Since viscosity is shear-dependent, it should always be given with the shear condition. Typical materials that show this behavior are coatings, glues, shampoos, polymer solutions and polymer melts. Shear-thinning behavior (or: pseudoplastic flow behavior) is characterized by decreasing viscosity with increasing shear rates (Figure 5.2, line 2). The SI system is the international system of units (French: système international d’unités). A previously used unit for the kinematic viscosity was 1 cSt = mm 2/s (centistokes), but cSt is not an SI unit and should not be used any longer. This applies, for example, to tests with flow cups, falling-ball and capillary viscometers. Kinematic viscosity is always determined if gravitational force or the weight of the sample is the driving force. A previously used unit was 1 cP = 1 mPas (centipoise, best pronounced in French), but cP is not an SI unit and should not be used any longer. Other units include 1 kPas = 1,000 Pas (kilo-Pas), 1 MPas = 1,000,000 Pas (mega-Pas). The unit for shear viscosity is 1 Pas = 1000 mPas (pascal seconds, milli-pascal-seconds). For calculations, please note: The units for density are kg/m 3 and g/cm 3, where 1000 kg/m 3 = 1 g/cm 3. The unit for kinematic viscosity is m 2/s = 10 6 mm 2/s. The ratio of 1:1000 can be illustrated with the following figure: The molecules are fish, each of them 10 cm (0.1 m) long, whereas the particles are ships with a length of 100 m (Figure 1.3).Īnother type of viscosity is the kinematic viscosity $\nu$ (pronounced: nu).ĭefinition: $\nu$ = $\eta$ / $\rho$ with shear viscosity η (in Pas) and density ρ (in kg/m 3). This means that the size ratio between molecules and particles is in the range from 1:100 to 1:10,000 (Figure 1.2). One nanometer (1 nm) equals 10-9 m one micrometer (1 µm) is equal to 10-6 m. 50 nm (coiled ball diameter at rest), and mineral particles approx. Molecules in fluids come in different sizes: solvent molecules approx. Why do different substances have different viscosities? Larger components present in a fluid are the reason for higher viscosity values. They develop a flow resistance caused by internal friction. When put into motion, molecules and particles are forced to slide along each other. Rheometry is the measuring technology used to determine rheological properties.Īll liquids are composed of molecules dispersions also contain some significantly larger particles. The term “rhei” indicates that the content of the bottle is liquid.). Exhibited in the German Apotheken-Museum, Heidelberg. The term originates from the Greek word “rhei” meaning “to flow” (Figure 1.1: Bottle from the 19th century bearing the inscription “Tinct(ur) Rhei Vin(um) Darel”. Rheologists describe the deformation and flow behavior of all kinds of material.
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