A paper/link to give a brief guidance on how to measure the viscosity using rheometer so that I can tackle questions like"
1)the geometry to be used
2)which test to be performed?
3)range of angular frequencies
4)settings that can reduce the measurement duration
5)procedure for volatile solvents
thank you
Hi Rahul,
The following link may be of some interest, as it gives an introduction to the principals behind rheological measurement and should help get you started. To access the material you will need to register.
http://www.malvern.com/en/support/resource-center/Whitepapers/WP160620BasicIntroRheology.aspx
Some general advice in response to your questions.
1- Each geometry has its own pros and cons. Typically, many rheologists prefer couette (concentric cylinder) or cone-plate geometries. This is because parallel plate can lead to inhomogenious shear, although can be used with varied sample thicknesses (cones and couette geometries are fixed.
2/3- Frequency sweep measurements are typically performed for understanding complex viscoelastic properties of your sample. If you are concerned with viscosity only, shear rate ramps will give you a measure of viscosity as a function of shear rate, and will indicate any non-Newtonian behaviour.
4- From my experience of rheological measurements, optimal settings can be highly sample specific. It may be worth carrying out some quick low resolution measurements to determine the interesting shear rates/angular frequencies that you might want to look into further.
5- Cooling your sample using the peltier control available on most rheometers should prolong the life of your sample, however a solvent trap may help limit the amount of evaporation of your volotile sample. These may be optional extra accessories for your particular rheometer, although I have seen an improvised solvent trap made from an old yogurt pot work surprisingly well.
Hi Rahul,
The advices given by Dr. Malm are very helpful. I can just supplement that concentric cylinders geometry is preferable if one needs to measure viscosity of diluted (low viscous) solution. In this case, torque generated at a rheometer shaft during shearing is low enough because of low resistance of liquid to the deformation applied. Higher contact area of the solution and geometry surface (in the case of concentric cylinders) causes of torque increase and more reliable viscosity values can be obtained as a result. In addition, the use of concentric cylinders allows to prolong testing time because the effect of solvent evaporation is slightly weaker in the case. However, this geometry is hardly suitable for testing of high viscous solutions because of some possible problems with sample loading (entrapped bubbles etc.).
As Dr. Malm mentioned shear rate ramps or shear rate sweep tests are optimal for steady-state shear viscosity measurement. I think, shear rates range of 1-10 1/s will be useful if you need to determine zero-shear rate (Newtonian) viscosity only and four or five points will be enough. Once again, the settings are strongly dependent on the sample under measurement. First, it would be useful to carry out time sweep test at a fixed shear rate to determine time window where the effect of solvent evaporation is negligible.
Best wishes
It is difficult to draw any conclusions just looking at the graphs. May be, data acquisition settings were not optimal. As was mentioned previously settings are strongly dependent on the sample under measurement. To my experience, I usually program 1-3 full cycles for conditioning (it is useful for high viscous systems) and 4-7 cycles for sampling at every point (fixed angular frequency) when programming oscillatory shear deformation tests. It makes test longer but more reliable data can be obtained as a result.
Hi Rahul,
As mentionned previously for such low viscosity sample with highly volatile solvents the bob and cup geometry would be the most suitable geometry.
For your experiments you will have to check that the stress is within the equipment transducer capabilities.
For your dynamic mechanical measurements you'll have to make sure that the strain at which you carry the frequency sweep is within the linear viscoelastic region. You can verify that by carrying out a strain sweep at the extremities of your frequency range. You can then find a good compromise to remain within the transducer range and within the linear viscoelastic region .
Kind regards
Guy
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