Free-standing films of the liquid-crystal compound C7 [4-(3-methyl-2-chloropentanoyloxy)-4’-heptyloxybiphenyl], which possesses a first-order smectic-A–smectic-C transition, are studied. In various films with thicknesses between 2 and 27 molecular layers, the average tilt angle, i.e., the order parameter of the transition, is measured by ellipsometry. In thick films of more than about 15 layers, a first-order transition with a discontinuous jump of the average tilt value is observed at the same temperature at which the smectic-A–smectic-C transition in the bulk sample occurs; in contrast to the bulk, a finite average tilt remains in the high-temperature phase, indicating that the transition takes place in the interior layers, whereas the surface layers are tilted in the whole temperature range. Films thinner than about 15 layers exhibit instead of the first-order transition a steep but continuous variation of the tilt with temperature. This feature becomes less pronounced with decreasing film thickness and is not observable in the two-layer film, which shows large tilt values in the whole temperature range, indicating the influence of a strong ordering field resulting from surface interactions.

In this paper, we review the theoretical and experimental progress in understanding the controversial phase behavior of binary liquids in dense porous media. Experimental observation of metastability and hysteresis in the phase-separation behavior has led to two widely different theoretical interpretations: the random-field Ising picture and the single-pore picture. We argue that the random-field model is inapplicable to binary liquids in low-porosity media such as Vycor, and discuss the available experimental evidence on such systems. Next, we present Monte Carlo studies of phase-separation kinetics of an Ising model in a pore. We find that the domain-growth kinetics slow down dramatically once the domain size becomes comparable to the pore size, as predicted by the single-pore model. In addition, we examine the influence of temperature and interfacial phase transitions on the kinetics, and show that the domain-growth rate slows down as the temperature moves further into the two-phase region. Finally, our results for small pores, only 20 spins across, suggest that macroscopic descriptions are surprisingly successful, even at short length scales.

A three-dimensional network model of a porous medium is used to compute relative permeabilities and capillary pressures in drainage and imbibition. In contrast, the invasion-percolation model of drainage with trapping does not make a sensible prediction for the relative permeability of either the displaced or injected phases since it fails to represent the fluid connectivity correctly. We describe two realistic trapping mechanisms which overcome this difficulty by representing the flow of the wetting phase along irregularities in the pore-gain surface. In imbibition, we simulate the rate-dependent competition between bulk filling of pores and film flow and show how different displacement mechanisms affect relative permeability. We verify percolation-theory results for the effects of buoyancy forces on trapped saturation by simulation and derive an expression for the correlation length in displacements perturbed by viscous forces. We can then demonstrate how relative permeabilities measured in quasistatic systems at capillary equilibrium are still meaningful in larger-scale displacements where viscous forces predominate.

The development of smectic-A ordering at the interface between isotropic liquid and vapor is studied using a density-functional theory, based on a mean-field approximation for the effect of attractive pair interactions and weighted-density treatment of repulsive-core contributions. In contrast to earlier microscopic theories of this phenomenon, the bulk liquid and vapor phases are in complete coexistence with each other, and no arbitrary ‘‘surface fields’’ independent of molecular pair interactions are invoked. The wetting behavior of the interface is studied as a function of the molecular-core anisotropy, which controls the relative stability of bulk nematic and smectic-A phases, under conditions where the attractive pair interactions favor complete wetting by the nematic phase. There is a change to incomplete wetting by the smectic-A phase when the latter preempts the nematic, in a process analogous to a triple-point wetting transition. This is consistent with experimental results, but the growth of smectic layers at the interface is always found to be continuous, with no evidence of the first-order layering transitions observed in experiment.

The complex viscosity of lipid vesicle dispersions as a function of frequency was monitored as the vesicles aged. Of the two relaxation processes that occur in fresh vesicle dispersions, the first (i.e., longest) relaxation time did not shift, while the second (i.e., shortest) relaxation time proved to increase dramatically in time. The first relaxation process pertains to an entropic relaxation process in which redistribution occurs of a shear-disturbed distribution of hard spheres. The second relaxation time pertains to vesicle deformation where the surface shear modulus μ of the bilayer plays a dominant role. We will show that μ decreases several orders of magnitude as the vesicles age. With increasing age, a third relaxation process became measurable. It is inferred that this pertains to vesicle deformation where the surface dilatational modulus κ plays a dominant role. We found some boundary values for the surface shear modulus, surface dilatational modulus, and the curvature modulus for fully aged bilayers. The process responsible for the changed bilayer mechanical properties is the peroxidation of the lipids. This process causes unsaturated lipids to break and modify, imposing many changes on the constitution of the bilayer.

We develop a phenomenological theory of the dynamic viscosity of colloidal suspensions, based on an extrapolation of the low-frequency behavior by use of a continued-fraction representation. In lowest approximation the dynamic viscosity depends on a small number of parameters, which may be determined experimentally. For semidilute suspensions the parameters may be found by theoretical calculation. The theory is tested by comparison with an exactly soluble model.

Complex-permittivity measurements of a reentrant nematic-liquid-crystal mixture, 4-n-hexyloxy-4’-cyanobiphenyl and 4-n-octyloxy-4’-cyanobiphenyl, for both the parallel and perpendicular components, have been performed by time-domain reflectometry in the frequency range between 300 kHz and 10 GHz and at temperatures from 58 °C (above the nematic and smectic-A phase-transition temperatures) down to 25 °C (below the smectic-A and the reentrant nematic phase-transition temperatures). It is shown that the molecular rotation around the short axis in the reentrant nematic phase is much hindered compared to that in the nematic phase, but the motion around the long axis is the same for both phases. It is also shown that, for the parallel component, the activation energy for the smectic-A phase is larger than that for the nematic phase but similar to that of the reentrant nematic phase. Finally, the transition between the reentrant nematic and the solid phases is discussed.

We have studied the spreading of liquid drops on a solid surface by molecular-dynamics simulations of coexisting three-phase Lennard-Jones systems of liquid, vapor, and solid. We consider both spherically symmetric atoms and diatomic molecules, and a range of interaction strengths. As the attraction between liquid and solid increases we observe a smooth transition in spreading regimes, from partial to complete to terraced wetting. In the terraced case, where distinct monomolecular layers spread with different velocities, the layers are ordered but not solid, with substantial molecular diffusion both within and between layers. The qualitative behavior resembles recent experimental findings, but the detailed dynamics differ. In particular, the layers exhibit an unusual spreading law, where their radii vary in time as ${\mathit{R}}^2$∼${\log }_{10}$t, which disagrees with experiments on polymeric liquids as well as recent calculations.

We study the reversible aggregation of polystyrene latex spheres (PLS) in one phase mixtures of 2,6 lutidine plus water at temperatures and solvent compositions near the solvent mixture’s critical point. The aggregation occurs only on the side of the critical composition rich in the liquid component which is nonpreferred when the system is in the solvent’s two-phase region. The liquid that is preferred can be changed by changing the surface charge density of the PLS. The aggregation region extends to temperatures well beyond the wetting temperature, and dynamic light scattering shows no evidence of a thick layer building up on individual particles before they join aggregates.

We present a numerical study of a Langevin-equation model for a simple fluid. We study the relaxation kinetics of the density-time-correlation function. We find a stretched exponential regime with an exponent β depending on wave vector in agreement with results obtained from mode-coupling theory and correlated with the wave-number dependence of the static-structure factor. At larger wave vectors, we find a transition to power-law behavior after a rapid decay.

We study the dynamics of defects generated in free-standing films of liquid crystals following a thermal quench from the smectic-A phase to the smectic-C phase. The defects are type-1 disclinations, and the strain field between defect pairs is confined to 2π walls. We compare our observations with a phenomenological model that includes dipole coupling of the director field to an external ordering field. This model is able to account for both the observed coalescence dynamics and the observed ordering dynamics. In the absence of an ordering field, our model predicts the defect density ρ to scale with time t as ρ lnρ∝${\mathit{t}}^{\mathrm{-}1}$. When the dipole coupling of the director field to an external ordering field is included, both the model and experiments show the defect coarsening proceeds as ρ∝${\mathit{e}}^{\mathrm{-}\mathrm{\alpha }\mathit{t}}$ with the strain field confined to 2π walls. The external ordering field most likely arises from the director’s tendency to align with edge dislocations within the liquid-crystal film.

We study the coarsening of the interfacial pattern separating two coexisting phases after the pattern becomes morphologically unstable. Shortly after the instability, the scale of the structure is given by the most unstable wave number in the linear regime. At later times, nonlinear interactions cause the structure to coarsen. Coarsening is studied by monitoring the time dependence of the two-dimensional power spectrum of the pattern, especially for wave vectors transverse to the direction of growth. Characteristic length scales of the pattern obtained from moments of the power spectrum are asymptotically linear in time. Furthermore, the power spectrum is seen to satisfy a scaling relation, in agreement with previous studies. A normal-velocity autocorrelation function is calculated and found to decay substantially over length scales that are of the order of the scale of the pattern. The issue of spatial anisotropy in the correlation functions is also discussed.

Molecular-dynamics simulation results are reported for systems of strongly interacting dipolar soft spheres. Calculations have been carried out along two isotherms and the structure of the liquid-crystal and solid phases obtained is described in detail. It is found that in addition to the ferroelectric nematic phase we previously reported [Phys. Rev. Lett. 68, 2043 (1992)], liquid crystals with columnar order can also be obtained. The model freezes to form a ferroelectric solid which is shown to have a tetragonal I crystal structure. The influence of different boundary conditions upon the simulation results is also discussed.

We present a theory of branched growth processes, notably diffusion-limited aggregation (DLA). Using a simple model of the dynamics of screening of competing branches, we compute statistics of the growth probability distribution. These statistics are multifractal only for stochastic models. Applying this general approach to diffusion-limited aggregation, we obtain results for the dimension D of a DLA cluster that are extremely close to the Muthukumar formula, D=(${\mathit{d}}^2$+1)/(d+1), in spatial dimensionality d [Phys. Rev. Lett. 50, 839 (1983)]. This formula is believed to be accurate for DLA clusters in dimensions d>2. The maximum growth probability of the cluster scales as ${\mathit{p}}_{\max }$∼${\mathit{r}}^{1\mathrm{-}\mathit{D}}$, with r the cluster radius, as predicted by Turkevich and Scher [Phys. Rev. Lett. 55, 1026 (1985); Phys. Rev. A 33, 786 (1986)]. We also discuss the scaling of the minimum growth probability, the behavior of nonstochastic models, and possible approaches to direct computation of the screening dynamics. Our results are in good qualitative, and in some cases good quantitative, agreement with numerical studies of the screening dynamics.

Scaling laws for plasma temperature, thickness of the heated region, and other important plasma parameters are discussed for high-intensity subpicosecond laser plasma interactions with solid targets. Effects of inhibited heat fluxes are taken into account following recent formulations of the nonlocal thermal transport theories. It is shown that for constant laser intensities in excess of ${10}^{17}$ W/${\mathrm{cm}}^2$, i.e., in the anomalous skin-effect regime, plasma temperature increases much faster and to higher values for the inhibited thermal conductivity as compared to heating rates with the classical thermal transport. It it also shown, however, that the dynamical evolution of the heating process is primarily influenced by the time evolution of the laser intensity, which on the scale of ultrashort laser pulses makes the evolution of a plasma temperature almost identical for the classical and inhibited thermal transports.

N-body simulations are used to assess the importance of discrete ion-ion Coulomb interactions in an electron-free plasma sheath, where ordinary Debye screening is ineffective. The angular dispersion ${\mathrm{\theta }}_{\mathrm{rms}}$ of the ion flux incident on the cathode is found to have a power-law dependence on a parameter Ω measuring the ‘‘graininess’’ of the sheath electric field, and an explanation for the observed exponent in this relation is postulated in terms of diffusion theory. In the Coulomb logarithm, a lower cutoff ${\mathit{b}}_{\min }$ equal to a fraction of the mean ion-ion distance at the plasma-sheath boundary is determined, while the upper cutoff ${\mathit{b}}_{\max }$ appears to be no more than about ten times the sheath thickness d; no sustained logarithmic growth of ${\mathrm{\theta }}_{\mathrm{rms}}^2$ with the lateral extent a over which Coulomb interactions are taken into account was observed. For sheath parameters representative of the low-pressure weakly ionized discharges used in very-large-scale-integration manufacturing processes, the dispersion due to Coulomb scattering in the sheath is typically ${\mathrm{\theta }}_{\mathrm{rms}}$≲0.1°.

A theoretical description of ultrashort ionizing wave pulses is presented by means of two different models where the ionization rate increases or decreases, respectively, as a function of the electric-field amplitude. We show that the pulse evolves either into a horseshoe- or a horn-type structure in the time-space domain. In some parameter regions the intensity of the pulse can also increase.

A self-consistent analytic model of the cathode region of a dc glow discharge is suggested. The description is based on the division of the discharge gap into a space-charge sheath and a plasma region and on an approximate kinetic analysis of different groups of electrons. A one-dimensional short discharge is considered for which the positive column is absent and the energy relaxation length ${\lambda }_{\mathrm{\varepsilon }}$ of slow untrapped electrons exceeds the gap length L. In this case, a point exists in the negative glow (NG) region where the plasma density has a maximum and the electric field changes sign. Three groups of electrons are treated separately. The first of them includes fast electrons with energies exceeding the first excitation potential ${\mathrm{\varepsilon }}^{\mathrm{*}}$. They are emitted by the cathode surface (primaries) or generated in the cathode fall region. These electrons are responsible for ionization and excitation processes and electron-current transport in the sheath and in the immediately adjacent plasma. The field reversal is caused by the nonlocal ionization in the plasma part of the NG, which is determined by the fast electrons. The slow electrons are subdivided into trapped and intermediate ones. The full energy ɛ (kinetic plus potential) of the trapped electrons is less than the anode potential e${\mathrm{\phi }}_{\mathit{a}}$. These electrons determine the plasma density but give no contribution to the electron current.

In the Faraday dark space, the electron current is due to diffusion of the intermediate electrons with energies e${\mathrm{\phi }}_{\mathit{a}}$<ɛ<${\mathrm{\varepsilon }}^{\mathrm{*}}$. A continuous-energy-loss model is used for description of the fast electrons. Simple analytic solutions for the electron-distribution function (EDF) and nonlocal ionization in the sheath and plasma are obtained from the constant-retarding-force approximation. The EDF of the intermediate electrons is close to isotropic. Analytic solutions for it are derived. Coulomb collisions lead in most cases to a Maxwell-Boltzmann distribution of the trapped electrons. Their temperature ${\mathit{T}}_{\mathit{e}}$ at ${\lambda }_{\mathrm{\varepsilon }}$>L does not depend on x. The plasma density profile is obtained from the ambipolar diffusion equation. The kinetic analysis of the trapped electrons is necessary only for the calculation of ${\mathit{T}}_{\mathit{e}}$ and ${\mathrm{\phi }}_{\mathit{a}}$ values. A criterion for the field reversal is proposed. The results are compared with experimental and simulation results of other authors.

The results from a series of experiments that characterize the x-ray flux transmitted through a laser-irradiated Au foil are presented. The purpose of the experiments was to develop a working model for an x-ray source that will create hot, dense plasmas with controllable gradients. These plasmas will be used as a test bed for the study of the complex radiative processes that are intrinsic to the evolution of moderate- and high-Z matter. The experiments quantitatively measured the time- and frequency-dependent energy transferred to the back of the foil. Angular information and the characterization of the flux as a function of foil thickness are also presented. Tables of the time-dependent flux are given.

The scalar model for collapse of Langmuir waves in plasmas is studied numerically in two and three dimensions, for both radially symmetric and anisotropic initial conditions. Using a dynamic rescaling method, singular solutions are shown to become isotropic and self-similar near collapse. In two dimensions, the self-similar profile is not universal. In the limit where the mass of the wave tends to its minimal value for collapse, the solution approaches a subsonic regime different from the generic singularity of the nonlinear Schrödinger equation.