Click here for more details and procedures on yield point evaluation.įor evaluating time-dependent flow behavior, shearing is kept constant. ![]() The yield point or yield stress is the minimum force that must be exceeded in order to break down a sample’s structure at rest, and thus make it flow.Ī sample’s superstructure can be imagined as being a stable, three-dimensional, consistent, physico-chemical network of forces interacting between the individual components of the sample for example, between the particles of a rheology additive in a dispersion (Figure 6.1). The yield point is the lowest shear-stress value above which a material will behave like a fluid, and below which the material will act like a – sometimes very soft – solid matter (according to prEN ISO 3219-1). With a presentation on a linear scale, however, this range can only be depicted to a limited extent. Often the biggest changes in viscosity just take place within the range of low shear rates, which is below $\dot \gamma$ = 1 s -1. The advantage of diagrams on a logarithmic scale is that a very large range of values (several orders of magnitude) can be illustrated clearly in one diagram. Materials that typically display such behavior include highly filled dispersions, such as ceramic suspensions (casting slurries), starch dispersions, plastisol pastes that lack a sufficient amount of plasticizer, dental filling masses (dental composites) as well as special composite materials for protective clothing.įor evaluating behavior in the low shear-rate range, it is beneficial to use a log-log plot for the diagrams of flow curves and viscosity curves. Shear-thickening (or: dilatant flow behavior) means increasing viscosity with increasing shear rates (Figure 5.2, line 3). Shear-thinning behavior is related to the internal structures of samples. 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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