I have a hydrogel of thickness ~ 4000 um and I want to perform a frequency sweep on it. How do I know that the plate is in contact with the material apart from visually seeing it in contact?
I noticed that as I changed the normal force applied on the material the G’ value changed. I have tried to understand what a normal force controlled exp is, but I don’t know what the ideal normal force would be for contact on my instrument an AR 2000 with a 40 mm Al Parallel plate. From the manual I see that, when I add the geometry I can perform a global normal force control to check if the plate is truly parallel to the surface for a disposable plate ETC.
Do you have any suggesitons on what I should do ? How do I ensure contact with my sample and what is the normal force range I should be looking at?
The first step is to ensure that the plates are truly parallel to each other by performing the global normal force control check. The second step is to pre-decide the gap level to run the frequency sweep. A little more sample should be loaded than what is needed to fill between the parallel plates in the gap. When the plates are brought together and excess sample scraped off, the gap should be slightly more than what has been pre-decided. The next step is to close in the gap to the pre-decided level. It should result in showing a slight bulge on the sample at the edge of the parallel plates. This ensures that the plate is in full contact with the material and the frequency sweep results should come out correct.
Normal force is the result of the presence of elastic forces in the material. Hydrogels would have an elastic component and hence seeing a significant level of normal force is natural. Just as shear stresses exemplify themselves as shear viscosity, the normal forces exemplify themselves as normal stress difference coefficients giving a measure of the elastic component of the material.
Hi Uma,
usually samples become more rigid due to an increasing axial load i.e. you will get lower values for tan(delta). Therefore you need to make sure to run your frequency sweeps with the same axial force in order to compare the data.
If you have properly set the normal force to zero without contact to the sample then the easy answer is: when moving the upper plate downwards, any increase in normal force indicates contact with your sample. The more difficult part is, which normal force is sufficient to avoid slippage and which normal force should not be exeeded to avoid damaging your hydrogel. This answer can only be given by you since you know the properties of your gels and can do some tests with them.
You could run an amplitude sweep in CD mode to detect the onset of slippage. If slippage occurs at strain values below or around the value to plan to use for the frequency sweep, increase the normal force and try again.
In order to avoid slippage, you should also carefully remove water from the contact surfaces of the gel with the measuring geometries if possible.
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