A Practical Approach to Rheology and Rheometry

Rheology
These creep and recovery curves indicate that “much” happens before the
sample really starts to flow, but it is this response that can be used to link
rheological behavior to a sample’s composition and homogeneity. The yield
value is an important rheological value but one must always keep in mind that
it is also a transient value and that it cannot be considered as a “constant”
completely characterizing a solid’s behavior once and for all. Fig. 25 compares
two ketchup samples indicating their difference in yield and rates of recovery.
deformation [%]
ketchup no.2
test time [s]
Fig. 125 CS-rheometer: Creep and recovery of 2 ketchup samples.
210
shear stress
ketchup no.1
8.2.7 The use of vane rotors for the measurement of yield values.
Making use of vane rotors – star-shaped rotors – provides a test method which
looks on first sight quite primitive, but it is in fact often the best method for the
testing of dispersions with a very weak thixotropic structure. It has been been
mentioned that placing such a thixotropic sample into the annular gap of coaxial
cylinders or in the gap of a cone-and-plate often partially destroys the thixotropic
structure. Waiting for a complete recovery of this structure can be minutes or
hours and for yoghurts this recovery never occurs since its biological structure
cannot be rebuilt once it has been mechanically broken by shear.
It is the great advantage of vane rotors that they may be submerged into
samples which may be left undisturbed in their commercial beakers as used
for yoghurts, ice-creams, desert jellies etc. In factories of these products one
can place 10 or 20 of these beakers into a refrigerator overnight, so that next
shear stress [Pa]