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Slippage: Detecting It and Dealing With It

Slippage: Detecting It and Dealing With It

Rheometer and viscometer manufacturers take great pains to get their spindles, cones and plates nice and smooth and shiny. However, this is often the last thing you need as it can allow slippage of the sample at the surface, giving you un-representative results from your test. Slippage happens when particles are forced into a configuration up against a smooth surface that leaves a layer of continuous medium depleted of the dispersed phase. As the dispersed phase volume fraction contributes significantly to the dispersion "bulk" viscosity this depleted layer has a reduced viscosity, thus leading to slippage, or to use the technical term: wall-depletion (See Figure 1)

wall depletion

When a viscosity / shear rate profile is performed on a sample that slips the effect often goes un-noticed. However, if you re-plot your results as viscosity against stress (see figure 2) we can often uncover the phenomenon which manifests itself as steps or shoulders at the low-stress end of the profile.

slippage warning

The easiest way to deal with slippage is to use serrated or crosshatched plates as an alternative to standard smooth plates or cones. These have channels cut into them in which the sample is entrapped during the test (see figure 3) effectively “gripping” the sample and avoiding slip. Similarly serrated lower covers can also be fitted to your rheometer’s lower plate to prevent slippage on the lower face of the sample. If concentric cylinders are being used then these can be grooved with vertical channels for the same effect

serrated plate

Serrated systems such as this can, however, cost a fair bit more than the standard systems. If you would like a cheap alternative you can always get your existing cylinders or plates lightly sandblasted to roughen them up a bit (I wouldn’t recommend you do this with a cone). And for a cheap but surprisingly effective lower plate alternative try sticking a fine-grade glasspaper to the plate with double-sided tape.

If you’re not getting the results you think you should from your instrument slippage may be the issue. Please feel free to contact me and I’ll do what I can to help you.

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