We present a computer-simulation study regarding the effect of system size on the isotropic-nematic transition in a molecular fluid model. Systems of 256, 500, and 864 molecules interacting through the Gay-Berne model are analyzed along an isotherm by using molecular dynamics in the NVT ensemble. For the three system sizes under consideration, the system exhibits a weak first-order isotropic-nematic transition. The transition densities are calculated by using thermodynamic integration. We find that the transition shifts to higher densities as the number of particles N increases. In addition, the transition weakens as N gets larger. These results are used to extract information on the transition properties in the infinite-system limit. The N dependence of thermodynamic properties are estimated from the simulation results and are compared with theoretical predictions.

We study the coarsening dynamics of defects in the uniaxial nematic liquid crystal 4-cyano-4’-n-pentylbiphenyl, subjected to a rapid pressure jump from the isotropic to the nematic phase. The defect tangle formed at the phase transition is dominated by type-1/2 strings (disclination lines), but also contains type-1 strings and monopoles. We also give a general discussion of the existence of scaling solutions to the nematodynamic equations for loop collapse and two-dimensional vortex-antivortex annihilation. We have measured the density of type-1/2 strings and loops and the monopole density as a function of time. We have also measured the rate at which the intercommutation of two strings takes place. Surprisingly, although the type-1/2 string density ρ scales as expected as a function of time, ρ∼${\mathit{t}}^{\mathrm{-}1}$, loop densities and monopole densities do not seem to scale over the time interval studied. A possible explanation is given.

The pattern structure and the scaling behavior of the surface width for two deposition models of two kinds of particles, particle A with a probability 1-P and particle C with a probability P, depositing on a (1+1)-dimensional substrate are studied. For model I, a randomlike deposition model, the pattern has a compact structure, and the surface width growth only depends on the time, W∼${\mathit{t}}^{1/2}$ for the early stage and W∼${\mathit{t}}^{\mathrm{\beta }\left(\mathit{P}\right)}$ for the intermediate time where β is a function of P, as well as W∼${\mathit{P}}^{\mathrm{-}\gamma }$ for the later time. For model II, a ballisticlike deposition model, the pattern and scaling behavior are similar to the ballistic deposition. The scaling of the surface width is W∼${\mathit{t}}^{\mathrm{\beta }\left(\mathit{P}\right)}$ for the early stage of growth and W∼${\mathit{L}}^{\mathrm{\alpha }}$ for the later stage. The exponent β is a function of P, while α is independent of P.

The temporal evolution of three-dimensional structure in an electrorheological fluid under nonequilibrium, high-field conditions is examined by a computer-simulation method similar to that of Klingenberg, van Swol, and Zukoski [J. Chem. Phys. 91, 7888 (1989)]. A variety of characteristic real-space properties (e.g., the radial distribution function) and the static structure factor S(q) are monitored during the simulation. The field-induced polarization of particles causes rapid chain formation followed by a kinetic trapping into a complicated gel-like state with no obvious lateral ordering. For all volume fractions considered, the first percolating chain appears at ten times the average time to first contact. Quenching of the gel is nearly complete on an order of magnitude longer time scale. The formation of chains results in the growth and narrowing of Bragg-like peaks in S(q) along the field direction. In the direction perpendicular to the field, the first peak in S(q) also grows and shifts to smaller q.

An aligned sample in the lamellar phase of the sodium dodecylsulphate–decanol–water system is investigated by orientation-dependent ${}_{}{}^{23}{}_{}{}^{}\mathrm{Na}$ NMR spin-relaxation experiments and x-ray diffraction. A pronounced relaxation anisotropy is observed, implying a nonclassical microstructure where a large fraction of the hydrocarbon-water interface is highly curved. Noting that the bilayer topology is either oil-continuous, bicontinuous, or water-continuous, we consider three classes of microstructures with curvature defects: a bilayer perforated by aqueous pores, a bilayer with aqueous slits and ribbonlike aggregates, and a bilayer fragmented into discrete discoidal micelles. The seven model-independent NMR observables impose severe constraints on the allowed microstructures: the discoid geometry can be rejected, while the pore and ribbon geometries are consistent with the data, but only for certain values of the relative defect dimensions. A simple analysis of the energetics of defect formation, using the deduced defect dimensions, favors the ribbon geometry.

We study a system of hard spherocylinders in contact with a hard wall, assuming that the length-to-width ratio of the spherocylinder tends to infinity. The Onsager approximation is applied to calculate the free energy of the system. The integral equation for the one-particle distribution function is solved in the uniaxial case, i.e., for bulk densities corresponding to the isotropic phase. We find the density and order parameter profiles and conclude that the particles prefer parallel alignment close to the wall. Finally, we study the stability of the uniaxial solution and find the bifurcation point corresponding to the nematic order close to the wall.

Monte Carlo simulations of aqueous charged colloidal suspensions interacting via an effective pair potential are carried out with a view to elucidate the mechanism of the reentrant phase transition reported in these systems. The computed pair-correlation functions show that a weakly interacting homogeneous suspension phase separates into dense and rare phases and thereafter reenters into a homogeneous state with crystalline or liquidlike order when the concentration of the ionic impurities is decreased. The cluster-size distribution indeed shows the existence of dense-phase droplets for the intermediate range of impurity-ion concentrations where the phase separation occurs. Some of the recent experimental results are discussed in the light of the present simulation results.

This paper presents and discusses the problem of quantitative determination of the percolation threshold on the basis of interactions within microemulsions. The model recently developed by J. Xu and G. Stell [J. Chem. Phys. 89, 1101 (1988)] was applied in the case of waterless microemulsions. The fit obtained is quantitatively highly satisfactory despite the approximation used to determine interactions from the cloud-point curve.

While severe multiple scattering precludes direct observation of the spatial arrangement of the particles suspended in most electrorheological (ER) fluids, both diffuse optical transmittance and diffusing-wave spectroscopy are useful noninvasive probes of particle motion in these materials. The temporal variation of the diffuse transmittance of a commercial ER fluid provides a measure of the time scale for field-induced structure formation. The response times observed with low static fields and with alternating fields are consistent with the expected induced-dipole attraction between particles. However, the field dependence of the response times measured with high static fields suggests that monopole forces play a role in the aggregation process in this fluid. The relevance of particle charge is confirmed by a novel application of diffusing-wave spectroscopy: detection of the oscillatory electrophoretic motion of the particles induced by alternating electric fields. Possible consequences of particle charge for the behavior and performance of electrorheological fluids are discussed.

We treat the theoretically predicted hexatic N+6 phase near the transition to the hexagonal discotic phase by means of a renormalization technique. In particular, we investigate one-loop corrections to the order-parameter correlation function. The condensation of the hexagonal two-dimensional lattice of the ordered phase from the bond-orientationally-ordered N+6 phase was described in a previous work [C. Giannessi, Phys. Rev. A 28, 350 (1983)] using a triple mass-density wave as an order parameter. The requirement of local invariance under rotations gives rise to gauge coupling between the order parameter and the bond-angle field Ω, introduced to fix the full orientational order of the hexatic phase. The peculiar tensorial structure of such a gauge coupling yields anisotropic corrections to the order-parameter correlation function, even in the plane orthogonal to the nematic director with respect to the directions of the sixfold crystallographic axes. As a consequence, the structure factor for x-ray diffraction should acquire sixfold anisotropy, revealing anisotropic scaling of correlation lengths. Actually, the occurrence of anisotropic scaling depends on the existence of a stable anisotropic fixed point of renormalization flow. For technical reasons it has not been possible to determine and analyze the fixed point controlling the phase transition, so that we can only state the possibility of anisotropic critical behavior characterized by correlation lengths in directions parallel and orthogonal both to the director and to the sixfold crystallographic axes diverging with different critical exponents.

A heterodyne light-scattering (LS) experiment that directly probes thermal fluctuations of the fundamental (lowest-order) bend director mode in homeotropic geometry, and a high-resolution Fréedericksz-transition (FT) experiment which excites the corresponding static mode magnetically were performed just above nematic–smetic-A criticality on the nonpolar wide-nematic-range material dihex- ylazoxybenzene over the reduced temperature ranges 3×${10}^{\mathrm{-}7}$≤t≤2.0×${10}^{\mathrm{-}3}$ and 2.7×${10}^{\mathrm{-}5}$≤t≤2.0×${10}^{\mathrm{-}2}$, respectively. It is found that over the nearly five decades of reduced temperature covered by these experiments, the enhancement of the bend elastic constant, ${\mathit{K}}_{33}$, obeys a simple power law with a critical exponent ${\mathrm{\rho }}_3$=0.825±0.008 (FT), ${\mathrm{\rho }}_3$=0.815±0.03 (LS); the error estimates include variations of ${\mathrm{\rho }}_3$ upon extensive range shrinking and gap expansion of the data sets. These studies cover a wider range of reduced temperature and extend two decades or more closer to the critical point than previous studies. Over the nearly five decades of reduced temperature covered by these experiments the bend constant ${\mathit{K}}_{33}$ increases by nearly four orders of magnitude to 5×${10}^{\mathrm{-}3}$ dyn corresponding to a longitudinal correlation length of ${\xi }_{\mathrm{\parallel }}$=7.9${\mathit{t}}^{\mathrm{-}0.82}$ Å (≃200 μm at t=3×${10}^{\mathrm{-}7}$). The measurement of correlation lengths of this magnitude may be unprecedented in the study of critical phenomena. In spite of accessing the regime where ${\xi }_{\mathrm{\parallel }}$ is comparable with the thickness of our films, ∼200 μm, finite-size and surface effects are not observed.

We also report the results of the above heterodyne experiment performed on another nonpolar material, hexyloxynonylbenzoate (6¯09), over a similar reduced-temperature range. This experiment yielded the same critical exponent within experimental uncertainty, ${\mathrm{\rho }}_3$=0.84±0.03, contrary to previously reported Fréedericksz-transition measurements which exhibited a crossover from ∼0.84 to 0.67 on range shrinking and were well represented over the entire experimental range of reduced temperature by a power-law term with ${\mathrm{\rho }}_3$=0.67±0.02 augmented by an anomalously large negative correction to scaling term. This discrepancy is discussed briefly in the context of surface phenomena and is currently under study. We compare our results with theoretical predictions about nematic–smectic-A criticality and with the director normal-mode spectrum which we derive from linearized Erickson-Leslie-Parodi theory including the effects of finite-director anchoring and surface dissipation.

We examine the fluctuation-induced interaction energy Δf between the surfaces of a layered hexatic film within a quadratic-functional-integral approach. Both discrete and continuum formulations are studied. We find large corrections to the asymptotic behavior Δf(l)∝${\mathit{l}}^{\mathrm{-}2}$ for finite l, where l is the film thickness. A small field becomes relevant beyond a length scale ξ(h), while an enhanced surface Frank constant is irrelevant above a length ξ(K). Possible relevance of these results to recent experiments on layer-by-layer surface ordering in liquid-crystal films is discussed.

We have studied the surface morphological instabilities of ${\mathrm{CBr}}_4$ crystals during growth from vapors. We have found that the crystal-growth morphologies depend not only on temperature and supersaturation, but also sensitively on the total pressure of inert gas in the vapor. With increasing inert gas pressure, at otherwise the same growth conditions, crystals with initially smooth surfaces first develop cellular patterns and then evolve into dendritic structures. Although these cellulars and dendrites bear morphological similarity with those found in melt and solution-growth experiments (i.e., the product ${\mathit{W}}^2$R=const, with W being either the width of a cellular or the radius of a dendritic tip, and R the growth rate at the corresponding solid front) the destabilizing mechanism appears to be more complex than that from bulk nutrient diffusion. We have found that the surface diffusion on the substrate also plays an important role in surface morphological instabilities in vapor crystal growth.

Study of single-phase fluid flow in a three-dimensional (3D) random capillary network on a regular cubic lattice has established a simple generalization of the Poiseuille law for the total flow. Results are discussed in the light of effective-medium theory and percolation theory. Detailed examination of the behavior of such networks near percolation threshold leads to an extended model which is appropriate for phase conductivities in two-phase flow. The simple expression for conductivity when combined with pore phase occupancy distributions from a rule-based percolation approach can be used to calculate relative permeabilities in 3D networks.

The dynamical diffusion of phospholipid tubules in highly concentrated solution has been studied as a function of temperature by high-field magnetic birefringence and by dynamic light scattering in zero field. Data at a temperature well below the lipid chain-melting transition T=${\mathit{T}}_{\mathit{m}}$ are analyzed using theoretical predictions for rigid-rod dynamics. While qualitatively successful, these lead to an estimate of the rotational diffusion constant which is larger than that anticipated when dynamical rod entanglement in a homogeneous solution is the only effect considered at high concentration. For T→${\mathit{T}}_{\mathit{m}}$, the results from both experimental techniques exhibit a marked temperature dependence, revealing dynamical studies to be a sensitive and potentially interesting probe of the phase transformation at ${\mathit{T}}_{\mathit{m}}$.

The surfaces of normal liquid alkanes (chain lengths between 15 and 18 carbon atoms) have been studied by light scattering from thermally excited capillary waves. These fluids have recently been reported to exhibit a surface-induced phase transition at temperatures (${\mathit{T}}_{\mathit{k}}$) near, but distinct from, their melting points. In all cases the propagation of the capillary waves showed abrupt discontinuities at this transition. While the data above ${\mathit{T}}_{\mathit{k}}$ are consistent with predictions based on the known properties of the liquids, below this temperature they suggest the presence of a structured surface layer which is viscoelastic in nature. Below ${\mathit{T}}_{\mathit{k}}$ both elastic and viscous parts of the surface excess transverse shear modulus are negative, indicating that the surface layer responds more easily to shear normal to the surface plane than does the bulk fluid. The light scattering thus suggests the existence of a surface layer which is much more ordered than the bulk fluid. This is consistent with the negative surface excess entropy density found in the new phase.

A phase-separated mixture of cyclohexane and acetic anhydride was subjected to positive linear temperature ramps while the capillary rise of the acetic anhydride–rich phase against a glass substrate was measured. The ramps, which drove the system out of thermal equilibrium, had the effect of producing large nonlinear changes in capillary rise with respect to ramp rate. These changes may be explained in terms of the nonequilibrium behavior of the wetting-layer thickness, and depended only weakly on the size of the ramps. The deviation of the capillary rise from its equilibrium value was found to attain a steady state after the temperature had been raised by ≊0.6 K from the starting temperature of ${\mathit{T}}_{\mathit{c}}$-T=5.9 K, independent of the ramp rate. This steady state may be indicative of the virtual disappearance of the wetting-layer. Unlike recent direct observations of wetting-layer thickness [Dean Ripple and Carl Franck, Phys. Rev. A 44, 8289 (1991)], our results could not be simply described as the ‘‘equilibrium’’ wetting behavior of a bulk nonequilibrium experiencing linear response to a weak temperature ramp.

The faceting transition of a crystal surface can be effectively simulated by a ferromagnetic Ising model on a simple-cubic lattice. In this paper, we study a planar interface in a fully three-dimensional Ising model, as well as an isolated bulk inclusion or droplet. We also demonstrate a method for identifying the faceting transition and compare it with the conventional techniques. Finally, we examine the effect of a driving force on the faceting transition of a droplet and the so-called dynamic or kinetic roughening of a moving planar interface. These results are compared with the current renormalization analysis of the faceting transition.

The steep phonon-dispersion curves found in computer-simulation studies on normal and supercooled water which have been interpreted as evidence of the existence of ‘‘fast-sound modes’’ of kinetic origin are considered. From the analysis of the dynamical structure factor of polycrystalline ice it is shown that such features are present in the solid calculated within the harmonic approximation, and the anomalous steep increase in frequency is shown to be originated by optical phonon branches, which cross the acoustical ones at wave vectors below the zone center, and result in a pull-out up to a maximum frequency at ${\mathit{Q}}_{\mathit{p}}$/2, which roughly coincides with that of the sharp translational edge.

We study the motion of an isolated terrace ledge on a crystalline surface within the framework of the terrace-ledge-kink model. We argue that for length scales larger than the diffusion length the step roughness is governed by the Kardar-Parisi-Zhang (KPZ) equation that predicts a broadening as ${\mathit{t}}^{1/3}$. For smaller length scales a variety of possibilities are explored. Their occurrence depends sensitively on the rates for the adsorption and desorption processes both on the terraces and at the ledge. The ledge could be unstable, developing a fractal, dendritic type of structure. If the ledge is stable, we obtain a crossover from a ${\mathit{t}}^{1/6}$ (conserved dynamics, model B) to a ${\mathit{t}}^{1/4}$ (nonconserved dynamics, model A) and a ${\mathit{t}}^{1/3}$ (KPZ) broadening.