Molecular Dynamics Simulations of Creep in Silica-Water Systems*

Geophysical processes are traditionally studied at the continuum level, but large scale molecular simulations now allow detailed studies of the underlying nanoscale mechanisms responsible for large scale behavior. An outstanding question in geoscience is the dynamics of water in thin water films in geological materials under high pressure. In this work, we demonstrate that molecular dynamics simulations provide new insights on creep in water-wetted silicates. We have performed nonequilibrium molecular dynamics simulations of the creep process in a pair of opposing quartz asperities under constant load. The results are compared with a microphysics-based model for thermally activated creep. The thermal activation energy and the time evolution of the system height agree with theory when there is a vacuum between the asperities. Replacing the vacuum with water drastically alters these results by introducing chemical effects which dominate the creep process. Pressure solution of silica and the formation of an amorphous gel at the asperity interface are found to greatly increase the creep rate.