Systematic study of the effect of disorder on nanotribology of self-assembled monolayers.

The adhesion and friction between pairs of ordered and disordered self-assembled monolayers on SiO2 are studied using molecular dynamics. The disorder is introduced by randomly removing chains from a well ordered crystalline substrate and by attaching chains to an amorphous substrate. The adhesion force between monolayers at a given separation increases monotonically with chain length at full coverage and with coverage for fixed chain length. Friction simulations are performed at shear velocities between 0.02-2 m/s at constant applied pressures between 200 and 600 MPa. Stick-slip motion is observed at full coverage but disappears with disorder. With random defects, the friction becomes insensitive to chain length, defect density, and substrate.

Surface-tethered chains entangled in a polymer melt: effects on adhesion dynamics.

The adhesion between a polymer melt and substrate is studied in the presence of chemically attached chains on the substrate surface. Extensive molecular dynamics simulations have been carried out to study the effect of temperature, tethered chain density (Sigma), tethered chain length (N(t)), and tensile pull velocity (v) on the adhesive failure mechanisms of pullout and/or scission of the tethered chains. We observe a crossover from pure chain pullout to chain scission as N(t) is increased. The value of N(t) for which this crossover begins approaches the bulk entanglement length N(e) as the temperature is lowered.

Granular flow down an inclined plane: Bagnold scaling and rheology.

We have performed a systematic, large-scale simulation study of granular media in two and three dimensions, investigating the rheology of cohesionless granular particles in inclined plane geometries, i.e., chute flows. We find that over a wide range of parameter space of interaction coefficients and inclination angles, a steady-state flow regime exists in which the energy input from gravity balances that dissipated from friction and inelastic collisions. In this regime, the bulk packing fraction (away from the top free surface and the bottom plate boundary) remains constant as a function of depth z, of the pile. The velocity profile in the direction of flow vx(z) scales with height of the pile H, according to vx(z) proportional to H(alpha), with alpha=1.52+/-0.05. However, the behavior of the normal stresses indicates that existing simple theories of granular flow do not capture all of the features evidenced in the simulations.

Liquid/vapor surface tension of metals: embedded atom method with charge gradient corrections.

Molecular dynamics simulations for three embedded atom method (EAM) function sets are used to determine the liquid/vapor surface tension gamma for Al, Ni, Cu, Ag, and Au. The three EAM models differ in both the functional forms employed and the fitting procedure used. All the EAM potentials underestimate gamma but one of the models performs consistently better than the others. We show that including a correction to the local charge density associated with gradients in the density together with exploiting the invariance of the EAM potentials to appropriate transformations in the charge density can lead to improved values for gamma, as well as for solid free surface energies, within existing EAM function sets.

Capillary waves at liquid-vapor interfaces: a molecular dynamics simulation.

Evidence for capillary waves at a liquid-vapor interface are presented from extensive molecular dynamics simulations of a system containing up to 1.24 million Lennard-Jones particles. Careful measurements show that the total interfacial width depends logarithmically on L(axially), the length of the simulation cell parallel to the interface, as predicted theoretically. The strength of the divergence of the interfacial width on L(axially) depends inversely on the surface tension gamma. This allows us to measure gamma two ways since gamma can also be obtained from the difference in the pressure parallel and perpendicular to the interface. These two independent measures of gamma agree provided that the interfacial order parameter profile is fit to an error function and not a hyperbolic tangent, as often assumed. We explore why these two common fitting functions give different results for gamma.

Existence of a flat phase in red cell membrane skeletons.

Biomolecular membranes display rich statistical mechanical behavior. They are classified as liquid in the absence of shear elasticity in the plane of the membrane and tethered (solid) when the neighboring molecules or subunits are connected and the membranes exhibit solid-like elastic behavior in the plane of the membrane. The spectrin skeleton of red blood cells was studied as a model tethered membrane. The static structure factor of the skeletons, measured by small-angle x-ray and light scattering, was fitted with a structure factor predicted with a model calculation. The model describes tethered membrane sheets with free edges in a flat phase, which is a locally rough but globally flat membrane configuration. The fit was good for large scattering vectors. The membrane roughness exponent, zeta, defined through h alpha L zeta, where h is the average amplitude of out-of-plane fluctuations and L is the linear membrane dimension, was determined to be 0.65 +/- 0.10. Computer simulations of model red blood cell skeletons also showed this flat phase. The value for the roughness exponent, which was determined from the scaling properties of membranes of different sizes, was consistent with that from the experiments.