PRINCIPLE OF MEASUREMENT

Vilastic measurements are based on controlled oscillatory flow in a cylindrical tube. The steady rotational method invented by M. Couette in the 19th century was sufficient for simple fluids. However, with the need to understand complex viscoelastic fluids, sensitive dynamic methods are required. Vilastic has developed the tube flow method to provide the sensitivity necessary to make these measurements.
Tube flow overcomes many problems of rotational
instruments that have been adapted to oscillatory flow, such as rotational inertia,
surface tension effects, dimensional reproducibility of fixtures, messy fluid handling,
difficulty of controlling temperature and lack of sensitivity.

The cylindrical tube is
an ideal sample confinement geometry because of its unvarying dimensions. It also provides
a maximum sample volume with the theoretically desired flow and a wide selection of
confinement materials, including glass. The Vilastic system is so sensitive that water is
used as a rheological standard for instrument calibration.

Measurements are performed using oscillatory flow generated at a selected frequency in a precision measurement tube. Sensors monitor the pressure drop across the tube and volume flow through the tube. Resolution of magnitude and phase of the pressure and volume flow allows calculation of the viscous and elastic components of the shear stress, shear rate and shear strain at the tube wall. From these values the viscosity, elasticity and storage and loss moduli are obtained. In addition, many applications are directly concerned with tube flow of rheologically important fluids and the Vilastic instruments provide direct information on pressure, volume flow, and impedance.

 

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