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 xi increases with decreasing temperature and becomes mesoscopic. The response involves particle rearrangements over xi and takes a time comparable to the structural relaxation time tau(alpha). Our finding suggests that the heterogeneous structural relaxation is of fundamental importance in anomalous viscous transport and viscoelasticity in supercooled liquids.