  We consider the hydrodynamic evolution of a quark-gluon gas with non-zero
quark masses and net baryon number in its phase diagram. For
far-off-equilibrium initial conditions the expansion trajectories appear to
violate simple rules based on the second law of thermodynamics that were
previously established for ideal or weakly dissipative fluids. For Bjorken flow
we present a detailed analysis within kinetic theory that provides a full
microscopic understanding of these macroscopic phenomena and establishes their
thermodynamic consistency. We point out that, for certain far-off-equilibrium
initial conditions, the well-known phenomenon of "viscous heating" turns into
"viscous cooling" where, driven by dissipative effects, the temperature
decreases faster than in adiabatic expansion.
