Solutions of the Navier-Stokes equation are not unique. We show that a maximum rate of entropy production is necessary admissibility condition to achieve a physical solution. For incompressible, constant density flow, the resulting solution is probably unique. For variable density flows, multiple distinct flow regimes are possible, and are determined by new physics added at the microscale. Front tracking has a role to enforce some examples of microphysical turbulent mixing regimes. A new multidimensional random choice method appears to be the ideal choice among competing sharp interface algorithms for this purpose.