where formula_46 is the density, formula_47 and formula_48 are the mass and heat fluxes, and formula_49 and formula_50 are the mass diffusivity and thermal conductivity. the fact that mass, momentum, and energy (heat) transport are among the most relevant processes in continuum mechanics is not a coincidence: these are among the few physical quantities that are conserved at the microscopic level in interparticle collisions. thus, rather than being dictated by the fast and complex microscopic interaction timescale, their dynamics occurs on macroscopic timescales, as described by the various equations of transport theory and hydrodynamics.