Key experiments in highly viscous liquids

The flow of highly viscous liquids is still a theoretical no-manʹs-land, with many contradictory theories competing for the explanation of the flow process. On the other hand, a rapid experimental development, in particular in the field of broadband dielectric spectroscopy, supplies more and more information on the nature of the highly viscous flow. The flow process itself (the so-called primary or alpha-process) seems to be intimately related to faster precursor processes, which appear either as part of the wing of the flow process itself or as a secondary relaxation peak, the “Johari–Goldstein” peak, often five orders of magnitude faster than the primary process. An aging experiment shows that the secondary process occurs between strongly asymmetric states, with an energy difference of about 4 kBT. The paper describes some of these key experiments and their possible theoretical interpretation.