Solvation forces in branched molecular liquids

The solvation force of squalane confined between a silicon tip and a graphite surface has been measured by atomic force microscopy. This highly branched molecule shows oscillatory force profiles similar to those of spherical and linear chain molecules. Squalane molecules closest to the substrate are tightly bound and finer details imply that interdigitation occurs. This agrees with computer simulations for branched molecules but differs qualitatively from force balance experiments. These differences arise from the smaller confinement area and the different chemical nature of the surfaces.     

Buoyancy forces in magnetic fluids

The ponderomotive force on a macroscopic body in a magnetic fluid is calculated by a hydrodynamic approach. The resulting equations are generally valid, neither small susceptibilities nor stationarity are assumed. The simple and widely-used formulaV(M-M ^bg )H is recoverd in linear order of ; magnetostrictive effects are shown to contribute in the order ³. The expressions derived here are definite and unambiguous, they do not depend on whether one starts from a theory in terms ofH, or in terms ofB: the correct evaluation of the contribution dV[-p] resolves the apparent contradiction between the force density expressions ₀ MH, orMB.     

Solvation force in hard ellipsoid fluids with HNW interaction

In present work, using density functional theory and extended restricted orientation model, the one particle density of hard Gaussian overlap fluid near the colloid walls is calculated. The hard needle–wall interaction between molecules and colloids are considered. Using non-linear equation, proposed by Grimson–Rickyazen, the solvation force of hard ellipsoidal molecular fluid with hard Gaussian overlap interaction is calculated. We could not find the exact or simulation results for comparison. The results in the case k = 2.0 are compared with the solvation force of one-dimensional hard rod fluids. The results are corresponded, qualitatively.     

Fluctuation-induced casimir forces in granular fluids

We numerically investigate the behavior of driven noncohesive granular media and find that two fixed large intruder particles, immersed in a sea of small particles, experience, in addition to a short-range depletion force, a long-range repulsive force. The observed long-range interaction is fluctuation-induced and we propose a mechanism similar to the Casimir effect that generates it: The hydrodynamic fluctuations are geometrically confined between the intruders, producing an unbalanced renormalized pressure. An estimation based on computing the possible Fourier modes explains the repulsive force and is in qualitative agreement with the simulations.     

Effective potential for three-body forces in fluids

We develop a model for an effective Axilrod–Teller–Muto (ATM) triple-dipole interaction based on accurate calculations of three-body effects on the third virial coefficient. The effective ATM interaction is written as a two-body density-dependent potential and is obtained by averaging the ATM function over the position of the third particle. It is shown that the addition of the mean ATM potential does not affect much the form of the binary interaction and so it can be incorporated as an effect on the minimum of the potential (position and depth). The underlying binary potentials are modelled by Approximate Non-Conformal (ANC) functions that have been proven to be highly accurate in accounting for effective pair interactions in many fluids of interest. The final total effective potential, binary plus ternary, is expressed in terms of the same ANC functions. The adequacy of the effective three-body force thus found is tested by looking at the pressure and specific heat of various fluids, formed by small nonpolar molecules, in the region of moderate densities where a third-virial approximation is reliable, and then comparing them against experimental results. The critical temperatures and volumes of those fluids are also calculated and the three-body effects on them are assessed.     

Self-diffusion in fluids with weak long-range forces

Diffusion of a test particle in a homogeneous classical fluid with weak long-range forces is studied. The dominant mean-field effect (Vlasov's theory) vanishes for symmetry reasons. Dynamical phenomena follow then from fluctuations of the effective potential energy felt by the propagating particle. The kinetic equation corresponding to this mechanism is derived with the use of the multiple-time-scale method. Its structure resembles very much that of the (linearized) Balescu-Lenard equation of hot plasma theory. It is shown that the kinetic equation holds only if no phase transition occurs in the system. The thermalization of the diffusing particle and the high-temperature and Lorentz gas limits are discussed.     

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.     

磁流体中Helmholtz和Kelvin力的界定

磁流体磁彻体力的两种简化形式Helmholtz力和Kelvin力具有一定的适用范围.在推导磁流体中的磁彻体力表达式基础上,分析Helmholtz力和Kelvin力在磁流体中的起源,得出两种形式的成立条件.计算结果表明:当磁流体磁导率与外磁场强度无关时,磁流体磁彻体力可由Helmholtz力表示;当磁流体中磁性颗粒的平均磁矩与磁流体比体积无关时,Kelvin力为磁彻体力的简化形式;在磁流体磁化系数与其密度成正比情况下,Helmholtz力可转换为Kelvin力. 关键词： 磁流体 磁彻体力 Helmholtz力 Kelvin力     

Decay of correlations in classical fluids with long-range forces

We show with simple arguments that, as a consequence of the Poisson equation, the correlations of a charged system at equilibrium decay faster than any inverse power, if they are integrable and monotonous at infinity. For all other longrange systems (with potential(x)b¦x¦^–s , ¦x¦ , 0v,s} 2), the decay is bounded below by an inverse power.Partially supported by the Swiss National Foundation for Scientific Research.     

Normal forces of magnetorheological fluids under oscillatory shear

The normal forces of magnetorheological fluids under oscillatory shear are investigated by a commercial magneto-rheometer with plate-plate geometry. At the constant strain amplitude and frequency, the normal forces almost keep a steady value with the testing time if the strain amplitude is smaller than the critical value. When a larger strain is applied, they will fluctuate periodically. Under the strain sweep mode, the relationships between normal forces and strain amplitude can be divided into three regions: linear viscoelastic region, nonlinear viscoelastic region and the viscoplastic region. Under the frequency sweep method, it is found that the angular frequency show little influence on the normal forces. At last, the normal forces increase with increasing of the temperature under a low magnetic field, while they decrease under a high magnetic field.     

Effective pair potentials in fluids in the presence of three-body forces

The physical properties of a fluid in which there is only a two-body potential, u_αβ , can be expressed in terms of the total correlation function, h ₁₂, which is a sum of all connected graphs with root-points on molecules 1 and 2, whose links are f bonds, where f_αβ = exp (-u_αβ/κT) - 1. It is shown that the total correlation function in the presence of a weak three-body potential, u_αβγ , is h ₁₂*, where h ₁₂* is the sum of all two-body connected graphs in which each f bond is replaced in turn by an f* bond, where

and where ? is a sub-set of the elementary graphs each of which contains one f_αβγ link. We call this sub-set the line-irreducible graphs, and its leading term is a graph discussed by Rushbrooke and Silbert.

The three-body potential is set equal to the dipole-dipole-dipole potential of Axilrod and Teller, and the analytic properties and numerical values of the first term ?₁ examined in detail.

Other effective potentials have been defined and the relations between them are elucidated. In particular it is shown that the first term in ? cannot be used to obtain the effective link f* at liquid densities, but that it can be compared with the dependence on density of the effective potential u* obtained by Mikolaj and Pings from the x-ray diffraction of compressed argon.     

Effective pair potentials in fluids in the presence of three-body forces. II

Earlier calculations are extended to the second order in density thus providing the non-additive contributions to the fourth virial coefficient in the presence of a weak three-body force given by the triple-dipole dispersion potential of Axilrod and Teller. We compare the Percus-Yevick approximations of Rushbrooke and Silbert and of Rowlinson at the level of the fourth virial coefficient and find that the former is more accurate. We also compare our calculations of the effective pair potential of liquid argon with the results of Mikolaj and Pings obtained from X-ray diffraction measurements.     

Effect of short- and long-range forces on the properties of fluids. III. Dipolar and quadrupolar fluids

Using realistic pair potential models for acetone and carbon dioxide, both the spatial and orientational structure of these two typical multipolar (i.e. dipolar and quadrupolar, respectively) fluids is investigated in detail by computing the complete set of the site-site correlation functions, multipole-multipole correlation functions, and selected 2D correlation functions. The effect of the range of interactions on both the structural and thermodynamic properties of these fluids is studied by decomposing the potential into short- and long-range parts in the same manner as for water [Kolafa, J. and Nezbeda, I., 2000, Molec. Phys., 98, 1505; Nezbeda, I. and Lísal, M., 2001, Molec. Phys., 99, 291]. It is found that the spatial arrangement of the molecules is only marginally affected by the long-range forces. The effect of the electrostatic interactions is significant at short separations and cannot be neglected but nevertheless the overall structure of the short-range and full systems is similar as well as their dielectric constants. These findings are also reflected in the dependence of the thermodynamic properties on the potential range with the short-range models providing a very good approximation to those of the full system.     

Dynamical pair correlations of classical and quantum fluids perturbed with weak long-range forces

We study time dependent correlation functions of ideal classical and quantum gases using methods of equilibrium statistical mechanics. The basis for this is the path integral formalism of quantum mechanical systems. By this approach the statistical mechanics of a quantum mechanical system becomes the equivalent of a classical polymer problem in four dimensions where imaginary time is the fourth dimension. Several non-trivial results for quantum systems have been obtained earlier by this analogy. Here we will focus upon particle dynamics. First ideal gases are considered. Then interactions, that are assumed weak and of long range, are added, and methods of classical statistical mechanics are applied to obtain the leading contribution. Comparison is performed with known results of kinetic theory. These results demonstrate how methods developed for systems in thermal equilibrium also is applicable outside equilibrium. Thus, more generally, we have reason to expect that these methods will be accurate and useful for other situations of interacting many-body systems consisting of quantized particles too. To indicate so we sketch the computation of the induced Casimir force between parallel plates filled with ions for the situation where the ions are quantized, but the interaction remains electrostatic. Further in this respect we establish expressions for a leading correction to ab initio calculations for the energies of the quantized electrons of molecules. To our knowledge these two latter applications go beyond earlier results.     

Solvation of large dipoles

A model system consisting of one strongly polar molecule dissolved in a pool of 499 moderately polar particles is studied in a molecular dynamics experiment. A pair potential of Stockmayer type is used. The structure of the solution has been analysed in terms of g(R), f(R), h _Δ(R), h _D(R), g(R, x), and Voronoi polyhedra. Solvent-solvent spatial correlations are presented and compared to solute-solvent properties already discussed in part I. Voronoi polyhedra analysis is also used to define structural relaxation around solute or solvent particles unambiguously. The dynamics of the system is described by means of angular momentum, orientational, and velocity autocorrelation functions. The solute's influence on the dynamics of the solvent is analysed by a polyhedral decomposition of time correlation functions.     

Effect of the attractive forces on the density change melting of 2D LJ fluids

The Weeks-Chandler-Andersen perturbation theory of fluids can be considered as a modern version of van der Waals'. Whereas in the old van der Waals theory the effect of the attractive forces on the thermodynamic properties is introduced through a constant, a, in Weeks-Chandler-Andersen theory this constant is replaced by a function of temperature and density α(β, ϱ). This function has been determined by means of Molecular Dynamics simulation of a two-dimensional Lennard-Jones fluid, and here is used to analyse the effect of attractive forces on the density change in melting. The influence is found to be slightly greater than the predictions of van der Waals theory.     

Physical Modeling of Aqueous Solvation

We consider the free energies of solvating molecules in water. Computational modeling usually involves either detailed explicit-solvent simulations, or faster computations, which are based on implicit continuum approximations or additivity assumptions. These simpler approaches often miss microscopic physical details and non-additivities present in experimental data. We review explicit-solvent modeling that identifies the physical bases for the errors in the simpler approaches. One problem is that water molecules that are shared between two substituent groups often behave differently than waters around each substituent individually. One manifestation of non-additivities is that solvation free energies in water can depend not only on surface area or volume, but on other properties, such as the surface curvature. We also describe a new computational approach, called Semi-Explicit Assembly, that aims to repair these flaws and capture more of the physics of explicit water models, but with computational efficiencies approaching those of implicit-solvent models.