In-line rheometry for highly filled suspensions through Doppler ultrasound

For chemical, cosmetic, pharmaceutical, and food industries the rheological properties of a product represent an important quality factor. Unfortunately such features are often difficult to estimate, in particular in a continuous production process. In this work, we present an ultrasound (US) Doppler method suitable for in-line rheological assessment of highly filled opaque suspensions. The velocity profile developed by highly filled and opaque polyurethane based adhesives and sealants flowing in a pipe was measured through a proprietary US board. The high resolution profile was then fitted to the power law model and the wall shear rate (WSR), a reliable rheological indicator in high shear-thinning suspensions, was obtained. The accuracy of the method was evaluated comparing the measured WSR to reference values obtained from off-line laboratory rheometry. The error from 78 measurements at flow rates between 1 and 35 ml/s featured a mean +3.6% bias and a standard deviation below 12%. Moreover, measurements are only minimally affected by the variation of several US parameters over a wide range of values demonstrating the robusness of the technique.