Hi, I am looking to conduct rheometric experiments on water or hydrogels whose viscosity is very less. Should I use Dynamic light scattering or a rheometer?
A rotational rheometer will be a good solution, but choosing the proper measuring system is crucial. For high shear rates, a double gap system with appropriate parameters will be best, and for low shear rates, I would recommend a cone-plate or plate-plate with a large area.
Please define the high shear rate range and the low shear rate range I should use
Richa Ghosh The exact range depends on the exact parameters of the measurement system. A universal solution will be to test the system on standardized solutions of known viscosity and check to what range of shear rate the result is within the range of acceptable tolerance.
Hi Richa Ghosh , most geometries can get you good results from 10 1/s to ~500 1/s. The challenges are that at the lower shears with a small geometry, you will be pushing the lower torque limit of the rheometer. At the higher shear rates and with low viscosity samples, you may generate turbulent flows which give an artificially high viscosity if your gap is wider.
So, with a cone 1°60mm you should be able to generate good data with *water* from 1 - 500 1/s. With a cone 1°50mm you will probably get good data from 5 - 1000 1/s. With a parallel plate 50 or 60mm you can get to higher shear rates with a narrow gap, as long as everything is well aligned.
For sample of 10 cP viscosity and similar density as water, you should be able to get 10 times as fast shear before turbulence begins. I attach a spreadsheet that may be helpful as well. To use it, you'll need to enable it in Excel, then enter the instrument type, geometry and gap in the yellow squares and the graph updates for you.
Philip Rolfe thank you for sharing the file with me.
As to the suggestion: 1°60mm you should be able to generate good data with *water* from 1 - 500 1/s. With a cone 1°50mm you will probably get good data from 5 - 1000 1/s.
Did you decide upon the range using the excel file you have shared? If yes, then which instrument type have you selected? I didn't get that range for any of the instrument settings given in the excel for the CP 1/60 or CP 1/50. Apart from instrument type modification, for geometry modification I have modified only the Cone plate to 40 mm later. I am using TA instrument's rheometer as well so don't know which instrument listed will come close to it.
Following your suggestion, for 40 mm CP geometry I have selected 10-1000 1/s. Will that work?
Hi @Richa Ghosh, the values i stated were just a guess, not from the spreadsheet. The TA instrument may be similar to ours, you’d need to check the torque range on the brochure on their website for that model. The Cone (2degree??) 40mm maybe ok for your testing - I’d try it. For water it’ll possibly give turbulence at the high end.
The best way to approach such tests is to tun a viscosity std before your sample and see if the reuskts lokk accurate. Ideally that std will have a similar viscosity as your samples.
Philip Rolfe I have cone 3 degree and 1 degree of 40 mm diameter. Which one in your opinion can give a good result and why?
I have run the test with viscosity standard samples. It gave pretty good results however, the viscosity present in the lab was of 101 mPa.s and hence I tried with the water sample as I am predicting the hydrogels I will be dealing with is of low viscosity.
Yes, for the TA instruments, the maximum Torque is 200 mN.m, minimum Torque steady is 0.05 uN.m (control rate mode) and 0.1 uN.m (control stress mode). Minimum torque oscillation=0.03 uN.m (control rate mode) and 0.1 uN.m (control stress mode). The torque resolution is 1 nN.m
If I use the Old Pro model, then the differene in maximum torque is by 50 mN.m and in the minimum torque oscillatory is by (0.02-0.05) uN.m range. Can this slight variation offset the predicted shear rate by much?
Richa Ghosh The viscosity of water is well documented. Experimental work will give you the conditions for an accurate determination. A set of experiments will tell you far more than any debate or any number of Greek letters…
You can fully measure that fluid by a Rheometer. I use RS600 rheometer (it`s a quite old rheometer).
BR.
Different test geometries are used for different viscosity ranges. For water-like substances a simple capillary rheometer is enough.
DLS gives particle size dispersed in some solvents like silica in ethanol. From Rheometer, we can find whether the fluid is viscoelastic by doing oscillatory experiments like amplitude sweep and frequency sweep. We can also do continuous shear experiments like viscosity wrt to shear rate and finds whether the fluid is shear thickening, Newtonian or shear thickening.
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