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Vane Test Methods (Vane Shear Tests)
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| Soft Solids Tester |
Vane measuring systems have been used for some years now for testing gels,
slurries, pastes and other soft solids. Their profile has risen in recent years with the launch of easy-to-use vane attachments
for research rheometers from TA Instruments, Haake etc and dedicated vane testers such as the Soft Solids Tester (developed
by myself for Brookfield - see photo) and the YR1 Yield Tester. The vane approach brings with it two big benefits:
- The sample is hardly damaged at all when you insert the vane prior to testing. Damage to sample “structure”
on loading is a real pain if you’re trying to assess the ongoing changes in a dispersion whilst, for example, on storage.
I would suggest that the vane insertion is one of the least destructive sample-loading processes you can find in rheometry.
- Slippage can be virtually eliminated, making the test method ideal for suspensions such as mineral slurries or ketchups,
or set gels and waxes that can’t be “gripped” by a smooth cone or plate.
The vane itself is a (usually) four-bladed paddle that, when inserted into a soft-solid sample and rotated slowly causes
the sample to deform and shear as if it were in a wide-gap concentric cylinder system that is made of the sample itself (see
fig).
Various test methods can then be performed with this arrangement. Depending on the “host” instrument’s
capabilities these can include:
- A slow controlled rate rotation is imposed on a gelled sample until it yields on the surface of the (imaginary) inner
cylinder (ie we cut a hole in it!). The stress on the sample at this point is then recorded as the yield stress.
- We apply a constant stress and perform a creep test to assess zero-shear viscosity and viscoelastic behaviour. This approach
works very well on “lumpy and chunky” samples; with the Soft Solids Tester I had some great results measuring
diabolically difficult samples such a pie-fillings, jams and even wet concrete!
- Oscillatory tests can also be performed for viscoelastic analyses.
Most vane testing is of this nature, slow and gentle, working with the sample within and up to it’s viscoelastic
limit or yield point. However, you can also perform what is known as mixer viscometry where vanes are rotated at higher RPMs
and flow curves are obtained. This can help assess pumpability of settling slurries etc.
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