At different solid concentrations, I'm not getting favorable results at lower shear rates from the rheology test.
But when I go to a higher shear rate, I'm getting favorable results.
Is this problem due to slip and sedimentation effects? If yes, then Is there any method to reduce this problem without changing its geometry?
When working with colloidal suspensions, slipping can be reduced by drying a layer of colloids onto the surface of your rheometer tools.
This was done to separate shear banding and slipping effects in the following paper.
https://www.researchgate.net/publication/265137770_Optical_Coherence_Tomography_Velocimetry_of_Colloidal_Suspensions
The rheometer plates were spin coated with a dilute suspension of colloids, and then baked in a vacuum oven to fix the coating in place.
Sedimentation can be controlled by varying the density of the solvent in the suspension. This process is described here in Section 4.2
https://www.era.lib.ed.ac.uk/bitstream/1842/5764/1/Harrison2011.pdf
Article Optical Coherence Tomography Velocimetry of Colloidal Suspensions
There are way to reduce slip. One is use oscillatory shear measurement compare to steady flow and you can use sand paper.
In the conditions of parallel plates attached with sand paper, treated sample, smaller diameter fixture, larger gap size, shorter shearing time, and normal force control loading method, the wall slip effects could be minimized.
Apart from roughened cylinders, using the vane geometry is a standard way to avoid slip. Standard data treatment is given in our paper in Applied Rhology: "Vane rheometry with a large finite gap" DOI: 10.3933/ApplRheol-12-81
You will always get slip using thin gap rheometry with suspensions. Going to a vane geometry and a cup to vane diameter ratio of greater than 3 will give consistent results. No other techniques are robust.
Avoiding slip is relatively easy as pointed out in several answers (roughened cylinder, vane geometry with at least 6 blades, sand paper...). You may also have wall depletion which is a different problem.
Sedimentation is a lot tougher to deal with, especially as its effects are not simple. In particular, when dealing with a concentrated suspension, sedimentation can produce a yield stress (Fall "Rheology of sedimenting pastes", J. Rheol.). To my knowledge there are two ways de diminish its importance: use a calibrated helicoidal ribbon or do very fast oscillatory measurements using the method described in Ghiringhelli Rheol Acta 2012.
In addition to the methods to eliminate slippage proposed by my colleagues, you can stick the fluid at the wall.
They need to understand the physical chemistry of the fluid.
Here are 3 examples of my work for silicone grease, cheese and aqueous gels (Carbopol).
Vane and plate-plate rheometry of cheeses under oscillations and wide strains: a comparative study and experimental analysis Conditions
DOI: 10.1016 / j.idairyj.2014.04.006
Stick-slip control of the Carbopol microgels is transparent polymethyl methacrylate smooth walls
DOI: 10.1039 / C2SM26244D
Shear rheometry of fluids with a yield stress
Journal of Non-Newtonian Fluid Mechanics 01/1987; 23: 91-106. DOI: 10.1016 / 0377-0257 (87) 80012-5
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