Viscosity

Viscosity measures a fluid’s resistance to flow. High viscosity means thick and slow (honey, molasses); low viscosity means thin and runny (water, alcohol).

Two common units:

• Pascal-second (Pa·s) — the SI unit. Water at 20°C is 0.001 Pa·s.
• Poise (P) or centipoise (cP) — cgs unit. 1 cP = 1 mPa·s. Water at 20°C is 1 cP, which makes it the natural reference value.

Reference viscosities (cP, at 20°C unless noted):

• Air: 0.018
• Water: 1.0 (by convention)
• Milk: 3
• Vegetable oil: 65
• Glycerine: 1,400
• Honey: 10,000
• Molasses: 100,000
• Peanut butter: ~250,000
• Pitch: 230 billion (famously dripping in the Pitch Drop Experiment, started 1927)

Viscosity drops sharply with temperature for liquids — honey straight from the fridge is unworkably thick; warmed honey pours like syrup. This is why engine oil has “multi-grade” ratings (5W-30, etc.): the W refers to cold-temperature viscosity, the second number to hot-temperature.