| Summary: | Measurements and modeling of the nonlinear viscoelastic properties of a high viscosity silicone oil (polydimethylsiloxane, PDMS) are reported. LAOS test were performed with a high precision rotational rheometer to probe the nonlinear response. The measurements show that the material can be safely considered linear below strain amplitude 1. The viscous Lissajous-Bodwitch curves indicate intracycle shear thinning, whereas the elastic Lissajous-Bodwitch curves indicate intracycle strain stiffening in the nonlinear regime. Secondary loops in some of the measured viscous stress curves are attributed to a non-sinusoidal shear rate signal. A multi-element White-Metzner model is used as a constitutive equation, which accurately describes the LAOS data in all measured cases. Based on the extension of the measured data by simulations, nonlinear properties are analyzed both for the elastic and for the viscous part. It is observed that the nonlinearity considerably increases the weight of the higher harmonics in the shear stress signal. It is predicted that the viscous nonlinearity has a maximum around 50 rad/s angular frequency, and that the elastic nonlinearity becomes nearly independent of the angular frequency above 30 rad/s.
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