(220h) Can We Define a Unique Microscopic Pressure in Inhomogeneous Fluids?

The estimation of the microscopic pressure in an adsorbed thin film on a planar surface remains a challenge in both experiment and theory. While the normal pressure is well-defined for a planar surface, the tangential pressure parallel to the surface at a point is not uniquely defined at the nanoscale. We show that by integrating the local tangential pressure over a small region of space, Δz, roughly the range of the intermolecular forces, it is possible to define an effective tangential pressure that is unique and free from ambiguities in the definition of the ‘virial-route’ local tangential pressure. Grand canonical Monte Carlo simulation results suggest that this characteristic length Δz for the adsorbed contact layer next to the wall has a weak correlation with the adsorbate-solid interaction strength, and the value of Δz converges for large slit pores. With that, we can also define an effective thickness for the adsorbed layer, the value of which agrees with the reported data. This unequivocal definition of the effective tangential pressure would help bridge the gap between experiment and theory in comparing and estimating the microscopic pressures in highly inhomogeneous systems.