Presence Of Attractive Forces Research Articles

The effect of rotational and translational energy exchange on tracer diffusion in rough hard sphere fluids

A study is presented of tracer diffusion in a rough hard sphere fluid. Unlike smooth hard spheres, collisions between rough hard spheres can exchange rotational and translational energy and momentum. It is expected that as tracer particles become larger, their diffusion constants will tend toward the Stokes-Einstein hydrodynamic result. It has already been shown that in this limit, smooth hard spheres adopt "slip" boundary conditions. The current results show that rough hard spheres adopt boundary conditions proportional to the degree of translational-rotational energy exchange. Spheres for which this exchange is the largest adopt "stick" boundary conditions while those with more intermediate exchange adopt values between the "slip" and "stick" limits. This dependence is found to be almost linear. As well, changes in the diffusion constants as a function of this exchange are examined and it is found that the dependence is stronger than that suggested by the low-density, Boltzmann result. Compared with smooth hard spheres, real molecules undergo inelastic collisions and have attractive wells. Rough hard spheres model the effect of inelasticity and show that even without the presence of attractive forces, the boundary conditions for large particles can deviate from "slip" and approach "stick."

Miscibility studies of PVC/PMMA and PS/PMMA blends by dilute solution viscometry and FTIR

The effect of polymer-polymer interactions on the miscibility of PVC/PMMA and PS/PMMA blends were studied in a broad composition range using viscosity and FTIR techniques. From the viscometric data, the presence of attractive forces among different polymers was evaluated according to Sun et al. (1990), by the determination of α parameter. Based on sign convention involved in the criterion, PS/PMMA system was found as immiscible while miscibility was observed in PVC/PMMA blend. FTIR analysis also supports the viscometric data. Key words: Viscosity, polymer blends, poly (methyl methacrylate), poly (vinyl chloride), polystyrene, fourier transform infrared spectroscopy.

Microrheology of Aggregated Emulsion Droplet Networks, Studied with AFM−CSLM

We studied the mechanical behavior of densely packed (up to approximately 30% v/v), sedimented layers of (1 microm) water-in-oil W/O emulsion droplets, upon indentation with a (10 microm) large spherical probe. In the presence of attractive forces, the droplets form solid like networks which can resist deformation. Adding a polymer to the oil phase was used to control droplet attraction. The droplet layers were assembled via normal gravity settling. Considering that both the network structure and the droplet interactions play a key role, we used a combination of atomic force microscopy (AFM) and confocal scanning laser microscopy (CSLM) to characterize the mechanical behavior. Here the AFM was used both as indentation tool and as force sensor. Indentation experiments were performed via a protocol consisting of approach, waiting, and retract stages. CSLM was used to observe the network structure at micron resolution in real time. Use of refractive index matched fluorescent droplets allowed the visualization of the entire layer. Upon compression with the probe, a markedly nonhomogeneous deformation occurred, evidenced by the formation of a dense corona (containing practically all of the displaced droplets) in the direct vicinity of the probe, as well as more subtle deformations of force-chains at larger distances. Upon decompression, both the imprint of the indenter and the corona remained, even long after the load was released. The force-distance curves recorded with the AFM correspond well to these observations. For each deformation cycle performed on fresh material, the retract curve was much steeper than the approach curve, thus corroborating the occurrence of irreversible compaction. Contrary to classic linear viscoelastic materials, this hysteresis did not show any dependence on the deformation speed. Our force-indentation approach curves were seen to scale roughly as F approximately delta(3/2). The pre-factor was found to increase with the polymer concentration and with the density of the network. These findings suggest that this new AFM-CSLM method could be used for rheological characterization of small volumes of "granular networks" in liquid. Our hypothesis that the mechanical resistance of the networks originates from interdroplet friction forces, which in turn are set by the interdroplet potential forces, is supported by the predictions from a new mechanical model in which the interdroplet bonds are represented by stick-slip elements.

Miscibility studies of PVC blends (PVC/PMMA and PVC/PEO) based polymer electrolytes

The miscibility of PVC blends (PVC/PMMA and PVC/PEO) was investigated. Experiments using viscosimetry and differential scanning calorimetry (DSC) were performed. The presence of attractive forces among polymers was evaluated according to the Sun theory by the determination of the α parameter from the viscosimetric data. The melting depression in a binary blend is an indication of miscibility. Viscosimetric and thermal analysis showed that the PVC/PMMA and PVC/PEO blends are miscible. The miscibility of the PVC/PMMA and PVC/PEO blends is explained in terms of donor–acceptor interactions between chlorine atoms (a weak acceptor species) of PVC and oxygen atoms (a donor species) of the PMMA and PEO.

Miscibility of PVC/PEO blends by viscosimetric, microscopic and thermal analyses

The miscibility of PVC with PEO, PVC/PEO blends, was investigated by viscosimetric, microscopic and thermal analyses. Three samples of PVC with different molecular weights were used. The presence of attractive forces among the polymers was evaluated according to the Sun theory by determination of the α parameter from the viscosimetric data. Thermal analyses were based on the depression of the melting temperature. Viscosimetric and thermal results showed that the PVC/PEO blends are miscible. However, the existence of PVC rich domains, smaller than 0.1 μm in blends of PVC/PEO, was observed by microscopic analysis. Polymer–polymer interaction parameters, determined by depression of the melting temperature are negative and showed to be dependent on the molecular weight of PVC. The miscibility of the PVC/PEO blend was explained as a result of donor–acceptor interactions between chlorine atoms of PVC, as a weak acceptor species, and oxygen atoms of the PEO, as a donor species.

ADSORPTION OF A SIMPLE FLUID IN A QUENCHED TWO-COMPONENT HARD SPHERE MATRIX REPLICA ORNSTEIN–ZERNIKE EQUATIONS

Using the replica Ornstein–Zernike (ROZ) integral equations, we investigate the adsorption of a simple fluid in a hard sphere quenched matrix made of two species. Our main focus is in the dependencies of the density of fluid species on the chemical potential in matrices with different microporosity and for several compositions. The simple fluid is considered by using a hard sphere model. The fluid-matrix interactions are assumed either solely repulsive or attractive of the Yukawa type. The ROZ equations are supplemented by both the Percus–Yevick (PY) and the hypernetted chain (HNC) closures. The PY closure is used to study the model with solely repulsive forces (reference system) and then the contribution of attractive forces into adsorption is included in the mean field approximation. On the other hand, the HNC approximation is used to get insight into the structure of adsorbed fluid and the fluid-matrix correlations in the presence of attractive forces.

The adsorption of nickel dimethylglyoxime complex on mercury

The adsorption of the Ni(II) dimethylglyoxime complex, Ni(DMGH) 2, on mercury was studied as a function of time by voltammetric and differential capacitance methods in ammonia and borate buffers. The influence of the adsorption potential and the complex concentration was also studied. The adsorption equilibrium shows a reversible behaviour. The experimental data conform to the Frumkin adsorption isotherm. The standard electrochemical Gibbs energy of adsorption and the interaction parameter obtained from the fit indicate the presence of attractive forces between adsorbed molecules and a standard electrochemical Gibbs energy of adsorption comparable with those of pentanol and hexanol. The experimental rate of adsorption, at short times, is independent of time. It is also linearly dependent on the complex concentration and exponentially dependent on the adsorption potential. The experimental rate law can be explained with a model considering the adsorption kinetics as being controlled by the adsorption of the complex.

Stacks and bunches of fluid membranes

Fluid membranes such as lipid bilayers often form oriented stacks or bunches. Within these bunches, the direct molecular forces are renormalized by thermally excited shape fluctuations. For purely repulsive forces between the membranes, this renormalization depends only weakly on the number N of the membranes within the bunch. In the presence of attractive forces, these bunches undergo continuous unbinding transitions. For freely suspended bunches, all membranes unbind simultaneously at a unique N-independent temperature T*s, but the effective critical behaviour depends on N over many length scales. For bunches that adhere to another surface, the membranes can peel off one after another in a sequence of transitions that occur in a finite temperature range above T*s.

The collision rate of small drops in linear flow fields

A dilute dispersion containing small, force-free drops of one fluid dispersed in a second, immiscible in a linear flow field is considered for small Reynolds numbers and large Péclet numbers under isothermal conditions. The emphasis of our analysis is on the effects of pairwise drop interactions on their collision rate, as described by the collision efficiency, using a trajectory analysis. Simple shear flow and uniaxial extensional or compressional flow are considered. For both flows, the collision efficiency decreases with increasing drop viscosity due to the effects of hydrodynamic interactions. It also decreases as the ratio of the smaller drop radius to the larger radius decreases. For uniaxial flow, finite collision rates are predicted in the absence of interdroplet forces for all finite values of the drop size ratio and the ratio of the viscosities of the drop and suspending medium. In contrast, several kinds of relative trajectories exist for a pair of drops in simple shear flow, including open trajectories, collision trajectories, and closed and semi-closed trajectories, in the absence of interdroplet forces. When the ratio of small to large drop diameters is smaller than a critical value, which increases with increasing drop viscosity, all of the relative trajectories that start with the two drops far apart remain open (no collisions), unless in the presence of attractive forces. Attractive van der Walls forces are shown to increase the collision rates.

Gravitational breather and topological properties of gravisolitons

It is shown that for a wide class of cosmological solutions to the vacuum Einstein equations the notions of gravisolitons and antigravisolitons with respect to some topological charge can be introduced. The presence of attractive forces between two gravitational solitons with charges of opposite signs and repulsive forces between solitons of the same charges is shown. The construction of the gravitational breather as a bound state of the gravisoliton and antigravisoliton is described.

On the role of attractive forces in the cage effect in simple liquids

Molecular dynamics calculations were performed in an attempt to characterize the mechanism by which attractive forces affect the velocity correlation function in simple Lennard-Jones liquids. The relationships between the presence of attractive forces, the rigidity of molecular cages, and the decay of velocity correlations were studied by molecular dynamics runs in which a single mobile particle moved in a rigid cage environment. Our results suggest that the principal role of the attractive forces is to enhance the cohesiveness of the cages and their ability to confine particles within them.

Numerical Experiments on the Stochastic Behavior of a Lennard-Jones Gas System

It is known that the hard-sphere gas exhibits strongly stochastic properties. For it, most initially close phase-space-trajectory pairs separate exponentially with time, and Sinai has used this so-called $C$-system behavior to develop a rigorous proof of ergodicity and mixing in the hard-sphere gas. In this paper we numerically integrate the equations of motion for a Lennard-Jones gas, which has attractive as well as repulsive interparticle forces, and we demonstrate that this exponential separation of initially close trajectory pairs persists even in the presence of attractive forces over a fairly wide range of particle densities. In our calculations, this range extends from the dilute-gas region up to densities at which three-body and four-body collisions become significant. Moreover for this density range, we show that an expression for the trajectory-pair rate of exponential separation, rather crudely derived for a hard-sphere gas, fits the empirical Lennard-Jones data quite nicely. Our evidence thus indicates that a gas system with attractive forces can exhibit $C$-system behavior similar to that of the hard-sphere gas. Finally we point out that the exponential separation of trajectories as empirically observed here involves an unusual type of correlated collision sequence.

Summability of the Many-Fermion Perturbation Series with Attractive Forces

The summability of the many-fermion perturbation series in the presence of attractive forces is considered. We find that, although it is an asymptotic series, the potential-strength perturbation series is summable. We find that some rearrangements to handle the hard-core potentials also lead to the correct sum; however, the ``hole-line'' summation procedure using Brandow's choice for intermediate state energies is either wrong or deceptive in its convergence.