I am trying to do oscillatory rheology of concentrated polymer suspensions (glass microspheres with mean diameter of 6.1 micron) and slip is observed at higher frequencies above 100 rad/s. I've tried using a smaller strain of 0.3% in stead of 3% that is within the LVR of the unfilled polymer. I've seen in the literature that sandpaper can be used on a parallel plate geometry, but I am wondering how to pick an appropriate grit of sandpaper.
Should the sandpaper grit be larger than the particle filler, smaller, or as close as possible?
Hello Derek,
These are great questions!
One answer is that the surface roughness of the geometries should be of sufficient depth (volume) to accommodate the quantity of continuous phase material that separates from the body of the sample. This then allows the geometry to grip the body of the sample and not only the slip layer.
In some materials there is a stress induced phase separation, as for instance with tomato ketchup, where the tomato fibers bend to release more juice when the test starts. These materials need very rough surfaces or even the use of a small vane tool in a large cup to test the bulk of the sample. However in other regular dispersions, such as the one you describe, the issue may be boundary concentration depletion (as described by Barnes et al, Article A review of the slip (wall depletion) of polymer solutions, ...
) and the surface roughness probably only needs to be more than the particle size at a minimum to prevent slip.To specifically answer your question, the normal procedure to identify what roughness is required to prevent sample slip, is to run the test with increasingly rougher surfaces until the shape of the viscometry flow curve becomes constant (indicating that slip has been prevented). Obviously with very rough surfaces, the gap errors are large, and so we would use a surface just rough enough to prevent slip, but also not to introduce other errors more than necessary.
One last thought is that the artifacts you are observing may not only be wall slip if you are seeing them at higher oscillatory frequencies. These tests can be challenging for the best of us, with the possibility of exceeding the Schrag "gap loading limit" of the sample in the test conditions and introducing a lot of geometry/motor inertia (which will ideally be corrected if everything is well calibrated). I attach a document that may help consider these other issues as well.
Best regards,
Phil
Thanks for the suggestions Philip Rolfe . I was able to try 400 grit sandpaper (18.3 um) since that is larger than the largest particle according to the size distribution. This grit eliminated the slip I was seeing for a 10wt% composite and I will be testing more concentrated suspensions soon.
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