Nonequilibrium fractional correlation functions and fluctuation–dissipation in linear viscoelasticity

Abstract

A simple model for time retarded fractional density and velocity hydrodynamic fluctuations correlations of a linear viscoelastic fluid under an external gradient is proposed. A fractional generalized Langevin equation with a Caputo’s fractional time derivative is formulated for a power-law memory kernel with long correlation noise. The fractional density and velocity fluctuations correlation functions, as well as the fractional light scattering spectra, and the fractional intermediate scattering function (FISF) are calculated analytically for power-law viscoelasticity. Their relation to the longitudinal modulus of the viscoelastic fluid is also obtained. We find that for different frequency intervals appropriate to perform isothermal light scattering experiments using Fabry–Perot interferometry and photon correlation, the non-equilibrium fractional structure factor may be smaller or larger than its fractional equilibrium values. We also discuss the validity of a fractional fluctuation–dissipation theorem for this model in terms of the asymptotic behavior of the equilibrium fractional correlation functions of the model.