Don’t Let Controlled-Stress Get Stressful

You know what it’s like, you’re trying to set up a controlled-stress ramp method for your product but you can’t seem to get a stress range that works; on one sample the cone or plate doesn’t budge but on another the instrument goes over-speed long before the test is over, threatening to expel the sample and add an unwanted stripe to your labcoat. How can you remedy this frustrating situation?

The answer is, don’t use a controlled-stress method; at least not yet. Virtually all good rheometers these days can operate in controlled rate mode as well so make use of it. A controlled shear rate sweep or loop is an excellent method of getting a quick handle on how a product flows, and while it isn’t particularly “good practice” from an experimental design viewpoint it can prove to be an effective comparative tool. Try the following:

Up-ramp: 0 to 100 s^-1 over two minutes,

Peak-hold: 100 s^-1 for one minute

Down-ramp: 100 to 0 s^-1 for two more minutes

Linear data collection throughout.

This should give you a much more controlled and predictable test.

The problem with the controlled stress approach is that in quick “non-equilibrium” tests the position of the yield stress in relation to the start and end stresses can lead to wildly differing degrees of shear being experienced by only moderately different samples. Keep controlled stress methods for static yield stress analysis or equilibrium flow curves – it’ll reduce the stress on you!