A Generalized Soft-Sphere for Monte Carlo Simulation

A new collision model, called the generalized soft-sphere (GSS) model, is introduced. It has the same total cross section as the generalized hard-sphere model [Phys. Fluids A 5, 738 (1993)], whereas the deflection angle is calculated by the soft-sphere scattering model [Phys. Fluids A 3, 2459 (1991)]. In virtue of a two-term formula given to fit the numerical solutions of the collision integrals for the Lennard-Jones (6-12) potential and for the Stockmayer potential, the parameters involved in the GSS model are determined explicitly that may fully reproduce the transport coefficients under these potentials. Coefficients of viscosity, self-diffusion and diffusion for both polar and nonpolar molecules given by the GSS model and experiment are in excellent agreement over a wide range of temperature from low to high.

A Physically-Constrained Source Model For FDTD Acoustic Simulation

The Finite Difference Time Domain (FDTD) method is becoming increasingly popular for room acoustics simulation. Yet, the literature on grid excitation methods is relatively sparse, and source functions are traditionally implemented in a hard or additive form
using arbitrarily-shaped functions which do not necessarily obey the physical laws of sound generation. In this paper we formulate
a source function based on a small pulsating sphere model. A physically plausible method to inject a source signal into the grid
is derived from first principles, resulting in a source with a near-flat spectrum that does not scatter incoming waves. In the final
discrete-time formulation, the source signal is the result of passing a Gaussian pulse through a digital filter simulating the dynamics of the pulsating sphere, hence facilitating a physically correct means to design source functions that generate a prescribed sound field.

Theoretical Treatment of Interaction Parameters in Multicomponent Metallic Solutions

A new theoretical equation for interaction parameter in multicomponent metallic solutions is developed using the pseudopotential formalism coupled with the free energy of the hard sphere system. The approximate expression for the pseudopotential term is given in terms of the heat of solution at infinite dilution, to allow easy evaluation of the interaction parameter in various multicomponent systems. This theory has been applied to 23 non-ferrous alloys based on Pb, Sn, Bi and indium. Comparison with the results of previous theoretical calculations using only the hard sphere model suggests that the inclusion of the pseudopotential term yields a quantitatively more correct prediction of interaction parameters in multicomponent metallic solutions. Numerical calculations were also made for 320 Fe-base solutions relevant to steelmaking and the agreement between calculation and experimental data appears reasonable, with 90% reliability in predicting the correct sign.

Use of polydispersity index as control parameter to study melting/freezing of Lennard-Jones system: Comparison among predictions of bifurcation theory with Lindemann criterion, inherent structure analysis and Hansen-Verlet rule

Using polydispersity index as an additional order parameter we investigate freezing/melting transition of Lennard-Jones polydisperse systems (with Gaussian polydispersity in size), especially to gain insight into the origin of the terminal polydispersity. The average inherent structure (IS) energy and root mean square displacement (RMSD) of the solid before melting both exhibit quite similar polydispersity dependence including a discontinuity at solid-liquid transition point. Lindemann ratio, obtained from RMSD, is found to be dependent on temperature. At a given number density, there exists a value of polydispersity index (delta (P)) above which no crystalline solid is stable. This transition value of polydispersity(termed as transition polydispersity, delta (P) ) is found to depend strongly on temperature, a feature missed in hard sphere model systems. Additionally, for a particular temperature when number density is increased, delta (P) shifts to higher values. This temperature and number density dependent value of delta (P) saturates surprisingly to a value which is found to be nearly the same for all temperatures, known as terminal polydispersity (delta (TP)). This value (delta (TP) similar to 0.11) is in excellent agreement with the experimental value of 0.12, but differs from hard sphere transition where this limiting value is only 0.048. Terminal polydispersity (delta (TP)) thus has a quasiuniversal character. Interestingly, the bifurcation diagram obtained from non-linear integral equation theories of freezing seems to provide an explanation of the existence of unique terminal polydispersity in polydisperse systems. Global bond orientational order parameter is calculated to obtain further insights into mechanism for melting.

Thickness And Component Distributions Of Yttrium-Titanium Alloy Films In Electron-Beam Physical Vapor Deposition

Thickness and component distributions of large-area thin films are an issue of international concern in the field of material processing. The present work employs experiments and direct simulation Monte Carlo (DSMC) method to investigate three-dimensional low-density, non-equilibrium jets of yttrium and titanium vapor atoms in an electron-beams physical vapor deposition (EBPVD) system furnished with two or three electron-beams, and obtains their deposition thickness and component distributions onto 4-inch and 6-inch mono-crystal silicon wafers. The DSMC results are found in excellent agreement with our measurements, such as evaporation rates of yttrium and titanium measured in-situ by quartz crystal resonators, deposited film thickness distribution measured by Rutherford backscattering spectrometer (RBS) and surface profilometer and deposited film molar ratio distribution measured by RBS and inductively coupled plasma atomic emission spectrometer (ICP-AES). This can be taken as an indication that a combination of DSMC method with elaborate measurements may be satisfactory for predicting and designing accurately the transport process of EBPVD at the atomic level.

等离子体效应对类氦氖Kα线系电偶极辐射的影响

采用离子球模型，通过自洽求解Boltzmann方程和Poisson方程，得到类氦氖离子Kα线系的两条电偶极辐射光谱能量随等离子体环境的漂移．结果显示，Kα线系电偶极谱线随等离子体电子密度增大发生红移，红移量与等离子体电子密度有近似的正比关系；随着等离子体电子温度的降低，光谱红移对等离子体电子密度的敏感性增大。另外，所研究的两条谱线间的能量间隔随等离子体电子密度的增大而减小，减小量随等离子体电子密度的变化也呈现出近似的线性规律。值得注意的是，类氦氖Kα线系中两条电偶极谱线分别为互组合线与共振谱线，而其能量差

The Flocculation of E. coli with Polyethyleneimine

A comprehensive study was made of the flocculation of dispersed E. coli bacterial cells by the cationic polymer polyethyleneimine (PEI). The three objectives of this study were to determine the primary mechanism involved in the flocculation of a colloid with an oppositely charged polymer, to determine quantitative correlations between four commonly-used measurements of the extent of flocculation, and to record the effect of varying selected system parameters on the degree of flocculation. The quantitative relationships derived for the four measurements of the extent of flocculation should be of direct assistance to the sanitary engineer in evaluating the effectiveness of specific coagulation processes.

A review of prior statistical mechanical treatments of absorbed polymer configuration revealed that at low degrees of surface site coverage, an oppositely- charged polymer molecule is strongly adsorbed to the colloidal surface, with only short loops or end sequences extending into the solution phase. Even for high molecular weight PEI species, these extensions from the surface are theorized to be less than 50 Å in length. Although the radii of gyration of the five PEI species investigated were found to be large enough to form interparticle bridges, the low surface site coverage at optimum flocculation doses indicates that the predominant mechanism of flocculation is adsorption coagulation.

The effectiveness of the high-molecular weight PEI species 1n producing rapid flocculation at small doses is attributed to the formation of a charge mosaic on the oppositely-charged E. coli surfaces. The large adsorbed PEI molecules not only neutralize the surface charge at the adsorption sites, but also cause charge reversal with excess cationic segments. The alignment of these positive surface patches with negative patches on approaching cells results in strong electrostatic attraction in addition to a reduction of the double-layer interaction energies. The comparative ineffectiveness of low-molecular weight PEI species in producing E. coli flocculation is caused by the size of the individual molecules, which is insufficient to both neutralize and reverse the negative E.coli surface charge. Consequently, coagulation produced by low molecular weight species is attributed solely to the reduction of double-layer interaction energies via adsorption.

Electrophoretic mobility experiments supported the above conclusions, since only the high-molecular weight species were able to reverse the mobility of the E. coli cells. In addition, electron microscope examination of the seam of agglutination between E. coli cells flocculation by PEI revealed tightly- bound cells, with intercellular separation distances of less than 100-200 Å in most instances. This intercellular separation is partially due to cell shrinkage in preparation of the electron micrographs.

The extent of flocculation was measured as a function of PEl molecular weight, PEl dose, and the intensity of reactor chamber mixing. Neither the intensity of mixing, within the common treatment practice limits, nor the time of mixing for up to four hours appeared to play any significant role in either the size or number of E.coli aggregates formed. The extent of flocculation was highly molecular weight dependent: the high-molecular-weight PEl species produce the larger aggregates, the greater turbidity reductions, and the higher filtration flow rates. The PEl dose required for optimum flocculation decreased as the species molecular weight increased. At large doses of high-molecular-weight species, redispersion of the macroflocs occurred, caused by excess adsorption of cationic molecules. The excess adsorption reversed the surface charge on the E.coli cells, as recorded by electrophoretic mobility measurements.

Successful quantitative comparisons were made between changes in suspension turbidity with flocculation and corresponding changes in aggregate size distribution. E. coli aggregates were treated as coalesced spheres, with Mie scattering coefficients determined for spheres in the anomalous diffraction regime. Good quantitative comparisons were also found to exist between the reduction in refiltration time and the reduction of the total colloid surface area caused by flocculation. As with turbidity measurements, a coalesced sphere model was used since the equivalent spherical volume is the only information available from the Coulter particle counter. However, the coalesced sphere model was not applicable to electrophoretic mobility measurements. The aggregates produced at each PEl dose moved at approximately the same vlocity, almost independently of particle size.

PEl was found to be an effective flocculant of E. coli cells at weight ratios of 1 mg PEl: 100 mg E. coli. While PEl itself is toxic to E.coli at these levels, similar cationic polymers could be effectively applied to water and wastewater treatment facilities to enhance sedimentation and filtration characteristics.

Ground-state energy shifts of H-like Ti under dense and hot plasma conditions

In this study, by adopting the ion sphere model, the self-consistent. field method is used with the Poisson-Boltzmann equation and the Dirac equation to calculate the ground-state energies of H-like Ti at a plasma electron density from 10(22) cm(-3) to 10(24) cm(-3) and the electron temperature from 100 eV to 3600 eV. The ground-state energy shifts of H-like Ti show different trends with the electron density and the electron temperature. It is shown that the energy shifts increase with the increase in the electron density and decrease with the increase in the electron temperature. The energy shifts are sensitive to the electron density, but only sensitive to the low electron temperature. In addition, an accurately fitting formula is obtained to fast estimate the ground-state energies of H-like Ti. Such fitted formula can also be used to estimate the critical electron density of pressure ionization for the ground state of H-like Ti.

Nested sampling for materials: the case of hard spheres

The recently introduced nested sampling algorithm allows the direct and efficient calculation of the partition function of atomistic systems. We demonstrate its applicability to condensed phase systems with periodic boundary conditions by studying the three dimensional hard sphere model. Having obtained the partition function, we show how easy it is to calculate the compressibility and the free energy as functions of the packing fraction and local order, verifying that the transition to crystallinity has a very small barrier, and that the entropic contribution of jammed states to the free energy is negligible for packing fractions above the phase transition. We quantify the previously proposed schematic phase diagram and estimate the extent of the region of jammed states. We find that within our samples, the maximally random jammed configuration is surprisingly disordered.

Pressure effect on vibrational frequency and dephasing of 1-alkyl-3-methylimidazolium hexafluorophosphate ionic liquids

Raman spectra in the range of the totally symmetric stretching mode of the [PF6]− anion, νs(PF6), have been measured for 1-alkyl-3-methylimidazolium ionic liquids [CnC1im][PF6], for n = 4, 6, and 8, as a function of pressure at room temperature. The ionic liquids [C6C1im][PF6] and [C8C1im][PF6] remain in an amorphous phase up to 3.5 GPa, in contrast to [C4C1im][PF6], whichcrystallizes above ∼0.5 GPa. Equations of state based either on a group contribution model or Carnahan-Starling-van der Waals model have been used to estimate the densities of the ionic liquids at high pressures. The shifts of the vibrational frequency of νs(PF6) with density observed in [C6C1im][PF6] and in [C8C1im][PF6] have been calculated by a hard-sphere model of a pseudo-diatomic solute under short-range repulsive interactions with the neighboring particles. The stochastic model of Kubo for vibrational dephasing has been used to obtain the amplitude of vibrational frequency fluctuation, ⟨Δω 2⟩, and the relaxation time of frequency fluctuation, τ c , as a function of density by Raman band shape analysis of the νs(PF6) mode of [C6C1im][PF6] and [C8C1im][PF6].

Radar scattering by aggregate snowflakes

The radar scattering properties of realistic aggregate snowflakes have been calculated using the Rayleigh-Gans theory. We find that the effect of the snowflake geometry on the scattering may be described in terms of a single universal function, which depends only on the overall shape of the aggregate and not the geometry or size of the pristine ice crystals which compose the flake. This function is well approximated by a simple analytic expression at small sizes; for larger snowflakes we fit a curve to Our numerical data. We then demonstrate how this allows a characteristic snowflake radius to be derived from dual wavelength radar measurements without knowledge of the pristine crystal size or habit, while at the same time showing that this detail is crucial to using such data to estimate ice water content. We also show that the 'effective radius'. characterizing the ratio of particle volume to projected area, cannot be inferred from dual wavelength radar data for aggregates. Finally, we consider the errors involved in approximating snowflakes by 'air-ice spheres', and show that for small enough aggregates the predicted dual wavelength ratio typically agrees to within a few percent, provided some care is taken in choosing the radius of the sphere and the dielectric constant of the air-ice mixture; at larger sizes the radar becomes more sensitive to particle shape, and the errors associated with the sphere model are found to increase accordingly.

Size, electrophoretic mobility, and ion dissociation of vesicles prepared with synthetic amphiphiles

Vesicles prepared with synthetic amphiphiles (dioctadecyldimethylammonium bromide and chloride, dihexadecyl phosphate and its sodium salt) were obtained by sonication, ethanol injections, and chloroform injections. The hydrodynamic diameter of vesicles (Dh), estimated from the diffusivity measured by quasielastic light scattering, ranged from 230 to 3000 Å. The electrophoretic mobility (Um) was measured by free-flow electrophoresis. The zeta potential (ζ) and the degree of counterion dissociation (α) of the vesicles were calculated from Um and conductivity data, α decreased with increasing Dh of the vesicles, probably due to the decreasing headgroup area and the increasing counterion association needed to relax the surface electrostatic potential. The electrophoretic mobility was also calculated (Uc) according to an impenetrable, nonconducting sphere model with a spherically symmetric charge distribution approximation. Within the limits of the experimental error(s) of the (different) methods employed and the assumptions made in the calculations, the fact that the Um/Uc ratio ranged from 1.3 to 7.5 was considered to be a good agreement between the calculated and the experimental values. © 1990 American Chemical Society.

Konfokalmikroskopie an stabilen und metastabilen kolloidalen Hartkugel-Fluiden : interne Struktur, Kristallnukleation und Kristallwachstum

Die vorliegende Forschungsarbeit behandelt konfokalmikroskopische Untersuchungen zur Strukturbildung in kolloidalen Suspensionen, die als experimentelles Modellsystem für harte Kugeln verwendet werden. Die lokale und globale Struktur wurde im stabilen und metastabilen Fluid bestimmt. Bereits unterhalb des Gefriervolumenbruchs wurden nah-geordnete Cluster vorgefunden, die mit zunehmendem Volumenbruch häufiger und größer werden. Das Kristallwachstum aus der metastabilen kolloidalen Schmelze konnte quantitativ analysiert werden und es zeigt sich eine Übereinstimmung mehrerer Kenngrößen mit Literaturdaten nahe dem Schmelzvolumenbruch. Die Untersuchung demonstrierte die Realisierbarkeit der quantitativen Analyse der Kristallisationskinetik mittels konfokaler Mikroskopie. Es zeigte sich ein mehrstufiges Nukleationsszenario bei dem zuerst nah-geordnete Cluster gebildet werden, die im weiteren Verlauf zu kristall-artigen Clustern transformieren. Die Beobachtungen belegen den Mechanismus der Precursornukleation in Hartkugelsystemen. Die heterogene Nukleation wurde an glatten und an hexagonal strukturierten Substraten untersucht. Anhand der Kristallisationskinetik und der direkten Messung der Benetzungswinkel konnte ein Übergang des Benetzungsverhaltens unter Variation des Substratgitterabstands nachgewiesen werden: An glatten und an kommensurabel strukturierten Substraten zeigten sich eine vollständige Bedeckung des Substrats mit der kristallinen Phase und ein sofortiges unidirektionales Wachstum. Bei inkommensurabel strukturierten Substraten wurde eine unvollständige Bedeckung des strukturierten Substrats mit der kristallinen Phase sowie ein verzögertes isotropes Wachstum bei fortwährender Kristallnukleation beobachtet.