Friction is intrinsically a multiscale process because it arises from molecular interactions at the contacts between sliding objects, but the geometry of the contacts depends on deformation at all larger scales. While models often consider two flat contacting solids, real surfaces are rough and may be separated by other bodies: fractured rock (gouge) in earthquake faults, debris or surface layers produced by wear at laboratory scales, or molecular layers that are adsorbed from the air or deposited as boundary lubricants. Whatever the scale of these objects, they can play similar roles in inhibiting sliding. Concepts of aging and rate dependence also span the range of length scales. Overviews of some of the major issues were provided at the kickoff conference (Dieterich, Robbins) and baglunch talks (Falk-Dieterich 8/29, Robbins 9/19)

One thrust of the workshop was the role of geometrical interlocking in producing friction either through jamming of particles confined between surfaces, or mating of surface geometries. Studies of jamming addressed molecules confined in nanometer thick layers with simulations (Robbins, Mueser, Urbakh) and experiments (Israelachvili, de Boer2, Carpick) as well as granular media in faults (Morgan, Marone, Tullis, Behringer). Qualitatively similar behavior is observed across scales, including dilatancy, scaling of friction with load, and the role of mating of the sliding surfaces. However, interparticle friction and rotation lead to quantitative changes and extra complexity in granular systems. For example, shear localization may be enhanced by the history dependence of frictional forces. Additional interlocking occurs when the opposing surfaces are mated, as in a fault. Sliding between mated rough surfaces leads to large local stresses and may nucleate fault branching (Dieterich).

Another focus was the role of area and load in determining friction. Contact between rough surfaces generally leads to a true area of contact that scales linearly with load, making it difficult to separate the two (Persson discussion, Robbins, Mueser, Dieterich). Area and load scale differently in single-asperity contacts (Goldsby, Carpick, Israelachvili). The importance of area grows with adhesion and decreases with material stiffness (Mueser-Carpick). Dissipation may also occur through time-dependent deformation in the bulk during sliding over a rough surface (Persson). Viscoelasticity (Persson), phononic and electronic modes (Krim), and dislocation motion (Needleman) may all lead to dissipation.

Friction forces are not a simple function of load and sliding velocity, but can have a complex dependence on past conditions. Rate-state laws attempt to incorporate this memory phenomenologically in terms of the evolution of a variable that characterizes the state of the system. Similar rate-state models describe earthquakes (Dieterich conf), laboratory experiments with bare surfaces or surfaces separated by gouge (Dieterich, Goldsby), contacts in micromachines (DeBoer 11/16) and the onset of flow in sandpiles (Charlaix). Two talks (Dieterich conference, Goldsby) gave an overview of the rate-state approach and of experiments that relate the state variable to the area of contact between two rough surfaces or in a single asperity (Goldsby). One topic of interest was the origin of the growth in area with time and its dependence on moisture: plasticity or plowing (Dieterich conf., Goldsby, Tullis discuss, Fineberg-McGuiggan 10/12), capillary condensation (Charlaix), friction effects in single contacts (Fineberg-McGuiggan 10/12), or slow changes in adhesion energy (Fineberg-McGuiggan 10/12, Tullis discuss). Aging of the packing of gouge or atoms at the interface can lead to logarithmic increases in strength (Rottler, Fineberg-McGuiggan 10/12, Tullis discuss) that are independent from the changes due to contact age. Both effects may be important in a complete model of rate-state effects in granular materials (Morgan).

Stick-slip motion is manifested from atomic to tectonic scales. Stress builds slowly to the static friction while the surfaces stick, and is released rapidly during sliding. This cycle may repeat stochastically or periodically and there may be creep or partial slip in the stick phase. While material properties, surface roughness, and length and time scales can be very different, there are common unifying aspects: The friction force must weaken as sliding begins, the system must be sufficiently compliant and the driving rate must be low (Israelachvili, Urbakh, discussion 11/10, Marone). Specific studies of stick-slip were presented based on atomistic (discussion 11/10), mesoscale (Urbakh) and granular (Morgan) simulations, and experiments with nano-scale AFM tips and micromachines (discussion 11/10, Carpick, de Boer2), micrometer scale SFA contacts (Israelachvili), and sheared granular systems (Marone).

Earthquakes are a case where stick-slip motion involves localized sliding in a ruptured region that propagates along the interface (Dieterich, Coker, Zoeller, Carlson, …). Rupture propagation was analyzed in simulations of atomic (Marder) and tectonic (Coker, Zoeller) systems, and examined in experiments with plexiglass (Fineberg), elastomers (Baumberger), and granular systems (Rosakis, Tullis,). Propagation may occur at low, subsonic or supersonic velocities. At high velocities new mechanisms of friction weakening may become important, including melting or gel formation due to flash heating (Rice) and reduction of the normal stress on gouge particles due to pore pressurization (Rice talk and discussion).

24 Talks in this area

8/16, 10:35am
Juli Morgan (Rice)
Particle Dynamics Simulations of Granular Friction
8/17, 8:30am
Jay Fineberg (Hebrew Univ)
Detachment Fronts and the Onset of Friction
8/18, 8:30am
Jim Dieterich (UC Riverside)
Macroscopic Friction and Earthquakes
8/18, 9:20am
Mark Robbins (Johns Hopkins)
Friction Mechanisms: From Atomistic to Macroscopic Scales
8/18, 11:10am
Demir Coker (Oklahoma State)
Dynamic Frictional Sliding Modes Along Interfaces Between Identical Materials
8/18, 11:45am
Michael Urbakh (Tel Aviv)
Atomic Scale Friction: From Understanding to Control
8/18, 2:00pm
Jacob Israelachvili (UCSB)
Sub-Ångstrom (Pico-Scale) Effects in Adhesion and Friction
8/18, 2:35pm
Jacqueline Krim (NCSU)
Fundamentals of Friction in Confined and Unconfined Geometries
8/18, 3:10pm
Maarten DeBoer (Sandia)
Friction in Surface Micromachined Interfaces
8/18, 4:15pm
Bo Persson (Juelich)
Rubber Friction and Crack Propagation in Rubber-like Materials
8/18, 4:50pm
Alan Needleman (Brown)
Discrete Dislocation Modeling of Contact and Friction
10/05, 12:00 p.m.
Dr. Elisabeth Charlaix (University Lyon)
Physics of Contacts, Capillary Condensation and Cohesion Effects in Granular Media
10/25, 2:00 p.m.
Dr. David Goldsby (Brown)
Creep and Friction of Microscopic Single Contacts in Minerals: Application to Earthquake Mechanics
10/26, 12:00 p.m.
Dr. Tristan Baumberger (University Paris)
Friction/Fracture of Hydrogels
10/27, 10:30 a.m.
Dr. Michael Marder (UT Austin)
Discussion: Slip Pulses
11/01, 10:30 a.m.
Dr. Terry Tullis (Brown)
Dramatic Reductions in Fault Friction at Earthquake Slip Rates
11/02, 12:00 p.m.
Dr. Robert Carpick (Wisconsin)
Friction at the Nanometer Scale: Recent Experimental Advances
11/08, 10:30 a.m.
Dr. Chris Marone (Penn State)
Friction of Sheared Granular Materials
11/09, 12:00 p.m.
Dr. Martin Mueser (University of Western Ontario)
Energy Dissipation Mechanisms at the Nanoscale: Computer Simulations
11/10, 3:30 p.m.
Discussion
Stick-Slip at Different Scales
11/16, 12:00 p.m.
Dr. Maarten de Boer (Sandia)
Adhesion and Friction in Surface Micromachined Interfaces
11/17, 10:30 a.m.
Dr. Terry Tullis (Brown)
Discussion: Possible Explanations for Evolution Effect
11/18, 10:30 a.m.
Dr. Bo Persson (IFF Research Center)
Contact Mechanics and Rubber Friction on Smooth Surfaces: Some New Results
12/07, 12:00 p.m.
Martin Mueser, Rob Carpick
Discussion: Friction Laws for Single Asperity Contacts