Friction and aging of silica: atomistic simulations for fundamental understanding of earthquake mechanics

项目意义：浅层构造地震是由发生在地壳断层上的不稳定性引起的。地震可以被认为是一个动态运行的剪切裂缝。裂纹表面的摩擦磨损决定了断裂带的稳定性。裂纹的表面是粗糙的，它们的滑动力学不仅受到表面特性（例如，拓扑结构，化学）的影响，而且受到裂纹界面附近大块岩石中发生的过程的影响。目前用于预测断层不稳定性的理论是基于描述断层时间演化的现象学定律。这些理论由于缺乏对摩擦滑移过程中实际发生的物理机制的理解而受到影响，因此这些理论的预测能力有限。PI将采用分子模拟来确定代表地壳断层的表面的摩擦和粘附，她将把摩擦反应与滑动过程中发生的基本化学和机械机制联系起来。确定地壳断层中摩擦的基本机制将带来对地震力学的物理见解，并将提供用于预测地震现象的本构定律的物理解释。对二氧化硅/水界面的摩擦和粘附的理解也将对其他科学领域产生重大影响。例如，在可靠的微纳米机电系统（MEMS/NEMS）设计中，不期望的粘附已被证明是禁止的。MEMS/NEMS通常由二氧化硅制成，在环境条件下迅速氧化并形成一层无定形二氧化硅。减少二氧化硅的粘附，特别是在潮湿环境中，是MEMS/NEMS设计的突出挑战之一。即使表面光滑的宏观表面在微观尺度上也是粗糙的。因此，虽然地震滑动的结果是在宏观尺度上观察到的，但断层的摩擦行为是由几十纳米到微米大小的小接触（凸起）控制的。目前还没有理论可以预测任何长度尺度下的摩擦系数。发展这一理论的挑战源于可能导致摩擦的能量耗散机制的多样性和复杂性，例如，位错辅助滑移或跨界面的化学键合。如果环境中存在水，问题的复杂性就会进一步增加，因为地壳断层很可能就是这种情况。由于摩擦过程发生在原子尺度上，这是分子模拟方法应用的一个特别令人兴奋的领域。在拟议的项目中，PI和她的团队将确定在单个粗糙度水平上控制二氧化硅表面摩擦的物理机制。这些机制将被确定为温度、湿度和ph值的函数。摩擦力和附着力将被研究，无磨损和磨损机制以及表面化学和地下变形对摩擦的贡献将被建立。先进的加速分子动力学技术，基于平行复制动力学，将被用于研究粘滑行为，并确定摩擦对速度的依赖在几十年的时间。原子建模的最新发展，包括模拟单个粗糙接触大小的大型系统的能力，加速分子动力学技术，以及模拟二氧化硅/水界面的可靠力场，为解决与地壳断层摩擦相关的挑战性问题创造了机会。私家侦探将与地质学家密切合作，地质学家正在进行实验项目，旨在回答本建议中确定的问题的补充问题。

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