Intrinsic profiles and the structure of liquid surfaces.

We present a brief review of the advances made in the characterization of liquid surfaces over the last decade. We focus particularly on the links between the capillary wave theory, the density functional formalism and the direct evaluation of the intrinsic density profiles from computer simulations. A new perspective of the liquid surfaces is appearing, with a sharper view of their molecular structure, which opens new challenges for theoretical and experimental studies. Novel results on the intrinsic interfacial structure of molten salt liquid-vapor interfaces are presented.

Constricting force of filamentary protein rings evaluated from experimental results.

We present a model of Z -ring constriction in bacteria based on different experimental in vitro results. The forces produced by the Z ring due to lateral attraction of its constituent parts, estimated in previous studies that were based on FtsZ filaments observed by atomic force microscopy, are in good agreement with an estimation of the force required for recently found deformations in liposomes caused by FtsZ. These forces are calculated within the usual Helfrich energy formalism. In this context, we also explain the apparent attraction of multiple Z rings in the liposomes initially separated by small distances, as well as the stable distribution of rings separated by distances greater than approximately twice the diameter of the cylindrical liposomes. We adapted the model to the in vivo conditions imposed by the bacterial cell wall, concluding that the proposed mechanism gives a qualitative explanation for the force generation during bacterial division.

Hydrodynamics of nanoscopic capillary waves.

The dynamics of nanoscopic capillary waves on simple liquid surfaces is analyzed using molecular dynamics simulations. Each Fourier mode of the surface is obtained from the molecular positions, and its time behavior compared with the hydrodynamic prediction. We trace the transition from propagating to overdamped modes, at short wavelengths. The damping rate is in very good agreement with the hydrodynamic theory up to surprisingly small wavelengths, of about four molecular diameters, but only if the wave number dependent surface tension is considered. At shorter scales, surface tension hydrodynamics break down and we find a transition to a molecular diffusion regime.

Beyond the dynamic density functional theory for steady currents: application to driven colloidal particles in a channel.

Motivated by recent studies on the dynamics of colloidal solutions in narrow channels, we consider the steady state properties of an assembly of noninteracting particles subject to the action of a traveling potential moving at a constant speed, while the solvent is modeled by a heat bath at rest in the laboratory frame. Here, since the description we propose takes into account the inertia of the colloidal particles, it is necessary to consider the evolution of both positions and momenta and study the governing equation for the one-particle phase-space distribution. First, we derive the asymptotic form of its solutions as an expansion in Hermite polynomials and their generic properties, such as the force and energy balance, and then we particularize our study to the case of an inverted parabolic potential barrier. We numerically obtain the steady state density and temperature profile and show that the expansion is rapidly convergent for large values of the friction constant and small drifting velocities. On the one hand, the present results confirm the previous studies based on the dynamic density functional theory (DDFT): On the other hand, when the friction constant is large, it display effects such as the presence of a wake behind the barrier and a strong inhomogeneity in the temperature field which are beyond the DDFT description.

Langevin computer simulations of bacterial protein filaments and the force-generating mechanism during cell division.

FtsZ is a bacterial protein that forms filaments that play an essential role in midcell constriction during the process of cell division. The shape of individual filaments of different lengths imaged with atomic force microscopy was modeled considering the protein monomers as beads in a chain and a few parameters to represent their effective interactions. The flexural rigidity and persistence length of the filaments were estimated. This latter value was comparable to the filament length, implying that these biological polymers are halfway between the perfectly stiff linear aggregate whose shapes are fully controlled by the angle between the monomers and highly flexible polymers whose shapes follow a random walk model. The lateral interactions between adjacent filaments, also estimated in the modeling, were found to play an essential role in determining the final shape and kinetics of the coiled structures found in longer polymers. The estimated parameters were used to model the behavior of the polymers also on a cylindrical surface. This analysis points to the formation of helical structures that suggest a mechanism for force generation and amplification through the development of FtsZ spirals at the midcell division point.

Critical analysis of the density functional theory prediction of enhanced capillary waves.

We present a critical analysis of the density functional description for capillary wave fluctuations on free liquid surfaces. The proposal made by Mecke and Dietrich, [Phys. Rev. E 59, 6766 (1999)10.1103/PhysRevE.59.6766], to obtain the effective wave vector dependent surface tension, and their prediction of an enhanced regime of capillary waves at mesoscopic scales, has had a large impact including claims of experimental observation [Fradin, Nature (London) 403, 871 (2000)10.1038/35002533; Mora, Phys. Rev. Lett. 90, 216101 (2003)10.1103/PhysRevLett.90.216101]. Our analysis shows that there is a qualitative problem in the convergence of the low q expansion used for that prediction, and that the assumed link between the equilibrium density functional description of the liquid surface and its capillary wave fluctuations leads always to the unphysical decrease of the surface tension for large wave vectors.

Dynamical correlations in Brownian hard rods.

We analyze the time decay of small amplitude density perturbations in systems of highly packed Brownian hard rods, relaxing towards a uniform density distribution. The results of Brownian dynamics simulations and those of the deterministic dynamic density functional (DDF) theory, are contrasted with a new theoretical approach beyond the DDF assumptions. We characterize dynamical correlation modes which, having the lowest relaxation time, determine the late time evolution of the system. The spectrum of possible time decays has a continuous band structure, with pockets of discrete values, near the minima of the DDF results, where the validity of that theory appears to be well established.

Density functional study of layering at liquid surfaces.

We analyze the density profiles for liquid-vapor interfaces within two density functional (DF) approximations, applied to simple fluid models which have low ratios between their triple and critical temperatures. The observation of layering structures at low T is discussed in relation with the Fisher-Widom line for each model. Although we find no apparent correlation between the amplitude of the oscillatory density decay mode and the approach to T(FW) , that temperature sets a threshold for the generation of nonmonotonic structures within a fixed distance of the interface. The rapid decay of the oscillatory mode amplitude with T may be interpreted as a result of the capillary wave (CW) damping of strongly structured intrinsic density profiles. The layering in the presence of gravitylike external fields indicate that the effective transverse size which might be built in the DF approximations is around 10+/-2 molecular diameters; however, that interpretation has to allow for an effectively reduced damping exponent, i.e., an effective surface tension for the CW Hamiltonian which is larger than the value obtained directly from the DF grand-potential minimization.

Intrinsic profiles beyond the capillary wave theory: a Monte Carlo study.

We develop and test an operational definition of the intrinsic surface for liquid-vapor interfaces. The application to the microscopic configurations along Monte Carlo computer simulations gives the statistical properties of the intrinsic surfaces and the intrinsic density profiles for simple fluid models. The results open a framework of quantitative description to close the gap between the mesoscopic capillary wave theory and the sharpest level of resolution for the intrinsic density distribution, relative to the first atomic layer in the liquid surface, as done in the interpretation of experimental x-ray reflectivity.

Dynamic density functional study of a driven colloidal particle in polymer solutions.

The dynamic density functional (DDF) theory and standard Brownian dynamics simulations (BDS) are used to study the drifting effects of a colloidal particle in a polymer solution, both for ideal and interacting polymers. The structure of the stationary density distributions and the total induced current are analyzed for different drifting rates. We find good agreement with the BDS, which gives support to the assumptions of the DDF theory. The qualitative aspect of the density distribution are discussed and compared to recent results for driven colloids in one-dimensional channels and to analytical expansions for the ideal solution limit.

Layering at free liquid surfaces.

We show that simple liquids, with appropriate choices of the isotropic pair interaction, may exhibit surface layering above the melting temperature. Results for the liquid surface have been obtained by Monte Carlo simulations in slab geometry. Surface layering appears at temperatures below approximately one-third of the critical temperature for very different choices of pair interaction. The high melting temperature of the Lennard-Jones crystal preempts the observation of the oscillatory density at the free liquid interface, while model pair interaction potentials, built to reproduce some properties of mercury and the alkali metals, have low melting temperature, uncovering the region of surface layering.

Aggregation models at high packing fraction

Dense phases of micellar aggregates have strong molecular correlation at two different levels: that of the molecules forming a micelle and that between micelles, leading to a possible phase transition from a micellar fluid to a micellar crystal. The global phase diagram may also include lamellar and other dense phases, which do not have a micellar structure. We present here a generic approach to deal with these systems through a two-level density-functional description, to first describe an isolated micellar aggregate and then the dense micellar system, obtaining the free energy in a self-consistent way from the molecular interactions. Nonmicellar dense phases are included with the same density-functional approach applied at the first level. The results are shown to be very accurate for a one-dimensional model with exact solution, and the method is then applied to a three-dimensional amphiphile model that had been successfully used to describe the properties of diluted amphiphile solutions.

Density functional for hard sphere crystals: A fundamental measure approach

A new free energy density functional for hard spheres is presented, along the lines of the fundamental measure theory, which reproduces the Percus-Yevick equation of state and direct correlation function for the fluid, with a tensor weighted density. The functional, based on the zero-dimension limit, is exact for any one-dimensional density distribution of the spheres. The application to the hard sphere crystals gives excellent results, solving all of the qualitative problems of previous density functional approximations, including the unit cell anisotropy in the fcc lattice and the description of the metastable bcc lattice.

Lattice-gas model driven by Hubbard electrons.

Self-consistent Monte Carlo simulations are undertaken for a lattice-gas model which is driven by the free energy of electrons described by a Hubbard model with electronic hopping restricted to ions at nearest-neighbor sites. Our previous work, an independent-electron tight-binding lattice-gas model (bcc or fcc), is modified to introduce two effects: the disorder of the dense system and, more importantly, the role of the electronic correlation. The first effect is achieved using an fcc lattice, but restricted so an occupied site can have no more than eight, instead of twelve, occupied nearest-neighbor sites. To treat correlations, the electronic intra-atomic repulsion is, at first, included via the Gutzwiller approximation at finite temperature; this approach is simple enough to be solved for all cases in the large, disordered systems used in our Monte Carlo simulations but can still give a good qualitative representation of the main effects of the electronic correlations. Then, the exact treatment of the Hubbard model for clusters with up to six atoms is integrated into the calculation. We obtain vapor-liquid coexistence curves and then, approximations to the electronic conductivities and paramagnetic susceptibilities at coexistence conditions. This simple model is, in part, motivated by experiments on the alkali-metal fluids.

Fracture of the manubrium with posterior displacement of the clavicle and first rib. A case report.

This report describes a patient with a fracture of the manubrium of the sternum of a type which has not been reported previously. We discuss the incidence, the mechanism of production and the site of these fractures, and their treatment and prognosis.

Melatonin levels in Parkinson's disease: drug therapy versus electrical stimulation of the internal globus pallidus.

The objective of our work was to measure plasma melatonin levels in patients with Parkinson's Disease (PD) following electrical stimulation of the internal globus pallidus (GPi), and to compare these levels with groups of PD patients under drug therapy and healthy controls. The levels of melatonin were measured twice daily at 1000 and 1200. The GPi stimulation at 130 Hz lowered melatonin levels, while no changes were observed in the absence of stimulation. The melatonin levels from healthy subjects were lower than those observed in PD patients. The melatonin levels from PD patients under drug therapy were also measured during the night (2000-2400-0400) and at 0800 in order to observe their circadian changes. The Internal Globus Pallidus (GPi) stimulation was effective in lowering the melatonin levels during the day and, therefore returned these levels to those observed in normal subjects.