Heterogeneous Activation, Local Structure, and Softness in Supercooled Colloidal Liquids.

We experimentally characterize heterogeneous nonexponential relaxation in bidisperse supercooled colloidal liquids utilizing a recent concept called "softness" [Phys. Rev. Lett. 114, 108001 (2015)]. Particle trajectory and structure data enable classification of particles into subgroups with different local environments and propensities to hop. We determine residence times t(R) between particle hops and show that t(R) derived from particles in the same softness subgroup are exponentially distributed. Using the mean residence time (t) over bar (R) for each softness subgroup, and a Kramers' reaction rate model, we estimate the activation energy barriers E-b for particle hops, and show that both (t) over barR and E-b are monotonic functions of softness. Finally, we derive information about the combinations of large and small particle neighbors that determine particle softness, and we explicitly show that multiple exponential relaxation channels in the supercooled liquid give rise to its nonexponential behavior.