Small Displacements Superposed on Viscometric Flow

The constitutive equation for nearly viscometric flow is put into a form that allows easier comparison with experiments involving small oscillations superposed on steady shearing. Part of the stress perturbation is evaluated directly in terms of the stress for the undisturbed motion. Two different dynamic viscosities, referring to oscillations parallel and perpendicular to the main flow, are discussed. The low-frequency limits of these viscosities are evaluated in terms of the steady-shearing viscosity, and it is pointed out why these two limits must usually be unequal.