Excitation and propagation of surface waves on a viscoelastic half-space with application to medical diagnosis

An analytical solution is developed for the problem of surface wave generation on a linear viscoelastic half-space by a finite rigid circular disk located on the surface and oscillating normal to it. The solution is an incremental advancement of theoretical work reported in articles focused on seismology. Since the application of interest here is medical diagnostics, the solution is verified experimentally using a viscoelastic phantom with material properties comparable to biological soft tissue. Findings suggest that prior estimates in the literature of the shear viscosity in human soft tissue may not be accurate in the low audible frequency range. Measurement of wave motion on the skin surface caused by internal biological functions or external stimuli has been studied by a few researchers for rapid, nonintrusive diagnosis of a variety of specific medical ailments. It is hoped that the developments reported here will advance these techniques and also provide insight into related diagnostic methods, such as sonoelastic imaging and other methodologies that utilize disease-related variations in tissue shear elasticity or variations in density due to gaseous inclusions.     

A comparative study of nitrogen physisorption on different C70 crystal structures using an ab initio based potential

Quantum mechanical calculations are performed using the recently developed hybrid method for interaction energies to determine atom site Lennard-Jones potential parameters for the interactions of molecular nitrogen with C(70) molecules. This ab initio based potential is used in grand canonical Monte Carlo simulations to predict surface adsorption properties of N(2) on five known C(70) structures: rhombohedral, fcc, ideal hcp, deformed hcp, and monoclinic crystals. Because of the presence of five-membered carbon rings and the surface curvature of C(70) molecule, the Lennard-Jones potential parameters for nitrogen-carbon interactions obtained from ab initio based calculations are found to be different from that with planar graphite. The simulation results obtained from these two sets of force fields are compared and shown to differ, particularly at low coverage, where the nitrogen-carbon interactions are more important than the nitrogen-nitrogen interactions. The surface area, monolayer capacity, and isosteric heat of adsorption are calculated for various C(70) crystals and found to change appreciably as a result of the shear-induced phase transformation from hcp to rhombohedral lattice.     

Vapor–liquid equilibria and critical points of the CO2 + 1-hexanol and CO2 + 1-heptanol systems

Vapor–liquid equilibria (VLE), vapor–liquid–liquid equilibria (VLLE) and critical point (CP) data for the carbon dioxide+1-hexanol (at 324.56, 353.93, 397.78, 403.39, 431.82 and 432.45 K up to 20 MPa) and carbon dioxide+1-heptanol (at 313.14, 333.16, 373.32, 411.99 and 431.54 K up to 21 MPa) systems are reported. Phase behavior measurements were made in a new equilibrium cell based on the static-analytic method and capable of measurements up to 60 MPa and 673 K. The Peng–Robinson equation of state (EoS) with the Wong–Sandler mixing rules and temperature independent parameters was able to correlate and extrapolate the VLE for the carbon dioxide+1-hexanol system. However, in order to obtain good agreement with experimental data for the carbon dioxide+1-heptanol system, the mixture EoS parameters were adjusted to the experimental VLE data at each temperature.     

Neutrino production of same-sign dimuons at the Fermilab Tevatron

The rate of neutrino- and antineutrino-induced prompt same-sign dimuon production in steel was measured using a sample of μ^?μ^? events and 25 μ⁺μ⁺ events withP _μ>9 GeV/c, produced in 1.5 millionv _μ and 0.3 million \(\overline {v_\mu }\) induced charged-current events with energies between 30 GeV and 600 GeV. The data were obtained with the Chicago-Columbia-Fermilab-Rochester (CCFR) neutrino detector in the Fermilab Tevatron Quadrupole Triplet Neutrino Beam during experiments E 744 and E 770. After background subtraction, the prompt rate of same-sign dimuon production is (0.53±0.24)×10^?4 perv _μ charged-current event and (0.52±0.33)×10^?4 per \(\overline {v_\mu }\) charged-current event. The kinematic distributions of the same-sign dimuon events after background subtraction are consistent with those of the non-prompt background due to meson decays in the hadron shower of a charged-current event. Calculations ofc \(\bar c\) gluon bremsstrahlung, based on improved measurements of the charm mass parameter and nucleon structure functions by the CCFR collaboration, yield a prompt rate of (0.09±0.39)×10^?4 perv _μ charged-current event. In this case,c \(\bar c\) gluon bremsstrahlung is probably not an observable source of prompt same-sign dimuons.     

Separation of CO2 and N2 by adsorption in C168 schwarzite: a combination of quantum mechanics and molecular simulation study

Using a hierarchical multiscale approach combining quantum mechanics and molecular simulation, we have investigated the adsorption of pure CO(2) and N(2) and their mixture at room temperature in C(168) schwarzite, as a model for nanoporous carbons. First, the adsorbate-adsorbent interaction potential is determined using ab initio quantum mechanics computations, and then the adsorption is predicted using full atomistic Monte Carlo simulations. The extents of adsorption, adsorption energies, and isosteric heats of pure CO(2) and N(2) simulated with the ab initio potential are found to be higher than those with the empirical Steele potential that had been developed from gas adsorption on planar graphite. The inclusion of the electric quadrupole moment of adsorbate in simulation has no discernible effect on N(2) adsorption but results in a larger extent of CO(2) adsorption at high coverages. The selectivity of CO(2) over N(2) in the C(168) schwarzite from a model flue gas is predicted to be significantly larger with the ab initio potential than with the Steele potential. This illustrates the importance of an accurate adsorbate-adsorbent interaction potential in determining gas adsorption and suggests that nanoporous carbons might be useful for the separation of flue gases. As a comparison, the adsorption and selectivity of CO(2) and N(2) in ZSM-5 zeolites are also simulated with the experimentally validated potential parameters. The selectivity in the C(168) schwarzite predicted with the ab initio or Steele potential is found to be larger than the selectivity in all-silica ZSM-5, but less than that in Na-exchanged ZSM-5 zeolites.     

Solvation free energy of amino acids and side-chain analogues

The solvation free energies of amino acids and their side-chain analogues in water and cyclohexane are calculated by using Monte Carlo simulation. The molecular interactions are described by the OPLS-AA force field for the amino acids and the TIP4P model for water, and the free energies are determined by using the Bennett acceptance method. Results for the side-chain analogues in cyclohexane and in water are used to evaluate the performance of the force field for the van der Waals and the electrostatic interactions, respectively. Comparison of the calculated hydration free energies for the amino acid analogues and the full amino acids allows assessment of the additivity of the side chain contributions on the number of hydrating water molecules. The hydration free energies of neutral amino acids can be reasonably approximated by adding the contributions of their side chains to that of the hydration of glycine. However, significant nonadditivity in the free energy is found for the zwitterionic form of amino acids with polar side chains. In serine and threonine, intramolecular hydrogen bonds are formed between the polar side chains and backbone groups, leading to weaker solvation than for glycine. In contrast, such nonadditivity is not observed in tyrosine, in which the hydroxyl group is farther separated from, and therefore cannot form an intramolecular hydrogen bond with, the backbone. For histidine we find that a water molecule can form a bridge when the intramolecular hydrogen bond between the polar group and the backbone is broken.