condensation because it has a significantly higher heat transfer coefficient. Nanopatterned surfaces are of interest because they have experimentally demonstrated higher heat transfer than their smooth counterparts, but recent heat transfer measurements on individual droplets have revealed discrepancies between theoretical predictions and experimental measurements for the smallest droplets. Interfacial properties on small length scales are often difficult to measure experimentally and are often used as fitting parameters in condensation models. The common assumptions used when modeling dropwise condensation are that (1) the condensing droplets are thermodynamically quasi-static and that (2) the heat and mass transport are uncoupled, that is, droplet motion and heat transfer are modeled independently of one another. In this dissertation, several continuum properties including the mass accommodation coefficient and interfacial mobility are computed allowing for the physical parameters to be known a priori for continuum scale models such as the Navier-Stokes-Cahn-Hilliard equations or interfacial resistances in condensation models. Furthermore, the two fundamental assumptions used in condensation models are examined in an attempt to resolve the theoretical and experimental discrepancies. This will be done by leveraging microscopic and nonequilibrium thermodynamic approaches to determine the validity of the condensation assumptions for planar and highly curved systems

Determination of Noncovalent Intermolecular Interaction Energy from Electron Densities

Noncovalent intermolecular interactions, widely found in molecular clusters and bio-molecules, play a key role in many important processes, such as phase changes, folding of proteins and molecular recognition. However, accurate calculation of interaction energies is a very difficult task because the interactions are normally very weak. Rigorous expressions for the electrostatic and polarization interaction energies between two molecules A and B, in term of the electronic densities, have been programmed: (see formula in document). Z is atomic charge, ρ0 is the electron density of the isolated molecule and Δρind is the electron density change of the molecule caused by polarization. With some approximations, procedures for electrostatic and polarization energy calculations were developed that involve numerical integration. Electrostatic and polarization energies for several bimolecular systems, some of which are hydrogen bonded, were calculated and the results were compared to other theoretical and experimental data. A second method for the computing of intermolecular interaction energies has also been developed. It involves a “supermolecule” calculation for the entire system, followed by a partitioning of the overall electric density into the two interacting components and then application of eq. (1) to find the interaction energy. In this approach, according to Feynman’s explanation to intermolecular interactions, all contributions are treated in a unified manner. The advantages of this method are that it avoids treating the supersystem and subsystems separately and no basis set superposition error (BSSE) correction is needed. Interaction energies for several hydrogen-bonded systems are calculated by this method. Compared with the result from experiment and high level ab initio calculation, the results are quite reliable

Representations of molecular force fields. I. Ethane: Ab initio and model, harmonic and anharmonic

The quadratic and selected cubic force constants for ethane have been computed, using single determinant molecular orbital wavefunctions at the 4‐31G level, with a view to testing and extending model consistent force fields (CFF) for ’’molecular mechanics’’ calculations. Results agree semiquantitatively with experiment, but experimental force constants of sufficient reliability to provide a definitive comparison are not yet available. In a comparison with the most rational general CFF available, that of Ermer and Lifson, the most significant discrepancies found to occur are those for certain stretch–bend couplings assumed to be zero in the CFF but shown to be appreciable by quantum calculation. It is observed that these couplings, but not the stretch–stretch couplings, are well accounted for by a steric interaction model. The ab initio cubic constants examined display the same pattern of conformity with a steric model. Bend–bend–bend and bend–bend–stretch but not all stretch–stretch–stretch interactions agree with those of the steric model. The partial success of the steric model shows that it is possible to represent a large number of interaction constants, quadratic and higher order, by a small number of parameters in molecular mechanics. The failure of the steric model to account for predominantly stretching interactions confirms that ’’classical’’ nonbonded interactions as embodied in conventional Urey–Bradley fields are not the only major contributors to off‐diagonal force constants. An alternative model, the anharmonic model of Warshel, as modified by Kirtman et al., was found to account well for pure stretches but not for bends or stretch–bend interactions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70643/2/JCPSA6-63-11-4750-1.pd