True Shear Parallel Plate Viscometer
- Wednesday, 01 September 2010
This instrument is designed to measure the viscosity of non-Newtonian liquids.
This viscometer (which can also be used as a rheometer) is designed for use with liquids over a large temperature range. The device consists of horizontally disposed, similarly sized, parallel plates with a precisely known gap. The lower plate is driven laterally with a motor to apply shear to the liquid in the gap. The upper plate is freely suspended from a double-arm pendulum with a sufficiently long radius to reduce height variations during the swing to negligible levels. A sensitive load cell measures the shear force applied by the liquid to the upper plate. Viscosity is measured by taking the ratio of shear stress to shear rate.
The bearing points of the suspended plate and the upper arms of the pendulum ensure any motion is constrained to one axis, and no friction or resistance from the apparatus contributes to the load. Unlike other viscometers, e.g., those using rotating cylinders or disks, this design follows the simplest mechanical model to measure viscosity. By using large vitreous quartz plates and small gaps, liquids with very low viscosities at low temperatures can be measured with the same tooling as viscous liquids, like glass at elevated temperatures. By maintaining a constant gap and driving the lower plate with a linear driver motor, the liquid between the plates undergoes a uniform amount, and therefore rate, of shear over its volume.
When using the vitreous quartz plates to contain the liquid, high-temperature operation up to 800 °C, or more, can be considered. The transparency of the plates permits a precise measure of the liquid area of contact on the plate with a camera or similar method. One can recover the gap dimension from knowledge of the liquid area and the amount of liquid volume used; alternatively, one can set the gap and measure the area of spread to determine the liquid volume if it is an arbitrary amount. Another advantage of this method is the ability to quickly adjust the gap, and to determine what it is precisely (from the liquid spread area) between measurements of viscosity.
Although not implemented in the prototype, the plates can be temperature-controlled to ensure the liquid in contact with them is at proper temperatures. By building heaters and temperature sensors into the plates, one can be certain the liquid in contact will have the proper temperature. The thickly spread liquid will respond to the substrate (heater plate) temperature very rapidly. The thermal lag will be confined to the heater plates themselves, but not to support structures or fixtures.
The pendulum mounting of the upper plate offers noise elimination because only the lateral shear force is read on the load cell. The fixture and sample mass do not interfere with the load measurement. Furthermore, particularly worthwhile with viscous glasses, only small displacements away from the rest position of the pendulum are required to make a measurement. This reduces instrument error to a minimum.