Creaming creep measurements

Simple calculation shows that is in the range 10" to 10" Pa, which implies that for prediction of creaming or sedimentation it is necessary to measure the viscosity at such low stresses. This can be achieved by using constant stress or creep measurements. 

The most useful method for predicting creaming is to use constant stress (creep) measurements, from which the residual (zero shear) viscosity j (0) can be obtained. 

Thanks to their relatively lower viscosity, food and cosmetic products allow this type of behavior to be easily documented, when performing experiments with controlled stress rheometers. There are commercial versions of such instruments, essentially rotating systems (parallel disks or cone-and-plate), which have the capability to measure extremely small rotation rates (in the 10 rad/s, i.e., one revolution in 20 years ). Experiments performed in such conditions are called creep testing using controlled torques as low as 10 Nm with a resolution of 10 Nm. Figure 5.5 shows an example of the shear viscosity function (vs. shear stress) measured on a typical cosmetic product (body cream), using such a controlled stress rheometer. 

---