Statistical mechanics of a lipid monolayer

Chapter 7 - Tribological experiments with friction force microscopy

A new approach to surface modeling and reconstruction is developed which overcomes some important limitations of existing surface representations methods. The approach features two components. The first is a dynamic self-organizing oriented particle system which discovers topological and geometric surface structure implicit in visual data. The oriented particles evolve according to Newtonian mechanics and interact through long-range attraction forces, short-range repulsion forces, and coplanarity, conormality, and cocircularity forces. The second component is an efficient triangulation scheme that connects the particles into a continuous global surface model that is consistent with the inferred structure. A flexible surface reconstruction algorithm is developed that can compute complete, detailed, viewpoint-invariant geometric surface descriptions of objects with arbitrary topology. The algorithms are applied to 3-D medical image segmentation and to surface reconstruction from object silhouettes. >

Splines and deformable surface models are widely used in computer graphics to describe free-form surfaces. These methods require manual preprocessing to discretize the surface into patches and to specify their connectivity. We present a new model of elastic surfaces based on interacting particle systems, which, unlike previous techniques, can be used to sptiL join, or extend surfaces without the need for manual intervention. The particles we use have longrange attraction forces and short-range repulsion forces and follow Newtonian dynamics, much tiie recent computational models of fluids and solids. To enable our particles to model surface elements instead of point masses or volume elements, we add an orientation to each particle’s state. We devise new interaction potentials for our oriented particles which favor locally planar or spherical arrangements. We atso develop techniques for adding new particles automatically, which enables our surfaces to stretch and grow. We demonstrate the application of our new particle system to modefing surfaces in 3-D and the interpolation of 3-D point sets.

We examine rotational transitions of HCl in collisions with H2 by carrying out quantum mechanical close-coupling and quasi-classical trajectory (QCT) calculations on a recently developed globally accurate full-dimensional ab initio potential energy surface for the H3Cl system. Signatures of rainbow scattering in rotationally inelastic collisions are found in the state resolved integral and differential cross sections as functions of the impact parameter (initial orbital angular momentum) and final rotational quantum number. We show the coexistence of distinct dynamical regimes for the HCl rotational transition driven by the short-range repulsive and long-range attractive forces whose relative importance depends on the collision energy and final rotational state, suggesting that the classification of rainbow scattering into rotational and l-type rainbows is effective for H2 + HCl collisions. While the QCT method satisfactorily predicts the overall behavior of the rotationally inelastic cross sections, its capability to accurately describe signatures of rainbow scattering appears to be limited for the present system.

The van Kampen analysis of condensation in a fluid of hard spheres with long-range attractions is extended to investigate the statistical mechanics of a fluid of rigid rod-like molecules which interact through both short-range repulsive and long-range attractive forces and which assume only a restricted set of three mutually orthogonal orientations. The thermodynamic properties are derived for the fluid near the liquid-gas critical point and at the nematic-isotropic transition. The model predicts that both isotropic and anisotropic attractive and repulsive interactions contribute to the stabilization of the nematically ordered fluid when a density change accompanies the transition. However, when the transition occurs at constant density, only the anisotropic contributions of the long-range dispersion energy and short-range repulsions are found to stabilize the transition in partial accord with the Maier-Saupe theory. Numerical results for a rigid-rod fluid with length-to-breadth ratio 3 at various packing...

The transport properties of excess electrons in non polar gases or liquids give important information on the electronic states in a disordered medium and on the relationship among the electron-atom interactions and the properties of the fluid. Anomalous density effects on electron mobilities were discovered in a number of compressed gases1. A negative density effect, i. e., the density-normalized zero-field mobility μ 0 N of electrons decreases with increasing the gas density N, is shown by gases such as He and Ne, whose interaction with electrons is dominated by short-range repulsive exchange forces and whose scattering length a is positive. On the other hand, a positive density effect, i. e., μ 0 N increases with N, is shown by Ar, Kr, and Xe, whose scattering length is negative owing to the overwhelming contribution of the long-range attractive polarization forces to the atom-electron interaction.

Interaction forces between charged interfaces in the presence of oppositely charged dendrimers are studied by experiment and simulation. The experiments involve direct force measurements with an atomic force microscope (AFM) between two negatively charged colloidal particles in the presence of adsorbed, positively charged globular dendrimers. The simulations are carried out by treating the macroions explicitly, while the small salt ions are treated implicitly through the Debye-Huckel approximation. The system undergoes overcharging, and at the isoelectric point long-ranged attractive electrostatic forces are present. The range of the attraction is on the order of half the Debye length at high salt concentration, but it becomes smaller at low salt concentration. Away from the isoelectric point, repulsive electrostatic forces are observed due to diffuse layer overlap. A semiquantitative agreement between experiment and simulation is obtained, despite the fact that the simple theoretical model does not involve any adjustable parameters. This study provides for the first time detailed comparison between experimental and simulation data of interaction forces between colloidal particles in the presence of multivalent macroions and monovalent salt ions.

This paper is aimed to find a connection between i-dimensional spaces (i=0,…, ‘n') and the long-range j-dimensional attractive forces (j=0,…, ‘m') creating these spaces. The connection is fundamental and unrelated to any processes going in the spaces being studied. A theorem is formulated and strictly proved showing in which cases the long-ranged attractive forces can form real spaces of different dimensions ( i=0,…,n). The existence of the attraction between masses is defined by divergence of the vector of interaction between masses. Weak anisotropic real spaces are studied by rotating an ellipsoid for (3 ζ)D-cases when its eccentricity e<<1. Such spaces cannot be in equilibrium, the time of their existence is substantially limited. The greater is anisotropy, the shorter is the lifetime of such substance. The latter cannot be in equilibrium, the time of their existence is substantially limited.

A quartz crystal microbalance was used to study the influence of nanobubbles on the adsorption of polystyrene nanoparticles onto surfaces coated with gold, or coated with dodecanethiol or mercaptoundecanoic acid self-assembled monolayers (SAMs). Adsorption of the nanoparticles onto the surface causes the resonant frequency of the quartz crystal to decrease. We found that particles were adsorbed onto the gold-coated quartz crystal in air-rich water, but not in degassed water. This finding supports the long-standing hypothesis that nanobubbles play a key role in the long-range attractive force between hydrophobic surfaces in aqueous solutions. When the experiments were conducted using quartz crystals coated with a hydrophobic dodecanethiol SAM, the nanoparticles were adsorbed onto the surface even in degassed water due to the short-range hydrophobic interactions between the nanoparticles and the dodecanethiol molecules. In contrast, the nanoparticles were adsorbed to a lesser degree onto the hydrophilic mercaptoundecanoic acid-coated crystals due to electrostatic repulsive forces.

Dissipative condensation of ion channels described by a Langevin-Kelvin equation

Elasticity of Mutant Myosin Subfragment-1 Arranged on a Functional Silver Surface

The forces between a tungsten tip and a self-assembled monolayer of hexadecylthiol (C16H33SH) on a thin gold film have been studied using a newly developed interfacial-force microscope. Imaging of the surface, combined with spatially resolved force versus separation measurements, allow for the distinction of topographical and chemical features of the surface. Several distinct regions are observed for this system. The first, characterized by a very weak interfacial interaction between tip and sample, is representative of the self-assembled monolayer. The other regions show relatively strong, long-range attractive forces, which are associated with gross defects in the film.

Small-angle neutron scattering experiments were performed on concentrated binary dispersions of sterically-stabilized poly(methy1 methacrylate) particles of size ratio -0.31 dispersed in a mixture of cisdecalin-hl8 and octane-dl8. Using a contrast match technique, we have, for the first time, studied the local structure of small spheres in a simple (hard-sphere) colloidal mixture. In particular, our experiments demonstrate that the small spheres show evidence of particle clustering. We show that this observation is in accord with a long-ranged attractive force between the small spheres resulting from the depletion of larger particles from between the surfaces of the small spheres.

The saponin escin, extracted from horse chestnut seeds, forms adsorption layers with high viscoelasticity and low gas permeability. Upon deformation, escin adsorption layers often feature surface wrinkles with characteristic wavelength. In previous studies, we investigated the origin of this behavior and found that the substantial surface elasticity of escin layers may be related to a specific combination of short-, medium-, and long-range attractive forces, leading to tight molecular packing in the layers. In the current study, we performed atomistic molecular dynamics simulations of 441 escin molecules in a dense adsorption layer with an area per molecule of 0.49 nm2. We found that the surfactant molecules are less submerged in water and adopt a more upright position when compared to the characteristics determined in our previous simulations with much smaller molecular models. The number of neighbouring molecules and their local orientation, however, remain similar in the different-size models. To maintain their preferred mutual orientation, the escin molecules segregate into well-ordered domains and spontaneously form wrinkled layers. The same specific interactions (H-bonds, dipole–dipole attraction, and intermediate strong attraction) define the complex internal structure and the undulations of the layers. The analysis of the layer properties reveals a characteristic wrinkle wavelength related to the surface lateral dimensions, in qualitative agreement with the phenomenological description of thin elastic sheets.

The collisional removal of the carbene CCl 2(X̃(0,0,0)) and CCl 2( [formula omitted](0,7,0)) by rare gases and simple molecules