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These water-based dispersions are mixtures of PEG (polyethylene glycol) and colloidal silica particles (60 weight-%, with d = 400 nm, mono-dispersely distributed). After initial shear-thinning behavior showing η = 100 Pas at γ ̇ = 0.001 s-1 and η = 2 Pas at γ ̇ = 40 s-1, the viscosity value at the “critical shear rate value” of γ ̇ crit = 50 s-1 immediately steps upwards to η > 500 Pas; see Figure 3.19. At this limiting value of the loading velocity, the silica particles are agglomerating, abruptly forming a rigid “hydro-cluster” due to interparticle interactions.

Applications: Shock-proof, stab-proof and bullet-proof protective clothing as a combination of this “nano-dispersion” with synthetic technical textile fabrics; reinforced technical polymers for special functions (e. g. as “nano-composite” STF-Kevlar”) [3.17].

For “Mr. and Ms. Cleverly”

Note 5: Increased flow resistance due to flow instabilities and turbulence

Increased flow resistance can also occur due to hydrodynamic flow instabilities which may lead to secondary flow effects and even to turbulent flow behavior showing vortices at high shear rates. In this case, flow curves and viscosity curves will display as well higher values for shear stress and viscosity as well as higher curve slope values compared to curves measured at regular (i. e. laminar) flow conditions, therefore giving at the first glance an impression of shear-thickening behavior.

When performing tests on liquids using concentric cylinder measuring geometries with a rotating inner cylinder (Searle method, see Chapter 10.2.1.2a) there is a critical upper limit between laminar and turbulent flow conditions in the circular gap. Exceeding this limit, secondary flow effects may occur for the reason of centrifugal forces or inertial forces due to the mass of the fluid. The critical limiting value can be calculated in the form of a Taylor number (Ta). The range of turbulent flow is also reached when the critical Reynolds number (Re) is exceeded. Re numbers represent the ratio between the forces of inertia and flow resistance. (More about Ta number and Re number: see Chapters 10.2.2.4 and 11.3.1.3.)