On the chemical potential and grand potential density of solids under non-hydrostatic stress

Abstract: Non-hydrostatic stress has a peculiar effect on the phase equilibrium between solids and liquids. This was already pointed out by Gibbs. Gibbs derived his formulation of the condition for liquid-solid coexistence applying a surface accretion process without imposing chemical equilibrium between liquid and solid. Adding particles to the bulk of a solid was not possible in his view at the time. Chemical potentials for solids were later introduced by material scientists. This required extending chemical and mechanical equilibrium with a third condition involving a relation between grand potential densities controlling the migration of the interface. These issues are investigated using a non-linear elastic continuum model (technically an open compressible neo-Hookean material) developed in a previous publication (M. Sprik, J. Chem. Phys. 155, (2021) 244701). In common with a liquid, the grand potential density of the model is equal to minus the mean pressure even if the stress is non-hydrostatic. Applying isothermal compression normal to a liquid-solid interface initially in hydrostatic equilibrium drives the system away from coexistence. We derive the Gibbs-Thomson correction to the pressure of the liquid required to restore phase equilibrium. We find that the coupling between chemical potential of the solid and shear stress is a purely non-linear effect.