Although large, spherical surfactant vesicles are generally unstable to either lamellar or micellar phases, mixtures of two surfactants can lead to spontaneous vesicle formation. We show theoretically how the energetic stabilization of mixed vesicles can occur by considering the curvature elasticity of the surfactant bilayer. Interactions between the two species (of the proper sign and magnitude) are crucial to stabilizing these vesicles. These interactions lead to composition asymmetries and effective spontaneous curvatures of the inner and outer layers that are of equal and opposite signs. The vesicles have a Gaussian distribution about an average size determined by the effective spontaneous curvature; the width of the distribution is calculated as a function of concentration. The stability of these vesicles with respect to a flat lamellar phase is estimated. The predictions of the ranges of stability of the various phases as a function of the three concentrations (solvent, e.g., water–and the two amphiphiles) are in qualitative agreement with recent experiments.

A dynamic model for neural networks that explicitly takes into account the existence of several time scales without discretizing the time is studied analytically via the use of path integrals. The maximum capacity of the network is found to be that of the Hopfield model divided by 1+${\mathit{a}}^2$, with a the ratio of the refractory period to the action-potential duration. We obtain the phase diagram as a function of a, the capacity, and the temperature. The overall phase diagram is rich in structure, exhibiting first-order transitions as well as continuous ones.

The fluctuation-dissipation theorem (FDT) in its kinetic form is generalized to anisotropic nonequilibrium states with more than one temperature. In a pseudoequilibrium, expressions for the generalized response functions and for the generalized correlation functions can be defined that preserve the form of a FDT in the classical limit.

For a two-dimensional shallow liquid layer open to air and subjected to a thermal gradient (Bénard problem), the threshold condition is given for the onset of a Korteweg–de Vries soliton as the result of an instability triggered by the Marangoni effect, i.e., by the variation of the interfacial tension with temperature or concentration of a surfactant. In the absence of buoyancy, in contrast with Bénard convection, the heating for standard liquids must be done from the air side. However, for liquids exhibiting a minimum in surface tension with temperature the heating can also be done from the liquid side when operating past the minimum where the surface-tension coefficient increases with temperature.

We consider a fluid layer (electrically conducting) heated from below in the presence of a strong external magnetic field. In the region of hard turbulence, we predict the existence of two different scaling regimes depending on the relative magnitudes of the Rayleigh number and the Chandrasekhar number.

A classical model for l mixing of Rydberg states in atomic collisions is described. The primary assumption of the model is that l changing occurs in a single hard-sphere collision between the Rydberg electron and the perturbing atom which is represented by the zero-energy scattering length L. Predictions of the model, which are in the form of cross sections as functions of principal quantum number n, are compared with data for l mixing of Na(nd) and Rb(nf) by rare-gas atoms. Agreement is, in most cases, quite good, comparable with that of other, more sophisticated, models. It is shown that, because the calculation must be carried out in the reference frame of the ion core, the energy deficit between low angular momentum states and the nearest hydrogenic manifold can be made up in a single collision between the Rydberg electron and the perturbing atom, an effect that is analogous to the gravitational slingshot effect used to alter the orbits of a spacecraft under the influence of the Sun’s gravitational field. Comparison of data for l mixing by ground-state Rb atoms with model calculations in which L is treated as a free parameter leads to an estimate of 10 a.u. for the zero-energy scattering length of Rb.

An experimental confirmation is given of the existence of a stretched-exponential relaxation of the form exp[-(t/τ${)}^{\mathit{x}}$] with x≃0.39 in a binary solution with an upper critical point. The nonlinear dielectric effect (NDE) method was used for this experiment. Results obtained are similar to those reported earlier by Piazza et al. [J. Opt. Soc. Am. B 3, 1642 (1986); Phys. Rev. B 38, 7223 (1988)] based on the Kerr-effect measurements in solutions with a lower critical point. Studies could be carried out in the immediate vicinity of the critical point, because the application of the NDE is not restricted by the appearance of the critical opalescence.

We devise a method based on an analysis of the non-Gaussian parameter in a finite time domain to determine the transition between Gaussian and non-Gaussian dynamics in a stochastic trapping model of a glass transition. We find evidence of the Gaussian–to–non-Gaussian transition in the supercooled region of soft-sphere fluids. We propose an alternative analysis of the incoherent scattering function for glass-forming systems to observe the transition.

Current wave forms of the first Trichel pulses and initial stages of negative point-to-plane breakdown in carbon monoxide have been measured at a pressure of 13.33 kPa. The results include a report of the intricate pulse wave form with a step on the pulse leading edge and a current hump on the pulse trailing part, which indicates that the physical mechanism for the Trichel pulse phenomenon is more complex than the generally accepted description in terms of Morrow’s theory [Phys. Rev. A 32, 1799 (1985); 32, 3821 (1985)]. A tentative explanation of the observed pulse wave form based on the presumed existence of a cathode-directed ionizing wave is outlined.

Populations of 3${\mathit{p}}^2$ ${}_{}{}^1{}_{}{}^{}$${\mathit{D}}_2$${,}^3$${\mathit{P}}_1$${,}^3$${\mathit{P}}_2$ levels in Si-like Cu, Zn, Ge, and Se ions have been deduced from the measurements of absolute intensities of magnetic dipole transitions within the 3${\mathit{s}}^2$3${\mathit{p}}^2$ ground configuration. Observations have been made in the Texas Experimental Tokamak (TEXT). The measured population ratios are compared with theoretical calculations based on the distorted-wave approximation for the electron collisions and a semiclassical approximation for the proton collisions. The observed deviation from the statistical distribution for the excited-level populations within the ground configuration along the silicon isoelectronic sequence is in agreement with theoretical prediction.

Negative dimensions in probabilistic fractal measures are analyzed using the concept of level-independent multiplier distributions. By suitably manipulating these distributions we compute the positive and negative parts of the f(α) function. It is demonstrated that the multiplier method extracts the f(α) function with exponentially less work, and that it is more accurate than conventional box-counting methods. The utility of this method is demonstrated by applying it to a binary cascade with a triangular multiplier distribution and the dissipation field of fully developed turbulence.

A quantum study of the dissociation dynamics of a kicked Morse oscillator reveals that the boundary separating the dissociation and nondissociation regions in the parameter space of the kicking strength and period is a highly oscillatory curve. Large fluctuations also occur in the lifetime distribution in the dissociation region. These features represent quantum manifestations of the fractal behavior seen in corresponding classical studies. Experimental relevance of this sensitive dependence on the field parameters is discussed.

Recently a set of new phenomena has been observed in the directional solidification of liquid crystals, which include solitary inclusions of traveling waves and spatial period doubling. We propose that these patterns can be naturally explained as arising from the resonant interaction of modes with wave vectors q and 2q. This picture is substantiated by numerical calculations employing the material parameters of the liquid-crystal system of Simon, Bechhoefer, and Libchaber [Phys. Rev. Lett. 61, 2557 (1988)]. Our approach correctly predicts period-doubled (mixed) modes and the parity-breaking bifurcation to the traveling-wave state. Possible causes for the disagreement of some stability assignments as compared with the experiments are sketched.

The statistical mechanics of a discrete-vortex system is considered in a coarse-grained sense, as a model for relaxation of two-dimensional turbulence at very large Reynolds numbers. The implications are qualitatively consistent with observations from recent numerical simulations of decaying homogeneous turbulence.

It is argued that asymmetric phase diagrams and other properties of colloidal systems can be generated by a system of spherical particles interacting via a sticky hard-sphere potential (Baxter model). This concept is illustrated by application to the cloud-point transition in nonionic surfactants. A detailed interpretation of neutron-scattering, light-scattering, and phase-diagram results for the (${\mathrm{C}}_8$${\mathrm{E}}_5$) system is presented.

We study the distribution n(α,M) of growth probabilities {${\mathit{p}}_{\mathit{i}}$} for off-lattice diffusion-limited aggregation (DLA) for cluster sizes up to mass M=20 000, where ${\mathrm{\alpha }}_{\mathit{i}}$==-${\mathit{p}}_{\mathit{i}}$/logM. We find that for large α, log n(α,M)∝-${\mathrm{\alpha }}^{\gamma }$/${\log }^{\mathrm{\delta }}$M, with γ=2±0.3 and δ=1.3±0.3. One consequence of this form is that the minimum growth probability ${\mathit{p}}_{\min }$(M) obeys the asymptotic relation log${\mathit{p}}_{\min }$(M)∼-(logM${)}^{(\gamma +1+\mathrm{\delta })/\gamma }$. We find evidence for the existence of a well-defined crossover value ${\mathrm{\alpha }}^{\mathrm{*}}$ such that only the rare configurations of DLA contribute to n(α,M) for α>${\mathrm{\alpha }}^{\mathrm{*}}$, while both rare and typical DLA configurations contribute for α<${\mathrm{\alpha }}^{\mathrm{*}}$.

We implement a parallel processing Monte Carlo simulation to explore RNA configuration space that takes into account fluctuations in base-pairing patterns. The choice of folding pathways is biased by the refolding events that occur as the chain is being assembled. We prove that fluctuations in the initial stages of folding might lead to either active or inactive emerging structures. As an illustration, competing pathways that are the result of fluctuation propagation are computed for the splicing YC4 intron (a segment of the mitochondrial RNA from fungi), and the emerging structures are proved to be biologically relevant.

The far-uv spectra of sodiumlike S VI and neonlike S VII are obtained following single- and double-electron capture into excited states of slow (∼10-keV/q) ${\mathrm{S}}^{7+}$ ions passing through thin ${\mathrm{H}}_2$ and He gas targets. The high charge-state and excited-state selectivity of the collision process facilitated line identification. New model-potential calculations using the SUPERSTRUCTURE code are compared with measurements for S VII. Changes in relative line intensities observed using the different target gases help to elucidate the collision mechanism via the relative initial-state populations. Capture takes place into n states as predicted by a classical model, while the distribution among l states is nonstatistical at this velocity (∼0.29 a.u.).

We have measured the Lyman-α radiation following foil breakup of hydrogen ionic clusters ${\mathrm{H}}_{\mathit{n}}^{+}$ (n=2 and n=3 to 61, odd) with velocities above and around the Bohr velocity. An enhancement of this radiation was observed and could reach a factor of 3 with respect to the proton case of the same velocity. Cluster mass number, velocity, and thickness dependences of the relative population of the 2p state in hydrogen fragments following ${\mathrm{H}}_{\mathit{n}}^{+}$ foil dissociation have been extracted. A specific collective effect on the 2p-state hydrogen has been observed and interpreted in terms of charge-exchange processes.

A systematic experimental and theoretical study of Lα emission in ${\mathrm{H}}^{+\mathrm{-}}$Li(2s), ${\mathrm{H}}^{+\mathrm{-}}$Na(3s), and ${\mathrm{H}}^{+\mathrm{-}}$K(4s) collisions is presented. Good agreement is found between absolute cross sections obtained by vacuum-ultraviolet spectroscopy and atomic orbital expansion close-coupling calculations.