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Shear-thinning, or pseudoplasticity, is a غير نيوتوني behavior where a fluid’s اللزوجة decreases with an increasing rate of shear stress. Many common substances, like ketchup or paint, exhibit this property. At rest, they are thick, but they flow more easily when shaken, stirred, or spread. This is often modeled by the Ostwald–de Waele power-law relationship.
Shear-thinning behavior is typically caused by the alignment of the fluid’s internal structure under shear. In polymer solutions, for instance, long-chain molecules are randomly coiled and entangled at rest, leading to high اللزوجة. When shear is applied, these chains begin to untangle and align in the direction of flow. This alignment reduces the interactions and friction between the molecules, causing a decrease in the apparent viscosity. The more the fluid is sheared, the more aligned the molecules become, and the lower the viscosity gets, until it potentially reaches a lower plateau.
This property is highly desirable in many applications. For example, paint should be thick enough in the can to prevent pigments from settling and to avoid dripping from the brush (low shear), but it must flow easily when brushed onto a surface (high shear). Similarly, printing inks need to flow from the printing press onto the paper but then stay put without smudging. In the food industry, products like yogurt and cream are designed to feel thick in the mouth (low shear) but be easy to swallow (higher shear). The opposite of shear-thinning is shear-thickening, where viscosity increases with shear rate.
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