Читать книгу The Rheology Handbook - Thomas Mezger - Страница 60

Figure 3.5 presents three viscosity functions of the same dispersion all curves are measured in the range between γ ̇ = 0.01 and 100 s-1.

Preset for the duration of each individual measuring point:

Test 1: t = 10 s = const (i. e., with the same time for each one of the measuring points),

Test 2: t = 60 to 5 s (i. e., with variable time, decreasing towards higher shear rates),

Test 3: t = 120 to 5 s (similar to Test 2, but beginning with a longer time)

Results: Dispersions and gels when showing stability at rest, thus, a gel-like viscoelastic structure, usually display a constantly falling η-curve from low to high shear rates, at least in the low-shear range (i. e. at γ ̇ < 1 s-1). This behavior is indicated by the curve of Test 3. In contrast, the η-curves of Tests 1 and 2 are showing “transient viscosity peaks”, since here, still time-dependent effects are measured. The shorter the selected measuring point duration in the low-shear range, the lower are the η-values determined in this range, and the higher are the shear rate values at the occurrence of the peaks.