Isotropic linear constitutive relations for nonsimple fluids

We investigate the general constitutive relation of an isotropic linear fluid when the stress tensor can depend on higher-order spatial gradients of the velocity. We apply the results to the case of second-grade and third-grade fluids, be they compressible or not. However, the expression of the general isotropic tensor can be a matter of interest also for other classes of nonsimple material.