A novel difference between strong liquids and fragile liquids in their dynamics near the glass transition

The systematic method to explore how the dynamics of strong liquids (S) is different from that of fragile liquids (F) near the glass transition is proposed from a unified point of view discussed recently by Tokuyama. The extensive molecular-dynamics simulations are performed on different glass-forming materials. The simulation results for the mean-nth displacement ^Mn(t) are then analyzed from the unified point of view, where n is an even number. Thus, it is first shown that in each type of liquids there exists a master curve Hn(α) as ^Mn(t)=^RnHn(α)(^vtht/R;D/R^vth) onto which any simulation results collapse at the same value of D/R^vth, where R is a characteristic length such as an interatomic distance, D a long-time self-diffusion coefficient, ^vth a thermal velocity, and α=F and S. The master curves Hn(F) and Hn(S) are then shown not to coincide with each other in the so-called cage region even at the same value of D/R^vth. Thus, it is emphasized that the dynamics of strong liquids is quite different from that of fragile liquids.