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

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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 based on the mean-field theory 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⁽ⁱ⁾ as Mₙ(t)=RⁿHₙ⁽ⁱ⁾(vtht/R;D/Rvth) onto which any simulation results collapse at the same value of D/Rvth, where R is a characteristic length such as an interatomic distance, D a long-time self-diffusion coefficient, vth a thermal velocity, and i=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/Rvth. Thus, it is emphasized that the dynamics of strong liquids is quite different from that of fragile liquids. A new type of strong liquids recently proposed is also tested systematically from this unified point of view. The dynamics of a new type is then shown to be different from that of well-known network glass formers in the cage region, although both liquids are classified as a strong liquid. Thus, it is suggested that a smaller grouping is further needed in strong liquids, depending on whether they have a network or not.