Kinetic Heterogeneities in a Highly Supercooled Liquid

We study a highly supercooled two-dimensional fluid mixture via molecular dynamics simulation. We follow bond breakage events among particle pairs, which occur on the scale of the $\alpha$ relaxation time $\tau_{\alpha}$. Large scale heterogeneities analogous to the critical fluctuations in Ising systems are found in the spatial distribution of bonds which are broken in a time interval with a width of order $0.05\tau_{\alpha}$. The structure factor of the broken bond density is well approximated by the Ornstein-Zernike form. The correlation length is of order $100 \sigma_1$ at the lowest temperature studied, $\sigma_1$ being the particle size. The weakly bonded regions thus identified evolve in time with strong spatial correlations.