Nonequilibrium thermodynamics of colloids

According to the common rules of classical nonequilibrium thermodynamics an Onsager coefficient like the viscosity is not allowed to depend on the shear rate. Such a dependence is experimentally well documented, however. In this paper it is shown, using nonequilibrium thermodynamics and the concept of internal variables as already introduced by Prigogine and Mazur in 1953, how it is possible that the effective viscosity coefficient of an isotropic colloidal system depends on both the shear rate and the frequency. For a low density suspension of axi-symmetric spheroids the various Onsager coefficients involved are calculated explicitly.