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J B Sokoloff

Publications and source records attributed to J B Sokoloff.

8 recordsLinked to original sources
PubMed2006

Theory of the effects of multiscale surface roughness and stiffness on static friction.

It is shown on the basis of simple scaling arguments that an interface between two three-dimensional elastic solids, consisting of completely flat disordered surfaces, which interact with interatomic hard core interactions, will be in a weak pinning regime, and hence exhibit negligibly small static friction. It is argued, however, that the presence of roughness on multiple length scales can lead to much larger friction (i.e., static friction coefficients not too much smaller than 1), as is characteristic of most solid surfaces. This approach suggests a possible way of understanding why coatings of materials with high elastic constants are often excellent lubricants.

Journal Article↗
PubMed2005

Adiabatic molecular-dynamics-simulation-method studies of kinetic friction.

An adiabatic molecular-dynamics method is developed and used to study the Muser-Robbins model for dry friction (i.e., nonzero kinetic friction in the slow sliding speed limit). In this model, dry friction between two crystalline surfaces rotated with respect to each other is due to mobile molecules (i.e., dirt particles) adsorbed at the interface. Our adiabatic method allows us to quickly locate interface potential-well minima, which become unstable during sliding of the surfaces. Since dissipation due to friction in the slow sliding speed limit results from mobile molecules dropping out of such unstable wells, our method provides a way to calculate dry friction, which agrees extremely well with results found by conventional molecular dynamics for the same system, but our method is more than a factor of 10 faster.

Journal Article↗
PubMed2005

Theory of lubrication due to collective pinning.

In collective pinning theory, the problem of two three-dimensional solids in contact is at its critical dimension. This implies that when the disordered forces acting between the two solids at the interface are relatively strong, the force of static friction should be large, but at smaller values of these forces, the system switches over to a regime of weak static friction. It is argued that this provides a mechanism for the reduction of friction in boundary lubrication. Lubricant molecules reduce static friction by smoothing the roughness of the surface, thus allowing the force pushing the surfaces together to be supported by more points of contact, which can switch the interface from the strong- to weak-static-friction regime.

Journal Article↗
PubMed2003

Friction in the zero sliding velocity limit.

Using an adiabatic approximation method, which searches for Tomlinson model-like instabilities for a simple but still realistic model for two crystalline surfaces, with mobile molecules present at the interface, sliding relative to each other, we are able to account for the virtually universal occurrence of "dry friction" at zero temperature. A modified version of this method allows us to calculate the kinetic friction at nonzero temperature as well. We have also considered the static friction, and have demonstrated that the model is able to account for static friction being larger than kinetic friction.

Journal Article↗
PubMed2003

Dry friction due to adsorbed molecules.

Using an adiabatic approximation method, which searches for Tomlinson-model-like instabilities for a simple but still realistic model for two crystalline surfaces, with mobile molecules present at the interface, sliding relative to each other, we are able to account for the virtually universal occurrence of "dry friction." The model makes important predictions for the dependence of friction on the strength of the interaction of each surface with the mobile molecules.

Journal Article↗
PubMed2001

Static friction between elastic solids due to random asperities.

Several workers have established that the Larkin domains for two three-dimensional nonmetallic elastic solids in contact with each other at a disordered but atomically flat interface are enormously large, implying that there should be negligible static friction per unit area in the macroscopic solid limit. In contrast, the present Letter argues that when the Larkin domains are calculated for disorder on the multiasperity scale, they are much smaller than the interface size. This can account for the virtual universal occurrence of static friction.

Journal Article↗
PubMed1990

Theory of acoustic mode vibrations of DNA fibers.

A previous model for acoustic mode vibrations of a DNA molecule in water is extended to the case of an array of many DNA molecules, as occurs in the fibers studied in most experimental work on DNA. The acoustic modes of this system are found to consist of coupled modes of water sound vibrations and DNA acoustic modes. This model is used to study the electrostatic coupling of acoustic vibrations to the relaxational modes of the orientational degrees of freedom of the water molecules. It is found that the long-range or macroscopic electric field generated by the acoustic mode vibrations of the water-DNA system gives too small a damping and frequency shift of the acoustic modes to account for the observations on DNA fibers. Therefore, the observed damping and frequency shifts are most likely due to either friction between the surrounding water and the vibrating DNA, or coupling to the water orientation degrees of freedom resulting from the short range (i.e., screened) Coulomb interaction. The latter explanation (which is most likely the correct one) implies that the relaxation time of the hydration shell water is longer than the observed relaxation time by a factor of the static dielectric constant of the hydration water.

Acoustics↗