The extensional viscosity is an important parameter for many applications, e.g. in sprays (coating and cosmetic), inks or jet fuels (obfuscation) and in food industry. This property is relevant if extensional flow prevails.
Always when the diameter of a flow channel changes the flow can’t be described by shear flow only: Extensional flow takes (at least partially) place. In this case extensional viscosity plays an important role for the flow behavior. The extensional viscosity differs from the shear viscosity; for samples with simple flow behavior this difference can be calculated. For complex samples like polymer containing mixtures the extensional viscosity has to be measured. A typical example for an extensional flow is the streaming (entering or leaving) of a liquid through a nozzle. The measuring principle of our extensional viscometer is based on the determination of flows and forces in such a nozzle-
We use a Rheometrics RFX Fluid Analyzer, whose measuring principle is opposed nozzles. Because of the arrangement of the nozzles, an uniaxial extensional flow can be achieved. The required extensional rate can be realized with the variation of the nozzle diameter and the pump velocity.
Schema of the extensional rheometer
The extensional flow rheometer Rheometrics RFX is suitable for the determination of extensional viscosities of liquids with shear viscosities ranging from 50 mPas to ca. 10 Pas. Depending on the viscosity of the sample extensional rates up to 10,000 1/s can be achieved.
Sketch of an uniaxial extension