Fragile-to-strong crossover in supercooled liquids remains elusive.

Transport properties of glass forming liquids change markedly around an onset temperature To. For temperatures T above To, these properties depend little with T , while for T < To these properties are super -Arrhenius – varying faster than exponentially in 1/T . Upon lowering temperature significantly further, theory [1] predicts that reversible transport in a supercooled liquid, if it can be observed, will ultimately cross over from superArrhenius to Arrhenius temperature variation. But this so-called “fragile-to-strong” (FS) crossover at a temperature Tx < To has proved difficult to observe because most bulk fluids fall out of equilibrium at a glass transition temperature Tg that is higher than Tx. Yet Mallamace et al [2] report the observation of Tx for a large number of supercooled liquids. In truth, they observe the onset to supercooled behavior, and the reported values of Tx are poor lower-bound estimates to onset temperatures To. This fact is consistent with transport decoupling appearing only below temperatures identified with the crossover in Ref. [2]. To illustrate this understanding about Ref. [2], I graph data in Fig. 1 for two typical supercooled liquids. The data is compared with the parabolic form for transport property τ (denoting either relaxation time or viscosity),