We present results of a molecular-dynamics study on liquid cesium, just above the melting point; i.e., we have performed a computer experiment on exactly the same system that was studied recently in a very accurate neutron-scattering experiment by Bodensteiner et al. In this simulation the interaction of the cesium atoms is based on an Ashcroft empty-core pseudopotential; the total simulation length extends over 100 000 time steps, i.e., 8×${10}^{\mathrm{-}10}$ s real time. Both static and dynamic structure factors are in very good agreement with experimental results: as in the experiment, we also get—within very good accuracy—a positive dispersion relation. However, concerning propagating sound modes beyond the main peak in S(q) (q≥${\mathit{q}}_{\mathit{p}}$∼1.4 Å) we cannot find sufficient evidence for their existence. Analysis of the transverse current correlation function shows that the corresponding decay mechanism is built up by two relaxation processes (a fast binary one and a slow collective one); starting from ∼1 A${\mathrm{̊}}^{\mathrm{-}1}$ the latter one is completely extinguished. The correlation functions are fitted to hydrodynamic and memory-function models that contain generalized thermodynamic and elastic quantities as parameters. In several cases—if statistics allows—the true physical values can be recovered; then agreement with experimental values is quite satisfactory (differences of 10–15 % are observed).

Diffusion of tagged particles in a dilute gas in a stationary state with temperature and velocity gradients (planar Couette flow) is analyzed. The results are obtained from a kinetic model for a multicomponent gas constructed in the same spirit as in the Bhatnagar-Gross-Krook equation for a single gas. The kinetic equation for the velocity distribution function of the tagged particles is solved by using a Chapman-Enskog-like expansion around the steady-Couette-flow state. Up to first order in the concentration gradient of tagged particles, explicit expressions for the self-diffusion coefficient and the partial pressure have been derived. Both quantities are nonlinear functions of the shear rate. In addition, the distribution function of tagged particles is explicitly obtained. It exhibits a highly nonlinear dependence on both the shear rate and the thermal gradient. In the absence of shear rate, self-diffusion in pure heat flow is also considered with more detail.

The effect of spatial coherence of light scattering by particles in a hydrodynamic velocity field has been studied both theoretically and experimentally. Integration of the scattered intensity over all scattering vectors that contribute to the active detector area gives a correlation function with a functional form strongly dependent on the detector size. These results are of central importance to understand the limitations of homodyne light scattering from particles in a convective velocity field.

Compressible turbulent shear flows are analyzed using a two-scale direct-interaction (propagator-renormalization) approximation with the density, velocity, and internal energy as the primitive variables. This method elucidates the effects of fluid compressibility on various important correlation functions that appear in the equations for the mean field. Specifically, density fluctuations are confirmed to be closely linked with compressibility effects. These results show that the current one-point turbulence model based on mass-weighted averaging, which is widely used in aeronautical studies, obscures many important features of fluid compressibility. On this basis, modeling based on the combined use of ensemble averaging and primitive variables is recommended as the one-point turbulence modeling applicable to the study of aerospace and astrophysical phenomena.

The standard paradigm for analysis of rock deformation arises from postulating the existence of ‘‘an equivalent homogeneous porous rock.’’ However, data on the pore-pressure dependence of fluid permeability for some rocks cannot be explained using any equivalent homogeneous porous medium. In contrast, a positive result shows that deformation measurements on both high-porosity sandstones and low-porosity granites can be explained adequately in terms of an equivalent two-constituent model of porous rocks, for which exact results have recently been discovered.

An equilibrium-molecular-dynamics study of the diffusion of atomic fluids in random pore systems is reported. The pore space is generated by the use of a simple percolation technique on a tessellation of three-dimensional space with periodic boundary conditions. Simulations are performed on the percolating cluster only. The substrate atoms are explicitly represented and the substrate-fluid and fluid-fluid interactions are modeled using Lennard-Jones potentials. Using this technique, results are presented for an argon-in-graphite system for different porosities at 140 K and a fluid density of 3.5 k ${\mathrm{gm}}^{\mathrm{-}3}$. The results, which include mean-square displacement, velocity-autocorrelation functions and their associated memory kernels, and the total-force autocorrelation functions, clearly indicate that mass diffusion in completely random pore systems is different from that in the bulk phase. Trends shown in the results here are similar to those reported in Monte Carlo, random-walk, and theoretical works elsewhere. Restriction of the technique to the study of atomic fluids and random pore systems is made here for simplicity; it is presently being extended to molecular fluids and mixtures in real micropore spaces. The substrate is assumed to be rigid; however, it is felt that the technique can be extended to include substrate-fluid energy exchange.

The dynamics near the symmetry-breaking transition in the sponge phase ($L_3$) of self-assembling surfactant solutions is considered. The surfactant motion is taken to be diffusive (conserved), while the order parameter for the transition ($\eta$) is assumed to follow two channels of relaxation: diffusion (conserved) and leakage (nonconserved). Our dynamical treatment is based on mean-field theory within a time-dependent Landau-Ginzburg approach, whose static limit reproduces an earlier successful theory of the static structure factor. We consider two main regimes: in the first, $\eta$ relaxes rapidly compared to the surfactant diffusion, while in the second, the opposite limit applies. We find that in the fast-$\eta$ regime the surfactant dynamical structure factor $S(k,t)\mathrm{}$ is exponential in time, but the relaxation rate shows an unusual logarithmic behavior. On the other hand, in the slow-$\eta$ regime, $S(k,t)\mathrm{}$ is very nonexponential in time (although the average relaxation rates show the conventional critical slowing-down effects). We argue that a crossover from the fast-$\eta$ to the slow-$\eta$ case occurs as the $k$ vector is increased. Implications for dynamic light scattering from sponge systems are discussed.

The diffusion of dye-labeled linear polystyrenes within porous media composed of fumed silica particles has been studied by using forced-Rayleigh-scattering (FRS) spectroscopy. Significant differences have been observed between results obtained by using silica R972 (made by the Degussa Corp.), which is known to adsorb labeled polystyrene chains, and silica R972-M, in which the surface hydroxyl concentration has been reduced to minimize surface adsorption. In the porous medium R972-M, the FRS signal was ‘‘normal,’’ and the decay rate 1/τ was proportional to the square of the scattering wave vector, an observation indicating diffusion by Brownian motion. In contrast, in the porous medium R972, the FRS signal was ‘‘abnormal’’ and there was an obvious curvature in the plot of 1/τ vs ${\mathit{q}}^2$. We studied the effect of porosity on the hindrance to diffusion in both types of porous fumed silica; the hindrance is characterized by the ratio of the diffusion coefficient inside the pores to that in the free solution, D/${\mathit{D}}_0$. For polymer diffusion inside R972-M, the hindrance can for the most part be attributed to geometric obstruction and hydrodynamic interaction with the silica surface; the experimental data are compared with predictions of the theories of Neale and Nader [Am. Inst. Chem. Eng. J. 19, 112 (1973)] and of Prager [Physica 29, 129 (1963)]. In the porous R972, adsorption dominated the polymer diffusion behavior, and the measured value of D/${\mathit{D}}_0$, which was also a weak function of the labeling ratio, fell within the range 0.2–0.4, an observation indicating very strong hindrance. The changing diffusion rate during gelation was also monitored, and a difference in the times required for stabilization of the diffusion coefficient was found in the two silica systems. This difference is related to the mechanisms that govern diffusion behavior in each system.

The liquid-glass transition dynamics of the molecular glass Salol has been studied by light-scattering spectroscopy from 383 to 198 K. Analysis of wide-frequency-range composite spectra revealed the relaxation dynamics from 0.2 GHz to 3.5 THz. A two-step relaxation process was observed near the crossover temperature ${\mathit{T}}_{\mathit{c}}$∼256 K which is about 40 K above the glass-transition temperature ${\mathit{T}}_{\mathit{g}}$. The β-relaxation process is well described by the scaling predictions of the mode-coupling theory in both the liquid and glass states. The α relaxation obeys the time-temperature superposition principle for T>${\mathit{T}}_{\mathit{c}}$. The light-scattering results, when combined with previous dielectric measurements, indicate a continuous evolution of the relaxation time and stretching of the α process from the liquid to the glass state. Qualitative and quantitative comparisons of the experimental results with the predictions of the mode-coupling theory and the configuration-space percolation theory are presented.

Extensive two- and three-dimensional computer simulations have been carried out to investigate the effect of a destabilizing external field (such as gravity) on invasion percolation processes. The displacement patterns found under these conditions are dominated by the growth of a single branch. This branch can be described in terms of a connected string of blobs of size ${\xi }_{\mathit{w}}$, which form a directed random walk along the direction of the field. On length scales smaller than ${\xi }_{\mathit{w}}$, the displacement figures have the structure of invasion percolation clusters without a destabilizing field. The dependence of the correlation length ${\xi }_{\mathit{w}}$ on the magnitude of the field gradient g is given by ${\xi }_{\mathit{w}}$∼‖g${\mathrm{\Vert }}^{\mathrm{-}\nu /(\nu +1)}$ (where ν is the ordinary percolation correlation length exponent) in accord with the theoretical arguments of Wilkinson [Phys. Rev. A 30, 520 (1984); 34, 1380 (1986)]. For continuous threshold distributions the exponent relating ${\xi }_{\mathit{w}}$ to g does not depend on the shape of the distribution. For discontinuous distributions the behavior is quite different.

We study the behavior of the contact angle of a middle-phase pendant drop at an oil-water interface in an amphiphilic system. A Landau theory with a one-component order parameter is employed. We find that as a weak amphiphilic system departs from its balanced state, the contact angle decreases monotonically to zero at a wetting transition as a critical end point is approached. In a stronger system, the angle initially increases with this departure before ultimately falling to zero. For a very strong system, the angle can increase to 180° before falling to zero. In such a case, three wetting transitions would be encountered.

The thermodynamic properties of the Yukawa model for colloidal suspensions are determined theoretically from the Rogers-Young integral equation for the fluid phase and from a recently introduced van der Waals–like theory for the solid phases. Very good agreement with the Monte Carlo simulations of Meijer and Frenkel [J. Chem. Phys. 94, 2269 (1991)] is found for both the fluid and the (fcc-bcc) solid phases. The location of the two-phase coexistences, however, is shown to involve such small free-energy and density changes that no definite statements about the phase diagram are possible within the present accuracy.

Experimental studies of the annihilation of positrons in xenon show an anomalously large decay rate coupled with unusual nonlinear dependence on the density. In this work we use the path-integral Monte Carlo technique to study the equilibrium properties of a positron that has thermalized in xenon fluid. The decay rate and the correlation functions that describe the equilibrium structure of the interacting positron-xenon system are calculated on the isotherm T=340 K for the range of xenon densities studied experimentally. In contrast to density-functional theory, the most successful theory to date, path-integral Monte Carlo results give an accurate description of the positron-fluid system over a wide range of thermodynamic variables. In direct agreement with experiment, the calculations show that the decay rate increases nonlinearly with xenon density and that the degree of localization of the positron is a continuous function of the fluid density. The anomalous behavior of the decay rate with fluid density, strongly nonlinear at low density and then saturating at higher densities, is demonstrated to be a manifestation of the clustering of xenon atoms around the positron at low to moderate densities. The calculations clarify the connection between the localization of a positron and its annihilation in a highly polarizable fluid.

A simple lattice model has been used to investigate (2+1)-dimensional ballistic deposition for angles of incidence θ in the range 0°≤θ<90°. In the oblique-incidence limit (θ→90°) a pattern of overlapping scales inclined at an angle of about 37.5° is formed. The individual scales can be characterized in terms of their characteristic length, width, and thickness which grow algebraically with increasing scale size. All of the scaling exponents characterizing the growing surface can be obtained from exponents ${\mathrm{\sigma }}_{\mathit{x}}$=1/3 and ${\mathrm{\sigma }}_{\mathit{y}}$=2/3 that describe the growth of the correlation lengths ${\xi }_{\mathit{x}}$ and ${\xi }_{\mathit{y}}$ parallel and perpendicular to the plane of incidence. The scaling properties of the columnar patterns can be understood in terms of the (1+1)-dimensional dynamics of the leading edges of the advancing scales. An anisotropic generalization of the finite-size scaling form of Family and Vicsek [J. Phys. A 18, L75 (1985)] is proposed and its validity is demonstrated for ballistic deposition at near-grazing incidence.

A mean-field and continuous approximation of an ‘‘exact’’ time-developing equation based on Witten and Sander’s diffusion-limited-aggregation model [Phys. Rev. Lett. 47, 1400 (1981)] is investigated by means of a numerical integration. From the power-law decay of the density, we estimated the fractal dimension, D=2.50±0.05 (d=3), D=3.44±0.05 (d=4), and so forth, which is in good agreement with a previous result obtained from the usual direct simulation. The universality is partially confirmed by showing that D is rather insensitive to the detailed structure of the basic equation.

The study of Kα line absorption by point-projection backlighting is presented for a laser-produced aluminum plasma at electronic densities below 5×${10}^{21}$ ${\mathrm{cm}}^{\mathrm{-}3}$. The appearance of ionic species as a function of distance from the target is measured absolutely by the use of a knife edge as a spatial fiducial. These data are used to determine the gradient of the average charge as a function of space. A non-local-thermodynamic-equilibrium ionization model has been developed to determine temperature and densities consistent with this measure. Comparisons with hydrodynamic codes are presented, along with a discussion of the inferred temperature.

It is shown that even a nonmagnetized plasma with temperature T sustains zero-frequency magnetic fluctuations in thermal equilibrium. Fluctuations in electric and magnetic fields, as well as in densities, are computed. Four cases are studied: a cold, gaseous, isotropic, nonmagnetized plasma; a cold, gaseous plasma in a uniform magnetic field; a warm, gaseous plasma described by kinetic theory; and a degenerate electron plasma. For the simple gaseous plasma, the fluctuation strength of the magnetic field as a function of frequency and wave number is calculated with the aid of the fluctuation-dissipation theorem. This calculation is done for both collisional and collisionless plasmas. The magnetic-field fluctuation spectrum of each plasma has a large zero-frequency peak. The peak is a Dirac δ function in the collisionless plasma; it is broadened into a Lorentzian curve in the collisional plasma. The plasma causes a low-frequency cutoff in the typical blackbody radiation spectrum, and the energy under the discovered peak approximates the energy lost in this cutoff. When the imposed magnetic field is weak, the magnetic-field wave-vector fluctuation spectra of the two lowest modes are independent of the strength of the imposed field. Further, these modes contain finite energy even when the imposed field is zero. It is the energy of these modes that forms the zero-frequency peak of the nonmagnetized plasma. In deriving these results, a simple relationship between the dispersion relation and the fluctuation power spectrum of electromagnetic waves is found. The warm plasma is shown, by kinetic theory, to exhibit a zero-frequency peak in its magnetic-field fluctuation spectrum as well. For the degenerate plasma, we find that electric-field fluctuations and number-density fluctuations vanish at zero frequency; however, the magnetic-field power spectrum diverges at zero frequency.

The collisionless boundary sheath of a plasma containing additionally negative ions and secondary electrons is investigated theoretically. Bohm’s criterion is generalized for such plasmas. The following effects are considered. (i) For negative-ion densities less than a critical value, the negative ions are captured by the positive ions through Coulomb collisions in the bulk plasma and in the presheath. As a consequence, the negative ions move together with the positive ions to the cathode. The critical density is determined by the collision frequencies of the negative ions with the positive ions and with the neutral atoms. (ii) Negative ions penetrating into the collisionless boundary sheath together with the positive ions are reflected by the electric field. As a consequence a double layer appears with a maximum of the electric field within the sheath. (iii) Secondary electrons emitted by the electrode reduce the field in the sheath and can form a virtual cathode. The theoretical results are in good agreement with experimental measurements.

We analyze the transverse dielectric response function and the collective excitations in a strongly coupled one-component plasma (OCP). The collective modes are either photonlike or phononlike. The latter, which are maintained by shear, are unique to the strongly coupled system. We employ the recently established quasilocalized-charge model to develop a theory of the shear mode in the strongly coupled OCP. We examine in detail the dispersion of the shear mode in the 20<Γ<225 domain, including the supercooled liquid state (Γ=${\mathit{Z}}^2$${\mathit{e}}^2$/${\mathit{ak}}_{\mathit{B}}$T, a is the Wigner-Seitz radius), and compare it with the result of molecular-dynamics simulations of Hansen, McDonald, and Pollock [Phys. Rev. A 11, 1025 (1975)]. The agreement between the theory and the simulation results is quite satisfactory in the low-k domain, but the theory is unable to account for the observed high-k splitting of the shear mode. We also establish the correspondence between the shear mode of the strongly coupled OCP liquid and the acoustic phonons of the Wigner crystal.

Similar to the three-dimensional (3D) one-component plasma (OCP), a two-dimensional (2D) OCP (electron film on the surface of liquid He, e.g.) can support transverse shear waves. We adapt the quasilocalized charge formalism of the preceding paper [Golden, Kalman, and Wyns, Phys. Rev. A 46, 3454 (1992)] to the analysis of the 2D transverse dielectric-response function and identify the shear mode, whose existence is unique to the strongly coupled system. We study the dispersion of the shear mode in the 22<Γ<120 domain (Γ=${\mathit{e}}^2$/${\mathit{ak}}_{\mathit{B}}$T, a is the 2D Wigner-Seitz radius) and compare it with the results of the molecular-dynamics simulations of Totsuji and Kakeya [Phys. Rev. A 22, 1220 (1980)]. Agreement between the theory and simulation results is quite satisfactory. We also establish the correspondence between the shear mode of the strongly coupled 2D OCP and the acoustic phonons of the 2D Wigner crystal.