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Water was measured at different temperatures using a double-gap measuring geometry. The limiting value of the shear rate range of ideal-viscous flow behavior was found at

γ ̇ = 1300 s-1 at T = +10 °C showing η = 1.3 mPas

γ ̇ = 1000 s-1 at T = +20 °C showing η = 1.0 mPas

γ ̇ = 800 s-1 at T = +30 °C showing η = 0.80 mPas

In each viscosity curve at the mentioned upper limit of the shear rate a clear bend was observed, followed by a distinctive increase in the slope of the viscosity curve, indicating the begin of the turbulent flow range.

Note 6: Observation and visualization of turbulent behavior

Using a special measuring device, flow behavior of dispersions at defined shear conditions can be observed simultaneously as well in the form of a measured viscosity function as well as visually, for example, in order to observe the onset of vortex formation. This process can be recorded via digital photography or video, measuring point by measuring point. (See also Chapter 10.8.2.3: Rheo-optics, velocity profile of a shear flow field, for example, using a measuring cell particle imaging velocimetry PIV, or particle tracking velocimetry PTV).

End of the Cleverly section

Note 7: Daniel wet point (WP), flow point (FP) , and dilatancy index [3.18] [3.19]

The Daniel WP and FP technique used for millbase premix pigment pastes (pigment powder and vehicle), dispersions, paints and other coatings with a high pigment concentration. It is a simple hand-mixing method for characterizing two consistency stages in the take-up of vehicle (mixture of solvent and binder) by a bed of pigment particles. WP is defined as the stage in the titration of a specified amount of a pigment mass (e. g. 20 g) with vehicle, where just sufficient vehicle as incorporated by vigorous kneading with a glass rod or a spatula is present to form a soft, coherent paste-like mass showing a putty-like consistency. FP is determined by noting what further vehicle is required to produce a mixture that just drops, flows or falls off under its own weight from a horizontally held spatula. Between WP and FP, the mass hangs on a spatula with no sign of flow. The unit of WP and FP is volume of vehicle per mass (weight) of pigment [cm3/g].

“Daniel dilatancy index” (DDI) is defined as DDI (in %) = [(FP – WP)/WP] ⋅ 100 %. This is the proportion of the additional vehicle required to reach the FP from the WP. A DDI of 5 to 15 % is considered strongly dilatant, does not disperse well although fluid, showing no tack; a DDI = 15 to 30 % is considered moderately to weakly dilatant, an excellent dispersion, showing some tack; and a DDI > 30 % is considered substantially non-dilatant, a dispersion obtained but with difficulty, showing tacky behavior.

Comment: These three test methods WP, FP and DDI are not scientific since this is a very simple and manually performed technique, and the result depends on the subjective evaluation of the testing person. Even for a given pigment mixture as well WP as well FP obtained may vary significantly if the same pigment paste is used.