Session U22: Focus Session: Friction

Fundamental aspects of energy dissipation in friction

Energy dissipation in friction is mediated by excitation of elementary processes including surface phonons and electronic excitations. These excitations couple through anharmonic interactions or by Frank-Condon nuclear motions to bulk substrate phonons, which ultimately appear as heat. This gives rise to numerous phenomena including friction anisotropy, velocity dependence, and dissipative surface charge motion. Friction anisotropy can appear when phonon modes with specific polarizations are forbidden in particular crystal directions. Electronic excitations have been discussed and investigated but never clearly and definitely identified as primary mechanisms in contact friction. I will discuss these topics using recent experimental results in my laboratory including the large friction anisotropy of Al-Ni-Co decagonal quasicrystals, the role of hydrogen bonding networks in determining the velocity dependence of friction and finally the control of friction by changing the carrier concentration near the surface of p and n semiconductors.   

A nanotribology study of self-mated \textit{vs.} unmated interfaces and local packing density effects for octadecyltrichlorosilane monolayers and silicon

We use atomic force microscopy (AFM) to determine the frictional properties of nanoscale single asperity contacts involving octadecyltrichlorosilane (OTS) monolayers and silicon. Quantitative AFM measurements are performed using both uncoated and OTS-coated silicon AFM tips and surfaces. Friction is reduced by the presence of the OTS coating, and the overall shape of the friction \textit{vs. }load plot strikingly depends on whether or not the substrate is coated with OTS, regardless of tip material. Uncoated substrates exhibit the common sublinear dependence, while coated substrates exhibit an unusual superlinear dependence. These results can be explained qualitatively by invoking molecular plowing as a significant contribution the frictional behavior of OTS. Direct \textit{in-situ }comparison of two intrinsic OTS structural phases of otherwise identical molecules on the substrate show that the lower packing density phase exhibits higher friction, decisively observed here in single, uninterrupted images on the same monolayer for the first time. The lateral stiffness of the two OTS structural phases are indistinguishable, which implies that the packing density directly affects the interface's intrinsic resistance to shear as opposed to simply modifying the stiffness of the monolayer.   

Dynamical noise and avalanches in quasi-static plastic flow of amorphous solids

We build a mean-field model of plasticity of amorphous solids, based on the dynamics of an ensemble shear transformation zones, interacting via intrinsic dynamical noise generated by the zone flips themselves. We compare the quasi-static, steady-state properties for two types of noise spectrum: (G) Gaussian; (E) broad distribution derived from quadrupolar elastic interactions. We find that the plastic flow proceeds via avalanches whose scaling properties with system size are highly sensitive to noise tails. Comparison with available data suggests that non-affine strain fields might be of paramount importance in the small systems accessible to molecular simulations.   

Dynamics of Phononic Dissipation at the Atomic Scale

Dynamics of dissipation of a local phonon distribution to the bulk is a key issue in boundary lubrication and friction between sliding surfaces. We consider a highly excited molecule which interacts weakly with the substrate surface. We study different types of coupling and substrates having different types of dimensionality and phonon densities of states. We propose three different methods to solve the dynamics of the combined system, namely the equation of mation technique, Fano-Anderson method and the Green's function method. Using this theoretical framework we present an analysis of transient properties of energy dissipation via phonon discharge at the microscopic level. The methods allow the theoretical calculations to be extended to include any density of states for the substrate including experimental ones and any type of molecule that represent the lubricant or the asperity.   

Friction and Viscous Forces in Sub-Nanometer Water Films

Water under nano-confinement is ubiquitous, with examples including clay swelling, aquaporines, ion channels, and water menisci in micro-electrical-mechanical-systems. However, the structural and rheological characteristics of nano-confined pure and ionized water continue to be the subject of discussion and debate. Here, we report an experiment in which an atomic force microscope tip approaches a flat solid surface in purified water, while small lateral oscillations are applied to the tip. Direct measurements of the lateral forces encountered by a nano-size tip approaching a solid surface in purified water are reported for tip-surface distances, 0$\pm $0.03 nm $<$ d $<$ 2 nm. We find that, for hydrophilic surfaces, the dynamic viscosity is measured to grow up orders of magnitude in respect to bulk water, whereas no significant increase in the viscosity has been detected when the confining solid surface is hydrophobic. The origin of the observed different behavior is discussed.   

Friction Reduction Using Self-Assembled Hydrogels

Friction of agarose-based hydrogels against bare glass is examined as a function of added linear polyelectrolyte using a stress rheometer to measure the angular velocity of a clean glass plate against the hydrogel surface as a function of applied torque and normal force. Incorporating linear dextran sulfate into 2 weight percent agarose hydrogel reduces friction on the hydrogel surface. The reduction of friction is a nonmonotonic function of dextran sulfate concentration: a 2 percent doping of dextran sulfate shows the minimum friction. Lubricity enhancement on the agarose doped with 2 percent dextran sulfate occurs at all normal forces examined (0.5, 1, 1.5, and 2 N) and is more pronounced at larger angular velocities. Rheological studies of agarose hydrogels doped with dextran sulfate suggest that the dextran sulfate does not interfere with the porous structure of the hydrogel when present in concentrations of 2 weight percent or less.   

High velocity sliding at a compressed Al(111)/Al(100) interface

We discuss high velocity sliding at a compressed Al(111)/Al(100) interface sliding in the $1\overline{1}0$ direction at a pressure of 15 GPa. Three temperatures are considered, T=232, 464 and 696 K. System sizes are $1.4 10^{6}$ atoms .We find that for velocities above a critical velocity, $v_{c}$, the frictional force scales as $(v/v_{c})^{-\beta}$ with $\beta\approx 3/4$. We discuss the temperature and size dependence of $v_{c}$. We find that below $v_{c}$ the frictional force is an increasing function of velocity with an initial linear dependence. Above $v_{c}$ there is a regime of interfacial instability characterized by a (100) transformation front moving into the (111) material. This is followed by a fluid regime for which a Couette flow profile develops at the interface, the thickness of which grows with increasing velocity.   

The Dynamics of Precursors to Frictional Sliding

The dynamics of frictional motion are governed by the nature of the interface separating two sliding materials. We report that the spatial profile of the contact-area along an interface is a dynamic quantity which evolves via a discrete sequence of rapid crack-like precursors to overall motion. These precursors, which are generated at stress levels much lower than the critical stress for sliding, significantly modify the initially uniform contact area profile. Thus, when overall sliding finally occurs, the contact area is highly non-uniform in space. These results suggest a fundamentally new view of the processes leading to frictional motion with ramifications to earthquake dynamics and material failure.   

Molecular Dynamics Simulation of Frictional Melting

Frictional melting produces lubricant at the sliding plane and changes the physics of dynamical sliding, which may play a key role on coseismic slipping. In this paper, molecular dynamics simulation is used to study the basic physics of fritional melting. Here, friction between a Lenard-Johns material and a rigid material is considered for simplicity. When the sliding velocity is low enough, there is no melting and the friction coefficient almost does not depend on the sliding velocity. On the other hand, when the sliding velocity is so high that frictional melting occurs, the friction coefficient decreases due to the melting lubricant. A preliminary result shows that the friction coefficient is roughly power-law of the sliding velocity. A discussion will also be given on the themodynamic balance between the frictional heating, cooling by latent heat, and conduction cooling.   

Molecular Dynamics Simulations of Nanotribology with Accurate Probe Tip Models

Results for extensive dynamical nanotribological simulations of amorphous silica tips in contact with alkylsilane self-assembled monolayers (SAMs) will be presented. The radius of curvature of the tips match experimental dimensions. Comparison with contact mechanics models indicate that the standard JKR and DMT models do not give the correct dependence of contact area on applied force. The dependence of the tribological response on the chain length of the SAM has been determined. For short chains and for long chains at low loads the SAM presents a disordered sliding surface to the tip and the chain length is irrelevant. This result is in agreement with our previous simulations for SAMs in contact with a flat surface. For longer chains at higher loads the tip penetrates the monolayer and the friction is dominated by a plowing mechanism. Sandia is a multiprogram laboratory operated by Sandia Corp., a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000.   

Molecular dynamics studies of friction between bare and oxidized silicon surfaces

Using molecular dynamics simulation we examine the friction between a bare silicon tip and a silicon surface under perfect vacuum conditions. The simulations utilize a Stillinger-Weber model for the Si-Si interactions. In the case of bare silicon (100) the proper surface reconstructed is verified. Silicon-silicon sliding leads to high friction and significant wear due to the strong adhesive force between tip and surface. Repeated adhesion and shearing produces a stick-slip motion. The quantity of material lost during sliding depends on the relative orientation of the dimer rows between the reconstructed surfaces of the tip and substrate. Little dependence on the temperature or the normal force is observed in this case. The frictional force does not change significantly even when an upward normal force is applied to the tip force, although the quantity of lost material depends on the magnitude of upward normal force. The geometry and elasticity dependence of the stick-slip motion has also been analyzed. We have also begun investigations of a similar geometry in which the silicon is coated with a thin oxide layer. A charge transfer potential having 3-body terms as well as pair-wise interactions is being used to model the amorphous silica interactions. These simulation results will be compared to the recent AFM experiments by Schirmeisen et al. measuring the frictional forces between an oxidized silicon tip and substrate.