Kinetic fragility

Kinetic fragility is a measure of how sharply a liquid's transport properties—viscosity, diffusivity, or structural relaxation time—deviate from Arrhenius behavior as temperature decreases. Introduced by C. Austen Angell in his classification of glass-forming liquids, kinetic fragility quantifies the non-linearity of the temperature dependence on an Angell plot. Strong liquids have low fragility and follow nearly Arrhenius kinetics; fragile liquids have high fragility and exhibit dramatic super-Arrhenius slowing.

The kinetic fragility is closely correlated with the liquid's thermodynamic fragility—the rate at which its configurational entropy approaches the Kauzmann temperature limit. Whether this correlation is fundamental or accidental is one of the unresolved questions in glass physics. Some theories argue that the same microscopic mechanism governs both kinetic and thermodynamic behavior; others treat the correlation as empirical without deep significance.