Shear thinning

Intrinsic viscosity, also known as shear thinning, is the property of a fluid to expand at high shear forces decreasing viscosity. That is, the stronger the shear is applied to the fluid, the less viscous ( thick ) it is. In English, such a fluid is therefore aptly called "shear - thinning " the Germans will " shear thinning ," which is sometimes used as a synonym for thinning.

The decrease in viscosity caused by a structural change in the fluid, which ensures that the individual fluid particles (e.g., polymer chains ) is better able to slide past each other.

As a pseudoplastic fluid, the viscosity does not remain constant with increasing shear, it is classified as a non-Newtonian fluid. Other fluids from this classification, inter alia, have the following properties:

• Dilatancy: the opposite of intrinsic viscosity,
• Thixotropy: viscosity decrease with the load duration
• Rheopexy: viscosity increase with the load duration

Examples

• In polymer solutions and melts, the individual polymer chains are entangled with each other ( hooked =). With increasing shear force, these entanglements and viscosity solve decreases. This effect plays an important role in the processing of thermoplastics.
• Non -drip wall paint does not drip from the roll because the shear thus the strength or viscosity is small and large, while it is easy to apply on the wall, because the thin layer between the wall and roll causes a large shear and thus the viscosity of small is.
• Blood has a high viscosity at low shear forces. With stronger shear forces, the red blood cells to deform rather elongated structures. This allows the blood to flow better through the small veins.
• Associative materials are systems in which self-assemble through physical interactions, such as hydrogen bonds or ion-dipole interactions, small molecules - to supramolecular systems. This shear (as opposed to covalent bonding ) weak links to be broken up, which lowers the viscosity. The special feature here is that the bonds after a certain material-specific time can fully regress (→ thixotropy). Technically important representatives are ionomers.

Flow limit

For systems with fillers occurs a special feature. Example, one can understand blood as a suspension of water and various solids ( red blood cells). If the solids agglomerate in a certain formation, results in a changed flow behavior. At low shear rates occur a strong flow obstruction, which is called as a yield point. A simple representative of these substances is the Bingham fluid.

The yield point is for example of importance when attempting to withdraw from a ketchup bottle that has not moved for a long time, a little ketchup. Either the applied shear stress is very low, then do nothing, or the yield point is overcome by strong stress, then comes out way too much.