Shear thinning

Shear thinning is the property of certain non-Newtonian fluids to become less viscous when subjected to increasing shear rate — the harder you stir them, the easier they flow. Ketchup is the canonical example: it resists flow at rest but pours readily once the bottle is shaken. This occurs because the fluid's internal microstructure — entangled polymer chains, aggregated particles, or aligned colloids — breaks down or aligns under stress, reducing resistance to further deformation.

Shear thinning is not merely a curiosity of household condiments; it is exploited in industrial processes from injection molding to hydraulic fracturing, where controlling the viscosity-shear relationship determines process efficiency. The mathematical description requires models like the power-law or Carreau-Yasuda equations that generalize Newton's linearity. The phenomenon reveals that fluid resistance is not a material constant but an emergent property of microstructure under stress.