Читать книгу The Rheology Handbook - Thomas Mezger - Страница 36

Low-molecular liquids (and this means here: with a molar mass below 10,000 g/mol) such as water, solvents, mineral oils (without polymer additives), silicone oils, viscosity standard fluids (of course!), blood plasma; but also pure and clean bitumen (without associative superstructures, and at a sufficiently high temperature).

Flow behavior is illustrated graphically by flow curves (previously sometimes also called rheograms) and viscosity curves. Flow curves are showing the interdependence of shear stress τ and shear rate γ ̇ . Usually, γ ̇ is presented on the x-axis (abscissa), and τ on the y-axis (ordinate). However, τ might also be displayed on the x-axis and γ ̇ on the y-axis, but this is meanwhile rarely used in industrial laboratories.

Viscosity curves are derived from flow curves. Usually, η is presented on the y-axis and γ ̇ on the x-axis. Alternatively, the function η(τ) can be shown with η on the y-axis and τ on the x-axis, however, this is less frequently carried out in industrial labs.

Generally, the slope value of each point (x; y) of a curve can be calculated as: y/x. This counts for each point of a flow curve with the pair of values ( γ ̇ ; τ). The result of this calculation again corresponds to the viscosity value, this is because: η = τ/ γ ̇ . Therefore, the η( γ ̇ )-curve can be calculated point by point from the τ( γ ̇ )-curve. Correspondingly, a steeper slope of the flow curve results in a higher level of the viscosity curve (see Figures 2.5 and 2.6). Usually today, this calculation is performed by a software program.

Figure 2.5: Flow curves of two ideal-viscous fluids

Figure 2.6: Viscosity curves of two ideal-viscous fluids

The values of the shear viscosity of ideal-viscous fluids or Newtonian fluids are independent of the degree and duration of the shear load applied.

Viscosity values of ideal-viscous liquids are often measured using flow cups, capillary viscometers, falling-ball viscometers or Stabinger viscometers (see Chapters 11.3 to 11.6). However, when using these simple devices, the results do not accurately mirror the more complex behavior of non-Newtonian liquids (see for example Chapter 11.3.1.2c: change of shear rates in capillaries).

For “Mr. and Ms. Cleverly”

Figure 2.7: The dashpot model to illustrate ideal-

viscous behavior

Figure 2.8: A shock absorber which can be loaded from both sides [2.24]