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Rheology - Geometries

Geometries - Pros and Cons

Text books contain a huge selection of geometries for your rheometer. But what should you buy? With a simple geometry costing in excess of €500 you want to buy what you are actually going to use and you simply can't afford to buy everything. Also consider having your geometries sandblasted, it helps reduce slip when testing emulsions and there is no real disadvantage, apart from the extra initial cost.

Cone and Plate

Pros:

Rheologically correct because the angled cone gives an even shear Field. Good for simple, single phase liquids.

Cons:

You can't alter gap. This limits the range of shear rates you can achieve.

If you use a cone for heating experiments then drying out is a problem. Our recommendation is to add oil and a solvent trap. Check for jumps in the curves that might suggest drying and sticking.

You shouldn't use if your sample has particles greater than 1/10 the gap distance. The gap distance is typically 50μ so don't use it if your particles are above 5μ.

Parallel Plate

Pros:

You can alter the gap which is useful if you have small particles in your sample or you want to change the shear rate range significantly. Get it sand blasted and you will have a very useful geometry.

Very popular in industry due to its flexibility.

Cons:

Rheologically shear varies across plate so its not as rheologically correct as the cone and plate.

Concentric Cylinder / Bob and Cup

Pros:

One of our favourite geometries. Rheologically correct and very useful for temperature sweeps as the small exposed surface area gives little evaporation and practically none with a thin layer of oil added.

Cons:

Somewhat messier to clean.

You probably cannot run temperature ramps as fast as you can on a plate or cone.

Double Gap Cylinder

Pros:

The idea behind this geometry is to incease the low stress measurement capability compared to a standard concenric cylinder.

Cons:

You probably have about double the surface area compared to a similar concentric cylinder. So a similar torque translates to double the stress. It sounds a lot but when most rheological curves are plotted with log-stress then it doesn't actually gain you as much as you might think.

Somewhat messy to clean.

You probably cannot run temperature ramps as fast as you can on a plate or cone.