NONEQUILIBRIUM ENTROPY OF A GAS

The Boltzmann entropy of a dilute gas under uniform shear flow is analyzed. The entropy variation associated with viscous heating is evaluated and compared with the local equilibrium expression. For interaction potentials other than the Maxwell potential, significant discrepancies are found that can be related to the difference between thermodynamic quantities, defined from the entropy, and kinetic quantities, defined by means of local equilibrium. The discrepancies change, but remain relevant, when artificial external forces are introduced in order to create an ideal stationary state.