The effects of external forces on the mechanical and transport properties of glasses and liquids
Description
The effects of external forces on the properties of glasses and liquids are investigated in the potential energy landscape formalism. The potential energy landscape is a multi-dimensional surface that describes the potential energy as a function of the atomic coordinates; it provides the underlying terrain over which the system traverses and it defines the system dynamics and determines the system properties Straining the potential energy landscape results in the disappearance of a local energy minimum. Several characteristics of the potential energy surface are monitored as a function of strain, including the normal-mode frequencies and barrier heights, and the curvature and orientations of local minima and barriers. These probes of the potential energy landscape establish the disappearance of the local minimum and elucidate the ensuing relaxation to a different local minimum. A disappearing local energy minimum results in a mechanical instability that is localized in space to approximately 50 atoms and leads to a gradient in the system that forces the atoms to rearrange to different minimum The atomic rearrangements resulting from mechanical instabilities associated with volume changes lead to structural transformations in the absence of thermal activation. These transformations are the cause of the coordination changes in amorphous silicates under high pressure The yielding behavior of ductile glasses under shear is shown to arise from many shear-induced localized instabilities. A simple model is developed in which a large number of mechanical instabilities is averaged. This model gives the macroscopic stress-strain properties including an initial elastic region, a yield stress, and a plastic flow region Shear-induced mechanical instabilities lead to a strain-induced diffusion mechanism that operates in addition to the conventional thermal-activation diffusion mechanism. This leads to an enhancement in the diffusion constant when a liquid is sheared