Evidence of Growing Spatial Correlations during the Aging of Glassy Glycerol

We have measured, as a function of the age tₐ, the aging of the nonlinear dielectric susceptibility χ₃ of glycerol below the glass transition. Whereas the linear susceptibility can be accurately accounted for in terms of an age dependent relaxation time τ_α(tₐ), this scaling breaks down for χ₃, suggesting an increase of the amplitude of χ₃. This is a strong indication that the number Ncorr of molecules involved in relaxation events increases with tₐ. For T=0.96×Tg, we find that Ncorr increases by ~10% when tₐ varies from 1 to 100 ks. This sheds new light on the relation between length scales and time scales in glasses.