Tip a jar of water and it rushes out. Tip a jar of honey and it crawls. Both are liquids, and both are pulled down by the same gravity. So what is holding the honey back?
Start with something you know
Rub your palms together and you feel resistance, because the surfaces drag on each other. Liquids do the same thing inside themselves. Picture a liquid as thin layers stacked like playing cards. When it flows, each layer slides over the one below, and every layer tugs on its neighbours. That internal friction is called viscosity.
The layers in honey grip each other far more strongly than the layers in water, so honey needs a much bigger push to flow at the same speed.
Try it
Try this
- Press Drop the balls and watch which liquid lets the ball through first.
- Warm the honey to 40 °C and drop again. How much faster is it now?
- Make the ball smaller. Does the order of the race change, or only the times?
What is happening
A falling ball has to push liquid out of its way. The liquid resists being sheared, so it pushes back on the ball with a drag force. The ball speeds up until the drag and the upward push of the liquid exactly balance its weight. From then on it falls at a steady speed called the terminal velocity.
For a small ball moving slowly, George Stokes worked out that steady speed in 1851:
- v
- steady falling speed (m/s)
- ρ
- density of the ball and of the liquid (kg/m³)
- g
- gravity, 9.81 m/s²
- r
- radius of the ball (m)
- μ
- dynamic viscosity of the liquid (Pa·s)
Viscosity sits at the bottom of the fraction. Make a liquid ten times more viscous and the ball falls ten times slower. That is exactly what the race shows.
How viscosity is measured
Isaac Newton pictured a liquid trapped between two plates. Slide the top plate and the liquid shears. The stress τ needed to keep it sliding is proportional to how quickly the speed changes across the gap:
Water at room temperature has a viscosity of about 0.001 Pa·s. Glycerine is about 1.4 Pa·s, and honey around 10 Pa·s, ten thousand times more than water. Divide by density and you get the kinematic viscosity, ν = μ ÷ ρ, which tells you how easily a liquid flows under its own weight.
Heat changes everything
Warm honey pours easily. In a liquid, heat makes the molecules jiggle harder, so they hold on to each other less and the viscosity drops quickly. Gases behave the other way round: hot gas molecules move faster and carry more momentum between layers, so a gas becomes slightly more viscous when heated.
Where you see it
- Engine oil gradesA label like 10W-40 tells you how the oil behaves when cold (10W) and when hot (40). It must be thin enough to reach every part on a cold start and thick enough to protect them when hot.
- Ketchup that will not come outKetchup gets thinner when you shake or tap the bottle. Liquids like this are called non-Newtonian, because their viscosity changes with how hard you push them.
- LavaRunny basalt lava can travel for kilometres, while thick lava piles up into steep domes close to the vent.
- BloodThe viscosity of blood affects how hard the heart has to work to pump it around the body.
Easy to mix up
- Viscosity is not density. Cooking oil floats on water because it is less dense, yet it is far more viscous.
- Sticking to your fingers and resisting flow are different things. Stickiness comes from adhesion to a surface; viscosity is friction inside the liquid.
Check your understanding
5 questions.Viscosity Quiz