High-frequency Rheology of a High Viscosity Silicone Oil Using Diffusing Wave Spectroscopy

Abstract

Measurements and modeling of rheological properties of a high viscosity silicone oil (polydimethylsiloxane, PDMS) at high frequency are reported. The linear viscoelastic properties are measured by small amplitude oscillation shear (SAOS) tests with a rotational rheometer. Furthermore, Diffusing Wave Spectroscopy (DWS) is used, which expands the angular frequency range of the measured loss and storage moduli up to 105 rad/s, in a temperature range of 20 - 70°C. Good agreement between both methods is found in the overlapping frequency region, especially at higher temperatures. The DWS data show that the elastic modulus stays dominant and increases with frequency, without a second cross-over point up till 108 rad/s. Flow curves, measured with rotational and with capillary rheometry up to a shear rate of 7.6 · 104 s-1, show shear thinning behavior, which implies nonlinear viscoelasticity. Comparison of the dynamic and complex viscosity shows that the Cox-Merz rule is valid in a frequency range spanning six orders of magnitude. A multi-element White-Metzner model is proposed as a constitutive equation, which accurately describes the nonlinear viscoelastic properties, including the decrease of the loss and storage moduli during amplitude sweeps in oscillatory shear measurements.

Keywords

• high frequency microrheology
• diffusing wave spectroscopy
• pdms
• cox-merz rule
• white-metzner model