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

该奖项是根据2009年美国复苏和再投资法案（公法111-5）项目重大意义浅层构造地震是由地壳断层中发生的不稳定性驱动的。地震可以看作是一个动态运行的剪切裂纹。裂纹表面的摩擦磨损决定了断裂区的稳定性。裂纹的表面是粗糙的，并且它们的滑动机制不仅受到表面性质的影响（例如，拓扑学、化学），但也受到裂纹界面附近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唯象定律。这些理论缺乏对摩擦滑动过程中实际演变的物理机制的理解，因此这些理论的预测能力有限。PI将使用分子模拟来确定代表地壳断层的表面的摩擦和粘附，她将把摩擦响应与滑动过程中发生的基本化学和力学机制联系起来。确定地壳断层摩擦的基本机制将为地震力学带来物理见解，并将为用于预测地震的本构律提供物理解释。地震现象对二氧化硅/水界面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，不期望的粘附已被证明在微和纳米机电系统（MEMS/NEMS）的可靠设计中是禁止的。MEMS/NEMS通常由二氧化硅制成，其在环境条件下快速氧化并形成无定形二氧化硅层。降低二氧化硅的粘附性，特别是在潮湿环境中，是MEMS/NEMS设计中的突出挑战之一。技术概述即使名义上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（粗糙）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。发展这样一个理论的挑战源于可能的能量耗散机制的众多和复杂性，这些机制有助于摩擦，例如，位错辅助滑移或化学键合穿过界面。如果环境中存在水，问题的复杂性甚至会进一步增加，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，这是一个特别令人兴奋的领域的应用分子模拟方法。在拟议的项目中，PI和她的团队将确定物理机制，在一个单一的粗糙度的水平上控制二氧化硅表面的摩擦。这些机制将被确定为温度，湿度和pH值的函数。摩擦和附着力将在这两个，耐磨和磨损制度和贡献摩擦的表面化学和亚表面变形将被建立。先进的加速分子动力学技术，基于平行复制动力学，将被用来研究粘滑行为，并确定在几十年的时间内摩擦对速度的依赖性。原子模型的最新发展，其中包括模拟大型系统的能力，在一个单一的凹凸接触，加速分子动力学技术，和可靠的力场模拟二氧化硅/水界面，创造了一个机会，以解决具有挑战性的问题，在地壳断层摩擦。PI将与地质学家密切合作，地质学家正在进行实验项目，旨在回答本提案中确定的问题的补充。

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.