Hello!
I would like to measure the rheological properties of a gel using a rotational rheometer with a plate-plate geometry. The sample is a cylinder with diameter D and thickness h. Normally I make two tests: first the amplitude sweep to determine the LVE range and then the frequency sweep (on a new sample).
How should I choose the optimum thickness h? Could I get reasonable data if my sample has h=D/2, or I should cut it into smaller pieces? Are there any recommendations?
It shouldnt really matter to be honest, simply ensure all the plate is covered by the gel. I would do a few thicknesses to check that it doesn't matter though to be on the safe side.
Note if you have particles present need to ensure height is greater than around 25 diameter of the particles, otherwise a chain of particles could bridge between the the two plates
paul
Use between 1mm and 2mm. It makes a small difference, depending on the diameter of the plates.
I would choose h = D/3 or D/4 at most. For example, in hydrogel systems I work with I often use D=25mm and h~1mm. Be careful of going too low as well though - pieces of dust can get trapped and ruin your measurement if your gels have a very low modulus. I agree with Paul - you could try a few to make sure your measurements are consistent.
There are other, more complicated reasons for choosing a particular gap size in certain measurement regimes, e.g. high-velocity flows or very low torque signal. I recommend Prof. Randy Ewoldt's book chapter on avoiding bad data for more information on those cases: http://ewoldt.mechanical.illinois.edu/pubs/Ewoldt_2015_SpringerChapter_Author-generatedPREPRINT.pdf
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