  Thermal energy agitates all matter and its competition with ordering
tendencies is one of the most fundamental organizing principles in the physical
world. Thus, it is natural to enquire if an effective temperature could result
when external energy input enhances agitation. Potentially this could extend
the insights of statistical thermodynamics to nonequilibrium systems, but
despite proposals that the effective temperature concept may apply to synthetic
active matter, biological motors, granular materials and turbulent fluids, its
predictive value remains unclear. Here, combining computer simulations and
imaging experiments, we design a two-component system of driven Janus colloids
such that collisions produced by external energy sources play the role of
temperature, and in this system we demonstrate quantitative agreement with
hallmarks of statistical thermodynamics for binary phase behavior: the
archetypal phase diagram with equilibrium critical exponents, Gaussian
displacement distributions, fluctuation-dissipation relations, and capillarity.
These quantitative analogies to equilibrium expectations, observed in this
decidedly nonequilibrium system, constitute an existence proof from which to
compare future theories of nonequilibrium, but limitations of this concept are
also highlighted.
