Viscosa

Meet Viscosa, a drop of honey who takes her time because she is thick.

Viscosity measures a fluid's resistance to flow, its internal friction. Layers of a flowing fluid slide past each other, and a viscous fluid resists that sliding strongly. Honey is thousands of times more viscous than water. For a small sphere moving slowly through a fluid, George Gabriel Stokes showed in 1851 that the drag force is F = 6πηrv.

Water at 20 °C has viscosity η ≈ 1.0 × 10⁻³ Pa·s. A sphere of radius 1 mm moving at 1 cm/s feels F = 6π × 0.001 × 0.001 × 0.01 ≈ 1.9 × 10⁻⁷ N. In honey the same motion would feel thousands of times more drag.

Viscosity matters in engine oil, paint, blood flow and the slow settling of sediment. Stokes' law is used to measure viscosity by timing falling balls, and to find the size of tiny particles from how fast they settle.

Stokes' law only works for slow, smooth flow. At higher speeds the flow becomes turbulent and drag grows faster. Viscosity also drops sharply when most liquids are warmed.