Effect Of Intermolecular Forces Research Articles

Linear stability of an expanding spherical liquid film

As a simple model of processes relevant to generation of foams, we consider the stability of an expanding spherical liquid film. Primary influences on stability considered are destabilizing effects of intermolecular attractive forces in the liquid, competing with stabilizing effects of surface tension and its gradients resulting from perturbations to surfactant distribution. Linear stability and growth rates are determined for a film, such as a soap bubble, that is expanding by an increase in the volume of the interior gas. A bubble with fixed total interfacial surfactant is found to become increasingly unstable with expansion by virtue of film thinning and surfactant dilution; the expansion flow exerts a stabilizing influence, but this effect is negligible in practical situations.

Effect of surface texture on the flying characteristics of pico sliders

The effect of surface texture on the flying characteristics of proximity recording sliders is investigated numerically. The effect of texture on slider flying behavior is studied as a function of the characteristics of the texture and the contour design of the slider air bearing. The results show that texturing decreases the flying height. However, pitch and roll are generally increased. It is suggested that texturing can be used to reduce the effect of intermolecular forces by reducing the area over which intermolecular forces act.

Intermolecular force, surface roughness, and stability of head-disk interface

To achieve 1-Tb∕in.2 magnetic recording areal density, the head/disk spacing, or the flying height (FH) of the slider has to be reduced to 2–3nm and the disk surface roughness and the slider air bearing surface (ABS) roughness need to be well considered when the flying height is in such a regime. This paper presents authors’ efforts in further study of the intermolecular force effects with the surface roughness in consideration and the investigation of the contact force caused by the surface roughness effects. A probability model is built to simulate the intermolecular force and the contact force, and these two forces are introduced into the modified compressible Reynolds equation governing the air bearing pressure of the slider. The simulation results show that the effects of slider/disk roughness on the intermolecular force are negligible. On the other hand, surface roughness plays important roles in the contact force and the contact vibration. Reducing the roughness of both the slider and the disk surfaces is the best solution for reducing the contact force. Moreover, reducing the disk roughness can still significantly reduce the contact force, if the roughness reduction of the slider surface cannot be at the same pace of the disk surface roughness.

Effect of Intermolecular Forces on the Static and Dynamic Performance of Air Bearing Sliders: Part I—Effect of Initial Excitations and Slider Form Factor on the Stability

The mechanical spacing between the slider and the disk has to be reduced to less than 5 nm in order to achieve an areal density of 1Tbit∕in2. Certain physical phenomena, such as those that can be caused by intermolecular and surface forces, which do not have a significant effect at higher flying heights, become more important at such low head-media separations. These forces are attractive for head-media separation as low as 0.5 nm, which causes a reduction in the mechanical spacing as compared to what would be the case without them. Single degree of freedom models have been used in the past to model these forces, and these models have predicted unstable flying in the sub-5-nm flying height range. Changes in the pitch and the roll angles were not accounted for in such models. A 3-DOF air bearing dynamic simulator model is used in this study to investigate the effect of the intermolecular forces on the static and dynamic performance of the air bearing sliders. It is seen that the intermolecular forces increase the level of flying height modulations at low flying heights, which in turn results in dynamic instability of the system similar to what has also been observed in experiments. The effect of initial vertical, pitch, and roll excitations on the static and dynamic flying characteristics of the slider in the presence of the intermolecular forces has also been investigated. A stiffness matrix is defined to characterize the stability in the vertical, pitch, and roll directions. The fly height diagrams are used to examine the multiple equilibriums that exist for low flying heights. Finally, a study was carried out to compare the performance of pico and femto designs based on the hysteresis observed during the touchdown-takeoff simulations.

Effect of Intermolecular Forces on the Static and Dynamic Performance of Air Bearing Sliders: Part II—Dependence of the Stability on Hamaker Constant, Suspension Preload and Pitch Angle

Intermolecular and surface forces contribute significantly to the total forces acting on air bearing sliders for flying heights below 5 nm. Their contributions to the total force increase sharply with the reduction in flying height, and hence their existence can no longer be ignored in air bearing simulation for hard disk drives. Various experimentally observed dynamic instabilities can be explained by the inclusion of these forces in the model for low flying sliders. In this paper parametric studies are presented using a 3-DOF model to better understand the effect of the Hamaker constants, suspension pre load and pitch angle on the dynamic stability/instability of the sliders. A stiffness matrix is used to characterize the stability in the vertical, pitch, and roll directions. The fly height diagrams are used to examine the multiple equilibriums that exist for low flying heights. It has been found that the system instability increases as the magnitude of the van der Waals force increases. It has also been found that higher suspension pre load and higher pitch angles tend to stabilize the system.

Time-dependent free-surface thin film flows over topography

The lubrication equation governing free-surface thin film flows over topography is solved numerically including the effects of inertia and intermolecular forces. We study the initial value problem for a variety of initial conditions and perturbations and demonstrate that the free surface is strongly stable and can only be destabilized with large values of the dimensionless Hamaker’s constant and large amplitude free-surface perturbations, both of which are difficult to achieve in practice. The strong stability of thin film flows over topography is in agreement with the recent analysis by Kalliadasis and Homsy [J. Fluid Mech. 448, 387 (2001)].

A nonlinear stability analysis of pattern formation in thin liquid films

The development of spontaneous stationary equilibrium patterns in thin liquid films is investigated by means of a hexagonal-planform weakly nonlinear stability analysis applied to the appropriate governing evolution equation for this phenomenon. In the long-wavelength limit the mathematical system modeling the liquid film can be reduced to such a single nonlinear partial differential time-evolution equation describing the layer thickness on an unbounded two-dimensional spatial domain and including the effects of gravity, intermolecular forces, and temperature-dependent surface tension. The main result of this analysis is that supercritical equilibrium patterns can occur for an interval of mean layer thickness with subcritical rupture occurring outside that interval. These patterns consist of surface ridges and hexagonal network-like cells or close-packed configurations of nanodroplets separated by relatively flat ultra thin films. In particular those morphological phase separation patterns are generated by the coupling between the long-range attractive and short-range repulsive intermolecular forces with cells being stable for the thicker layers; nanodroplets, for the thinner ones; and ridges, for layers of intermediate thickness. These theoretical predictions are in accord with both relevant experimental evidence involving thin liquid polymer, crystal, and metal films coating a solid substrate and numerical simulations of similar model equations as well as being consistent with dewetting-type rupture occurring for such situations by hole formation in relatively thick layers but by drop formation in thinner ones.

Three-Dimensional Nonlinear Rupture Theory of Thin Liquid Films on a Cylinder

The three-dimensional nonlinear rupture theory of thin liquid films on a cylinder is presented and studied in this note. The thin liquid film with the effect of intermolecular forces was modeled by a continuum theory, and the three-dimensional evolution equation of liquid films on a cylindrical surface was derived based on a long wavelength approximation. Both linear stability theory and nonlinear numerical method were adopted to solve this evolution equation. The linear stability analysis fails to distinguish the three-dimensional mode from the two-dimensional one in terms of maximum disturbance growth rate and always yields a rupture time larger than the nonlinear solution. In contrast, the nonlinear numerical results clearly show that among three disturbance modes, the two-dimensional annular disturbance one yields the longest rupture time, the two-dimensional axisymmetric disturbance one yields the second longest, and the three-dimensional disturbance one does the shortest. Accordingly, it can be concluded that the rupture status of thin films on cylinder is most likely evolved in the three-dimensional disturbance mode as predicted.

Effects of intermolecular forces on deep sub-10 nm spaced sliders

This paper presents the authors' efforts in deep sub-10 nm flying height sliders considering the influence of intermolecular forces. The intermolecular forces are analyzed, modeled, and introduced to the current head/disk interface model. Simulation results indicate that when the flying height is in the range of deep sub-10 nm, the influence of intermolecular forces on the flying height of a slider can be significant and not negligible. A new type of femto slider design, with good performances in reducing the effects of intermolecular forces, is put forward and studied in detail.

Nuclear spin relaxation in ligands outside of the first coordination sphere in a gadolinium (III) complex: Effects of intermolecular forces

This article describes paramagnetic relaxation enhancement (PRE) in systems with high electron spin, S, where there is molecular interaction between a paramagnetic ion and a ligand outside of the first coordination sphere. The new feature of our treatment is an improved handling of the electron-spin relaxation, making use of the Redfield theory. Following a common approach, a well-defined second coordination sphere is assumed, and the PRE contribution from these more distant and shorter-lived ligands is treated in a way similar to that used for the first coordination sphere. This model is called “ordered second sphere,” OSS. In addition, we develop here a formalism similar to that of Hwang and Freed [J. Chem. Phys. 63, 4017 (1975)], but accounting for the electron-spin relaxation effects. We denote this formalism “diffuse second sphere,” DSS. The description of the dynamics of the intermolecular dipole–dipole interaction is based on the Smoluchowski equation, with a potential of mean force related to the radial distribution function. We have used a finite-difference method to calculate numerically a correlation function for translational motion, taking into account the intermolecular forces leading to an arbitrary radial distribution of the ligand protons. The OSS and DSS models, including the Redfield description of the electron-spin relaxation, were used to interpret the PRE in an aqueous solution of a slowly rotating gadolinium (III) complex (S=7/2) bound to a protein.

Apparent and partial ideal molar isentropic compressibilities of binary liquid mixtures Electronic Supplementary Information available. See http://www.rsc.org/suppdata/cp/b1/b104974g/

The excess apparent molar isentropic compressibilities of binary liquid mixtures, which are derived from ultrasonic speed measurements, are frequently used to probe the macroscopic effects of intermolecular forces. The composition dependence of the apparent and partial molar isentropic compressibilities of components of ideal mixtures is explored, with special interest being shown in the infinite aqueous dilution limit. Attention is also paid to convenient methods for estimating the effects of solvation and of cosphere overlap in water-rich mixtures. In particular, it is shown that the difference between excess partial molar and excess apparent molar isentropic compressibilities gives a strong indication of the magnitudes of the effects of solute–solute interactions.

Simulation of singularities and instabilities arising in thin film flow

We present a finite element scheme for nonlinear fourth-order diffusion equations that arise for example in lubrication theory for the time evolution of thin films of viscous fluids. The equations are in general fourth-order degenerate parabolic, but in addition singular terms of second order may occur which model the effects of intermolecular forces or thermocapillarity. Discretizing the arising nonlinearities in a subtle way allows us to establish discrete counterparts of the essential integral estimates found in the continuous setting. As a consequence, the algorithm is efficient, and results on convergence, nonnegativity or even strict positivity of discrete solutions follow in a natural way. Applying this scheme to the numerical simulation of different models shows various interesting qualitative effects, which turn out to be in good agreement with physical experiments.

Determination of the quench velocity and rewetting temperature of hot surfaces. Part I: analytical solution of the micro-scale hydrodynamic model

The hydrodynamic micro-scale model, developed previously, is used to solve the non-isothermal interface equation. The complex interface equation is simplified in a coordinate frame that moves with the three-phase contact line. This equation accounts for effects of evaporation, thermo-capillary and intermolecular forces. The new non-isothermal interface equation provides generalization of de Gennes’ equation that applies to the isothermal case. The simplified third-order differential equation is solved numerically, and the effect of numerical parameters and selection of boundary conditions on solution convergence are established for a wide range of properties of solid–liquid pairs. In contrast to the smooth isothermal interfaces, non-isothermal interfaces are characterized by an undulating or wavy geometry. This behavior is a reflection of evaporation and mass transfer occurring across the interface, and unique capillary and thermocapillary effects that arise under non-isothermal conditions. A parametric study of the interface solution shows that increase of the capillary, C, and thermocapillary, Cθ 0 2/ F numbers produces steeper interface profiles, whereas the factor N, evaporation coefficient S, and the Hamaker constant Ā, produce the reverse effect. Larger values of N, S and Ā result in higher undulation frequencies. These effects intensify and become dominant under rewetting conditions. The new interface equation provides an advanced tool for further studies of hydrodynamic mechanisms that govern the motion of thin liquid films on hot solid surfaces, that involve high temperature gradients and intense evaporation. This furnishes a hydrodynamic foundation for analysis of rewetting phenomena, and the definition of rewetting temperature and quench velocity, that are presented in a subsequent paper.

Contact behavior of spherical elastic particles: a computational study of particle adhesion and deformations

When two elastic particles are brought into contact, the effect of short-range intermolecular forces may become significant especially in the contact region. Due to the intermolecular interaction forces around the contact region, the elastic particles usually deform from their original shape. The deformation of the particles changes the distance between interacting molecules that in turn alters the force of interaction. Thus, the contact behavior of elastic particles constitutes a non-linear mathematical problem that defies the traditional analytical methods for general solution. In theoretical analyses, the behavior of contacting particles has often been considered by approximating the shape of undeformed particles as spheres. The discrepancy between simplified models for elastic spheres based on different analytical treatments led to a heated debate over many years. Computer-aided numerical solutions were resorted to as a final arbitrator. However, the existing numerical results for contacting elastic spheres are quite limited because of the requirement of large number of iterations with primitive computational techniques. The purpose of the present work is to demonstrate an efficient computational method that yields each converged solution in a few iterations. Using an arc-length continuation algorithm enables tracking solution branches around turning points to accurately determine jumping-on and jumping-off behavior of contacting surfaces when the value of Tabor's parameter is not small. Detailed features such as secondary turning points associated with the secondary hysteresis are revealed in load–approach curves when the value of Tabor's parameter becomes large.

Self-assembly of 1-nitronaphthalene on Au(111)

We report low-temperature scanning tunneling microscopy and near-edge X-ray absorption fine structure investigations of the organic molecule 1-nitronaphthalene adsorbed on the reconstructed Au(111) surface. Depending on the coverage, molecular structures of distinct dimensionality are observed. After saturation of the step edges with molecules, clusters form on the substrate terraces at coverages up to 0.15 monolayers (ML). The smallest clusters consist of three molecules and are stable at specific surface reconstruction sites only. In contrast, pinwheel-shaped clusters comprised of ten molecules are observed on various inequivalent sites of the reconstructed Au surface. For coverages larger than 0.2ML one-dimensional molecular double chains prevail. The structure of the decamers and the chains is determined by intermolecular electrostatic forces, whereas their position and orientation are affected by the surface reconstruction. At saturation coverage a periodic two-dimensional close-packed arrangement of molecules that is independent of the surface reconstruction occurs simultaneously with one-dimensional structures along the uniaxial reconstruction domains. The structural features are discussed in terms of competing effects of intermolecular forces and the interaction with the substrate.

Thermodynamic and hydrodynamic peculiarities of a foam lamella confined in a cylindrical pore

The paper investigates a specific behavior of foam films: lamellae confined in a cylindrical prewetted pore. Since both the lamella and wetting film are ultrathin (100 nm), classical thermodynamics is generalized to account for the effects of intermolecular forces. The conditions of coexistence of a lamella and wetting films are established. The problem of lamella motion is considered as a free boundary problem. The theory predicts three sliding regimes, depending on the priority of viscous, long-range surface, and capillary forces. At low speeds, the long-range surface, and capillary forces dominate the viscous one. The friction force acting on the lamella is then proportional to the speed. The pressure drop required to keep the lamella moving can be of the same order of magnitude as the critical pressure drop for lamella rupture.

Nonequilibrium statistical thermodynamic theory for viscoelasticity of polymers

In this paper, we propose a new molecular relaxation mechanism for polymers by considering the change in the actual microscopic structure under macroscopic stress fields. The effects of both intramolecular and intermolecular forces on the inner rotation and the relative slippage of links are taken into account. A constraint potential function, along with a constraint tensor, is introduced to describe the constraint exerted by the surrounding medium. A unified three-dimensional constitutive framework for the viscoelasticity of polymers including thermal effects is established by making use of nonequilibrium statistical thermodynamics, which can be reduced to James and Guth's (1943) non-Gaussian polymer network theory for rubber elasticity. The model compares well with the experimental data for PMMA and plasticized PVC over a wide range of temperatures.

The effect of intermolecular forces on the glass transition of solute-polymer blends: 2. Extension to different solutes

The effects of intermolecular forces on the glass transition temperature ( T g) of various dye solutes in numerous polymer matrices have been investigated. The T gs of these dye-polymer blends were found to depend on the polymer T g, the dye T g, their relative concentrations and the dye-polymer affinity. Attempts have been made to correlate increases in T g with dye-polymer affinity using solubility parameters as a predictive tool. Poor correlations were obtained using global solubility parameters. Excellent correlations were made between elevation in T g and enhanced dye-polymer affinity using averaged polar/hydrogen bonding solubility parameters. For all dye solutes, the T g was low when there was a large mismatch in solubility parameters, increased as dye and polymer became more compatible and was at a maximum when the solubility parameters of both components were equal. The results also indicate that longer range polar and hydrogen bonding forces have a controlling influence on T g.

The effect of intermolecular forces on the glass transition of solute-polymer blends

The effects of dye-polymer interactions on the T g of dye-polymer blends have been investigated using a disazothiophene dye solute in a variety of polymer structures. The dye was found to decrease the T g of some polymers (plasticization) while increasing the T g of others (antiplasticization). The T g of a dye-polymer blend depended on the polymer T g, the dye solute T g, their relative concentrations and dye-polymer affinity. Increasing dye-polymer affinity produced stronger interaction between the dye and polymer, leading to higher T g of the dye-polymer blend. Excellent correlations were established between T g enhancement and the dye-polymer solubility parameter match which was used to predict dye-polymer affinity. Solubility parameters indicated that the combination of polar and hydrogen bonding forces controlled the elevation of T g, since the best correlation was obtained by neglecting dispersion forces.

CONSTRAINED VAPOR BUBBLE

The equilibrium and nonequilibrium phenomena associated with a very thin liquid film formed between a vapor bubble and a container are discussed. Numerical examples of the effects of temperature and intermolecular forces on the film thickness and transport processes are presented. For relatively thick films, the sensitivity of experimental measurements to extremely small temperature gradients is found to be large. For extremely thin films, the sensitivity is small. Large superheats and heat fluxes are possible near the contact line. A small amount of adsorbed contamination has a large effect on the film profile. The constrained vapor bubble has both basic uses in the study of thermophysical properties and transport processes, and has applied uses as a heat exchanger.