Viscosity
A liquid's resistance to flow, arising from internal friction between adjacent layers of molecules.
Formula / equation
F = ηA (dv/dx)
Unit
Pa·s
In depth
Viscosity increases with stronger intermolecular forces, larger molecules and greater molecular entanglement, and it falls sharply as temperature rises because molecules gain enough energy to slip past one another. Hydrogen bonding makes glycerol and honey far more viscous than water.
Examples in the real world
Water is 0.89 mPa·s at 25 °C, olive oil about 80, and glycerol roughly 950; engine oil thins on a hot day.