Fragile vs. strong liquids:: A saddles-ruled scenario

In the context of the energy landscape description of supercooled liquids, we propose an explanation for the different behaviour of fragile and strong liquids. Above Goldstein's temperature T-x, diffusion is interpreted as motion in the phase space among saddles of the potential energy. Two mechanisms of diffusion then rise: mechanism A takes place when the system overcomes potential-energy barriers long stable uphill directions, while mechanism B consists in finding unstable downhill directions out of saddle. Depending on the mutual efficiency of A and B, the usual classification of liquids in fragile and strong is recovered. Moreover, this scenario naturally predicts the possibility of fragile-to-strong crossover when lowering the temperature.