ABSTRACT

This award is funded under the American Recovery and Reinvestment Act of 2009
(Public Law 111-5)

PROJECT SIGNIFICANCE
Shallow tectonic earthquakes are driven by instabilities that take place in crustal faults. Earthquake may be thought of as a dynamically running shear crack. Friction and wear at the crack surface determine the stability of the faulted region. The surfaces of the crack are rough and the mechanics of their sliding is affected not only by surface properties (e.g., topology, chemistry), but also by processes taking place in the bulk of the rocks near the crack interface.

Theories that are currently used to predict faulting instabilities are based on phenomenological laws that describe time evolution of the fault. These theories suffer from lack of understanding of what physical mechanisms are actually evolving during frictional slip and therefore predictive capabilities of these theories are limited. The PI will employ molecular simulations to determine friction and adhesion of surfaces representative of crustal faults and she will correlate the frictional response with fundamental chemical and mechanical mechanisms taking place during sliding.

Identifying fundamental mechanisms underlying friction in crustal faults will bring physical insights into the mechanics of earthquakes and it will provide a physical interpretation of constitutive laws that are used for prediction of earthquake phenomena. Understanding of friction and adhesion at the silica/water interface will also have a significant impact on other areas of science. For instance, undesired adhesion has been shown to be prohibitive in a reliable design of micro- and nano-electromechanics system (MEMS/NEMS). MEMS/NEMS are typically made of silica, which in ambient conditions quickly oxidizes and forms a layer of amorphous silica. Reduction of adhesion in silica, particularly in humid environments, is one of the outstanding challenges in MEMS/NEMS design.

TECHNICAL SUMMARY
Even nominally smooth macroscopic surfaces are rough at the microscale. Therefore, while the outcome of an earthquake slip is observed on a macroscale, friction behavior of faults is controlled by small contacts (asperities) that are tens of nanometers to micrometers in size. There is currently no theory that would allow prediction of friction coefficient at any length scale. The challenge in developing such a theory stems from the multitude and complexity of possible energy dissipation mechanisms that contribute to friction, e.g., dislocation assisted slip or chemical bonding across the interface. The complexity of the problem is increased even further if water is present in the environment, as it is likely the case with crustal faults. Since frictional processes occur at the atomic scale, this is a particularly exciting area for the application of molecular simulation methods.

In the proposed project, the PI and her group will identify physical mechanisms that govern friction of silica surfaces at a level of a single asperity. These mechanisms will be determined as a function of temperature, humidity, and pH. Friction and adhesion will be studied in both, the wearless and wear regime and contributions to friction from surface chemistry and subsurface deformations will be established. Advanced accelerated molecular dynamics techniques, based on parallel replica dynamics, will be employed to study stick-slip behavior and to determine the dependence of friction on velocity over a few decades of time.

Recent developments in atomistic modeling, which include the ability to simulate large systems at the size of a single asperity contact, accelerated molecular dynamics techniques, and reliable force fields for simulating silica/water interface, create an opportunity to address the challenging issues related to friction in crustal faults. The PI will closely collaborate with geologists, who have on-going experimental projects aimed at answering questions complementary to the ones identified in this proposal.

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