- resistance of a fluid to flow under applied force
- measured in pascal-seconds (Pa-s) or poise
- water has low viscosity (~0.001 Pa-s at 20 C), honey high (~2-10 Pa-s)
- blood is intermediate and exhibits shear-thinning (non-Newtonian) behavior
- decreases with rising temperature in liquids, increases in gases
- dynamic viscosity vs kinematic viscosity (dynamic / density)
- governs lubrication, pipe flow, sedimentation, and magma movement
- Reynolds number relates viscosity to flow regime: laminar vs turbulent
- critical in industrial processes: coatings, food processing, petroleum extraction