Direct evidence of heterogeneous mechanical relaxation in supercooled liquids.

Dynamic heterogeneity is now considered to be an intrinsic kinetic feature of a supercooled liquid. Here, we access the nonlocal complex modulus of a glass-forming liquid using molecular dynamics simulations. We find that the shear-stress relaxation exhibits a marked wave number dependence in a supercooled state. This dependence provides direct evidence that slow mechanical relaxation takes place heterogeneously in space. Its characteristic length ξ increases with decreasing temperature and becomes mesoscopic. The response involves particle rearrangements over ξ and takes a time comparable to the structural relaxation time τ(α). Our finding suggests that the heterogeneous structural relaxation is of fundamental importance in anomalous viscous transport and viscoelasticity in supercooled liquids.