Interpreting Viscosity Isotherms In Materials With Significant Structural Disorder

Numerical simulations of viscosity isotherms are presented for a model system with substantial structural disorder. The system consists of an ensemble of metastable two-level subsystems, characterized individually by an elementary activation moment mu and a double well potential with a dissipation barrier W(d)=mu H(d) and a level splitting W(s)=2 mu H(s), and collectively by a distribution of equivalent fields p(H(d),H(s))=(2 pi sigma(2)(d)H(d)(2))(-1/2) exp{-[ln(H(d)/H(dm))](2)/2 sigma(2)(d)}x(2 pi sigma(2)(s))(-1/2) exp(-H(s)(2)/2 sigma(2)(s)). The disorder is modeled with broad dispersions sigma(d)=sigma(s)/H(dm)=1.0. Under these conditions, a typical experimental observation window 10 s <= t <= 10(4) s reveals only a short quasilogarithmic fragment of the complete relaxation isotherm. In the current investigation, we present a prescription for evaluating the thermal fluctuation field H(f)=kT/mu based on a plot of T ln(t(r)/tau(0)) versus H(a), where t(r) is the "reversal time" at which an isotherm changes sign (or passes through m=0), and we discuss the merits of this approach with respect to scaling strategies. (C) 2008 American Institute of Physics.