Hydrodynamics from free-streaming to thermalization and back again

We study the evolution of the Knudsen and Reynolds numbers in (0+1)-dimensionally expanding fluids with Bjorken symmetry for systems whose microscopic mean free path rises more quickly with time than usually assumed. This allows us to explore within a simple 1-dimensional model the transition from initially thermalizing to ultimately decoupling dynamics. In all cases studied the dynamics is found to be controlled by a hydrodynamic attractor for the Reynolds number whose trajectory as a function of Knudsen number makes a characteristic turn as the dynamics changes from thermalizing to decoupling. We argue that this feature is robust and should also manifest itself in realistic 3-dimensional simulations of expanding heavy-ion collision fireballs. (C) 2019 The Authors. Published by Elsevier B.V.