  The relationship between the microscopic arrangement of molecules in a
supercooled liquid and its slow dynamics at low temperature near glass
transition is studied by Molecular Dynamics (MD) simulations. A Lennard-Jones
liquid with polydispersity in size and mass of constituent particles is chosen
as the model system. Our studies reveal that the local structure (that varies
with polydispersity) plays a crucial role both in the slowing down of dynamics
and in the growth of the dynamic heterogeneities, besides determining the glass
forming ability (GFA) of the system. Increasing polydispersity at fixed volume
fraction is found to suppress the rate of growth of dynamic correlations, as
detected by the growth in the peak of the non-linear density response function.
The growth in dynamical correlation is manifested in a stronger than usual
breakdown of Stokes-Einstein relation at lower polydispersity at low
temperatures and also leads to a decrease in the fragility of the system with
polydispersity. We show that the suppression of the rate of growth of the
dynamic heterogeneity can be attributed to the loss of structural correlations
(as measured by the structure factor and the local bond orientational order)
with polydispersity. While a critical polydispersity is required to avoid
crystallization, we find that further increase in polydispersity lowers the
glass forming ability (GFA).
