USING INSTRUMENTAL INERTIA IN CONTROLLED STRESS RHEOMETRY

We describe a new method for characterizing the non-linear behavior of complex fluids at both small and large deformations. For creep measur ements, we use the coupling between the instrumental inertia and the m aterial's elasticity to follow the rheological behavior of a solution of iota carrageenan both above and below the yield stress. It is shown that this coupling selectively excites one particular frequency of th e relaxation spectrum. An analytical calculation is used to quantify t he non-linear behavior near the yield stress. The ''free'' oscillation s observed during the first few seconds allow us to choose the most ap propriate mechanical model. Comparison with experiment shows that even above the yield stress, a linear model can still give independently r eliable information about the changes in each element of the mechanica l model. A comparison of free and forced oscillations in controlled st ress rheometry shows both experimentally and theoretically the conditi ons under which the use of free oscillations is advantageous.