Hydrodynamic fluctuation forces

We consider the fluctuating hydrodynamics of Landau and Lifshitz for a fluid confined by hard walls at finite distance. By considering the non-linearity of the stochastic fluid equations of motion, we show that there can be an inhomogeneous average stress set up throughout the fluid. The average stress corresponds to a force density on the fluid which is expressed in terms of the Green's function for the fluid in the linearized theory. For simple geometries we obtain the average stress explicitly as a long range pressure field. The effect can be interpreted as a long range effective force acting between the fluid boundaries. In this sense it might have observable consequences in thin films or in suspensions of hard colloid particles. The effect is strongest in incompressible fluids. It is greatly weakened by compressibility but relaxation of the fluid viscosity prevents the effect vanishing.     

First-principles theory of van der Waals forces between macroscopic bodies

We present a first-principles method for the determination of the van der Waals interactions for a collection of finite-sized macroscopic bodies. The method is based on fluctuational electrodynamics and a rigorous multiple-scattering method for the electromagnetic field. As such, the method takes fully into account retardation, many-body, multipolar, and near-fields effects. By application of the method to the case of two metallic nanoparticles, we demonstrate the breakdown of the standard 1/r(2) distance law as the van der Waals force decays exponentially with distance when the nanoparticles are too close or too far apart.     

Laser-induced forces in metallic nanosystems: the role of plasmon resonances

We present calculations of the laser-induced force between metallic nanospheres, similar and dissimilar in character, and that between a metallic nanosphere and a planar surface. When the separation between these objects is in the 0.5-2 nm range, we find very strong resonances in the laser-induced force associated with excitation of plasmon resonances. Measurement of such forces will provide direct access to the plasmon enhancements of laser fields so critical to optical spectroscopy in the nanoenvironment.     

First-principles phase-coherent transport in metallic nanotubes with realistic contacts

We present first-principles calculations of phase coherent electron transport in a carbon nanotube (CNT) with realistic contacts. We focus on the zero-bias response of open metallic CNT's considering two archetypal contact geometries (end and side) and three commonly used metals as electrodes (Al, Au, and Ti). Our ab initio electrical transport calculations make, for the first time, quantitative predictions on the contact transparency and the transport properties of finite metallic CNT's. Al and Au turn out to make poor contacts while Ti is the best option of the three.     

Process designing for laser forming of circular sheet metal

Laser forming is a new type of flexible manufacturing process that has become viable for the shaping of metallic components. Process designing of laser forming involves finding a set of process parameters, including laser power, laser scanning paths, and scanning speed, given a prescribed shape. To date, research has focused on process designing for rectangular plates, and only a few studies are presented for axis-symmetric geometries like circular plates. In the present study, process designing for axis-symmetric geometries-with focus on class of shapes-is handled using a formerly proposed distance-based approach. A prescribed shape is achieved for geometries such as quarter-circular and half-circular ring plates. Experimental results verify the applicability of the proposed method for a class of shapes.     

De Sitter and double irregular domain walls

A new method to obtain thick domain wall solutions to the coupled Einstein scalar field system is presented. The procedure allows the construction of irregular walls from well known ones, such that the spacetime associated to them are physically different. As consequence of the approach, we obtain two irregular geometries corresponding to thick domain walls with dS expansion and topological double kink embedded in AdS spacetime. In particular, the double brane can be derived from a fake superpotential.     

Inelastic scattering and local heating in atomic gold wires

We present a method for including inelastic scattering in a first-principles density-functional computational scheme for molecular electronics. As an application, we study two geometries of four-atom gold wires corresponding to two different values of strain and present results for nonlinear differential conductance vs device bias. Our theory is in quantitative agreement with experimental results and explains the experimentally observed mode selectivity. We also identify the signatures of phonon heating.     

Dynamics of a polymer chain confined in a membrane

We present a Brownian dynamics theory with full hydrodynamics (Stokesian dynamics) for a Gaussian polymer chain embedded in a liquid membrane which is surrounded by bulk solvent and walls. The mobility tensors are derived in Fourier space for the two geometries, namely, a free membrane embedded in a bulk fluid, and a membrane sandwiched by the two walls. Within the preaveraging approximation, a new expression for the diffusion coefficient of the polymer is obtained for the free-membrane geometry. We also carry out a Rouse normal mode analysis to obtain the relaxation time and the dynamical structure factor. For large polymer size, both quantities show Zimm-like behavior in the free-membrane case, whereas they are Rouse-like for the sandwiched membrane geometry. We use the scaling argument to discuss the effect of excluded-volume interactions on the polymer relaxation time.     

Geometry and spectrum of Casimir forces

We present a new approach to the Helmholtz spectrum for arbitrarily shaped boundaries and general boundary conditions. We derive the boundary induced change of the density of states in terms of the free Green's function from which we obtain nonperturbative results for the Casimir interaction between rigid surfaces. As an example, we compute the lateral electrodynamic force between two corrugated surfaces over a wide parameter range. Universal behavior, fixed only by the largest wavelength component of the surface shape, is identified at large surface separations, complementing known short distance expansions which we also reproduce with high precision.     

A generalized wall boundary condition for smoothed particle hydrodynamics

In this paper we present a new formulation of the boundary condition at static and moving solid walls in SPH simulations. Our general approach is both applicable to two and three dimensions and is very simple compared to previous wall boundary formulations. Based on a local force balance between wall and fluid particles we apply a pressure boundary condition on the solid particles to prevent wall penetration. This method can handle sharp corners and complex geometries as is demonstrated with several examples. A validation shows that we recover hydrostatic equilibrium conditions in a static tank, and a comparison of the classical dam break simulation with state-of-the-art results in literature shows good agreement. We simulate various problems such as the flow around a cylinder and the backward facing step at Re = 100 to demonstrate the general applicability of this new method.     

Repulsive and attractive critical Casimir forces

When confining vacuum fluctuations between two identical walls, the Casimir force manifests itself as a mutual attraction of the walls. When confining concentration fluctuations of a binary liquid mixture, an analogous force should exist near the critical temperature T_C; it is called the critical Casimir force. Here we show experimentally that this purely entropic force can be either attractive or repulsive, depending on the boundary conditions for the fluctuations. For symmetrical boundary conditions an attractive force is found while asymmetrical ones lead to a repulsive force. This is observed directly by confining the fluctuations in a thin wetting film. Depending on the boundary conditions either a thinning or a thickening of the film is observed when T→T_C.     

Macroscopic coherent rectification in Andreev interferometers

We investigate nonlinear transport through quantum coherent metallic conductors contacted to superconducting components. We find that in certain geometries, the presence of superconductivity generates a large, finite-average rectification effect. Specializing to Andreev interferometers, we show that the direction and magnitude of rectification can be controlled by a magnetic flux tuning the superconducting phase difference at two contacts. In particular, this results in the breakdown of an Onsager reciprocity relation at finite bias. The rectification current is macroscopic in that it scales with the linear conductance, and we find that it exceeds 5% of the linear current at sub-gap biases of a few tens of microelectronvolts.     

Extremal black holes and holographic c-theorem

We found Bogomol?nyi type of the first order differential equations in three-dimensional Einstein gravity and the effective second order ones in new massive gravity when an interacting scalar field is minimally coupled. Using these equations in Einstein gravity, we obtain analytic solutions corresponding to extremally rotating hairy black holes. We also obtain perturbatively extremal black hole solutions in new massive gravity using these lower order differential equations. All these solutions have the anti-de Sitter spaces as their asymptotic geometries and as the near horizon ones. This feature of solutions interpolating two anti-de Sitter spaces leads to the construction of holographic c-theorem in these cases. Since our lower order equations reduce naturally to the well-known equations for domain walls, our results can be regarded as the natural extension of domain walls to more generic cases.     

Spherical and non-spherical bubbles in cosmological phase transitions

The cosmological remnants of a first-order phase transition generally depend on the perturbations that the walls of expanding bubbles originate in the plasma. Several of the formation mechanisms occur when bubbles collide and lose their spherical symmetry. However, spherical bubbles are often considered in the literature, in particular for the calculation of gravitational waves. We study the steady state motion of bubble walls for different bubble symmetries. Using the bag equation of state, we discuss the propagation of phase transition fronts as detonations and subsonic or supersonic deflagrations. We consider the cases of spherical, cylindrical and planar walls, and compare the energy transferred to bulk motions of the relativistic fluid. We find that the different wall geometries give similar perturbations of the plasma. For the case of planar walls, we obtain analytical expressions for the kinetic energy in the bulk motions. As an application, we discuss the generation of gravitational waves.     

An experimental confirmation of longitudinal electrodynamic forces

According to the conventional views of electromagnetic theory, as these are expressed in the Lorentz force law, all the forces which act on a current carrying metallic conductor are perpendicular to the current streamlines. However, over the years, from Ampère through Maxwell until the present day, there have been persistent claims that when current flows in a metallic conductor, there are mechanical forces acting along current streamlines which subject the conductor to tensile stress, and which are therefore capable of performing work in the direction of current flow. The problem of substantiating these claims has always lain in the difficulty of designing an experiment in which the effects are unambiguously demonstrated. The present paper describes an experiment which to a large extent removes these ambiguities, and which provides a compelling novel demonstration of forces acting along current streamlines. A force calculation based on Ampère's original electrodynamic force law is found to be consistent with the observed behaviour. Received 15 November 2000 and Received in final form 12 March 2001     

Use of an anisotropic absorber for simulating electromagnetic scattering by a perfectly conducting wire

A perfectly matched anisotropic absorber (PMAA) is used as a new type of medium in a formalism previously developed for investigating diffraction from an inhomogeneous isotropic aperture in a thick metallic screen. In this paper we consider that the aperture consists of five homogeneous regions: PMAA, transparent dielectric, perfect conductor, transparent dielectric and PMAA. This configuration could allow us to simulate a conducting cylinder of rectangular section in open space and it sets the basis for further extensions of the method to other geometries. We present near and far field simulations for different geometrical and constitutive parameters of the PMAA regions. The results seem to indicate that this new medium could be used to represent a lateral open space, thus enhancing the modal method and making it suitable to simulate scattering problems of finite objects.     

On near-wall effects in hard disk packing between two concentric cylinders

Monodisperse packing of disks in confined geometries is rarely addressed in the literature. We present some theoretical aspects of the microstructural properties of 2D static packing of disks between two concentric cylinders. In the core of the geometry, the packing is intact, while it is dilated in the vicinity of the walls as a result of the removal of overlapping disks. We have derived the formula for average packing density in concentric cylinders geometry with given ratio of the particle diameter to the gap between cylinders. It is verified and analyzed further by constructing ordered and disordered packs of monodisperse disks in computer. In this context, Voronoi tessellation is performed for the resulting packs to demonstrate the thickness of the static boundary layer where the packing is influenced by the presence of cylindrical walls.     

Linear and non-linear theories of solvation forces in fluids

From an exact expression for the free energy of a non-uniform classical fluid, due to Saam and Ebner, a closure is used to develop a non-linear theory for the density and solvation force between two planar walls. In the linear limit these expressions reduce to ones used successfully elsewhere. Numerical solution of the equations for a hard sphere fluid shows that while the density profiles predicted by the two theories are markedly different, the solvation forces are similar.     

Effect of number of walls on plasmon energy in interaction of charged particles with double walled metallic nanotubes

The interaction of charged particles with double-walled metallic nanotubes (DWMNTs) has been investigated theoretically. Numerical results for energy dispersion relations as a function of the wave vector are presented when the charged particle is located outside of the nanotube. Calculations show that there are four modes for each angular mode (m) in DWMNT. Calculations show that the plasmon energy of DWMNTs depends on interwall spacing between two walls.     

Simulations of counterions at charged plates

Using Monte Carlo simulations, we study the counterion distribution close to planar charged walls in two geometries: i) when only one charged wall is present and the counterions are confined to one half-space, and ii) when the counterions are confined between two equally charged walls. In both cases the surface charge is smeared out and the dielectric constant is the same everywhere. We obtain the counterion density profile and compare it with both the Poisson-Boltzmann theory (asymptotically exact in the limit of weak coupling, i.e. low surface charge, high temperature and low counterion valence) and the strong-coupling theory (valid in the opposite limit of high surface charge, low temperature and high counterion valence) and with previously calculated correction terms to both theories for different values of the coupling parameter, thereby establishing the domain of validity of the asymptotic limits. Gaussian corrections to the leading Poisson-Boltzmann behavior (obtained via a systematic loop expansion) in general perform quite poorly: At coupling strengths low enough so that the Gaussian (or one-loop) correction does describe the numerical deviations from the Poisson-Boltzmann result correctly, the leading Poisson-Boltzmann term by itself matches the data within high accuracy. This reflects the slow convergence of the loop expansion. For a single charged plane, the counterion pair correlation function indicates a behavioral change from a three-dimensional, weakly correlated counterion distribution (at low coupling) to a two-dimensional, strongly correlated counterion distribution (at high coupling), which is paralleled by the specific-heat capacity which displays a rounded hump at intermediate coupling strengths. For the case of counterions confined between two equally charged walls, we analyze the inter-wall pressure and establish the complete phase diagram, featuring attraction between the walls for large enough coupling strength and at intermediate wall separation. Depending on the thermodynamic ensemble, the phase diagram exhibits a discontinuous transition where the inter-wall distance jumps to infinity (in the absence of a chemical potential coupling to the inter-wall distance, as for charged lamellae in excess solvent) or a critical point where two coexisting states with different inter-wall distance become indistinguishable (in the presence of a chemical potential, as for charged lamellae with a finite fixed solvent fraction). The attractive pressure decays with the inter-wall distance as an inverse cube, similar to analytic predictions, although the amplitude differs by an order of magnitude from previous theoretical results. Finally, we discuss in detail our simulation methods and compare the finite-size scaling behavior of different boundary conditions (periodic, minimal image and open). Received 6 November 2001