Temperature dependence of three-point correlation functions of viscous liquids: the case ofglycerol

Abstract

What causes the dramatic slowing down of flow and relaxation that leads to glassformation in liquids as temperature decreases is hardly understood so far andis the subject of intensive research work. It is tempting to ascribe the strongtemperature dependence of the dynamics, irrespective of molecular details, to acollective or cooperative behavior characterized by a length scale that grows asone approaches the glass transition. To access this length experimentally, weuse the recently introduced three-point dynamic susceptibility, from which thenumber of molecules dynamically correlated during the structural relaxation,Ncorr, can be extracted. The three-point functions are related to the sensitivity of the averagedtwo-time dynamics to external control parameters, such as temperature and density. We studiedNcorr values in an important temperature range for a large number of liquids, and found that itsystematically grows when approaching the glass transition. Here we specially emphasizethe case of glycerol for which we combined dielectric and neutron spin echo spectroscopy tocover more than 16 decades in relaxation time.