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Rheology - understanding flow behaviour

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article image Haake CaBER extensional rheometer.

THE food, cosmetic and pharmaceutical industries deal with the pumping, mixing and processing of diverse fluid streams, including the filling of the final product into containers for final sale or shipment.

Often, these systems exhibit a 'stringy' behaviour delaying production or filling operations.

This property of a fluid often results in the necessity for cleaning of the containers prior to labelling and shipping, adding waste and another expense to an already expensive and time consuming process. The same property clearly heavily influences the mouth or skin-feel of any product.

The stringy behaviour of these fluids is related to a flow property known as extensional viscosity. Products with high extensional viscosity will tend to form threads or strings when poured, chewed or otherwise stretched.

Although extensional viscosity may be related partly to viscoelastic properties, it can only be properly measured in extension, not by the traditional techniques of shear (rotation) or dynamic (oscillation) rheometry.

Until recently, there has not been any method available for these measurements outside of a few research institutes and universities.

Now, however, these properties can be quickly, scientifically and reproducibly measured for the first time using the thermo electron Haake CaBER1 capillary break-up extensional rheometer.

This instrument, available from Rheology Solutions , can provide information on the apparent (relative) extensional viscosity, as well as directly measuring the time to break-up of liquid strands.

With knowledge of the surface tension of the liquid, actual extensional viscosity can be computed.

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