Why does honey pour so slowly?

Water splashes out of a cup in a second. Honey takes its time. Some liquids are slower still: since 1927, a funnel of tar pitch at the University of Queensland has let fall fewer than ten drops. What makes a liquid slow, and what can you do to make it flow faster?

Grade 8 science · FluidsViscosity and flow rateAbout 12 min

Part 1 of 4 · The race

Drop a ball, start the clock

Scientists measure "thickness" with a falling-ball test: drop a steel ball into a tube of liquid and time how long it takes to fall 20 cm. The more the liquid resists flowing out of the ball's way, the slower it falls. That resistance is called viscosity.

Predict first: tap the tubes from the fastest fall to the slowest.

Your order: none yet. (You can also just drop them.)

0.00 s

Liquid	Fall time	Viscosity	× water

Part 2 of 4 · Thick is not heavy

Two properties, not one

Lots of people think thick liquids are always denser. Pour vegetable oil onto water: it floats on top, so it's less dense. Yet it fell slower in the race, so it's more viscous. Sort these liquids on both properties. Water sits in the middle of the map.

Try: tap a liquid, then tap its box.

Part 3 of 4 · Turn up the heat

Warm honey, faster pour

Honey particles cling to their neighbours. To flow, they have to keep breaking free and sliding past each other. Warming them makes them jiggle harder, so they break free more easily.

Try: pick a temperature and pour for 5 seconds. Test at least three temperatures.

Lines show particles holding onto a neighbour. Warmer: fewer holds, more sliding.

Honey temperature

Viscosity

× water

Part 4 of 4 · Engineer the flow

The pipe lab

At a honey packer, honey is pumped through a pipe from a big tank to the bottling machine. Four things set the flow rate: the push from the pump (pressure), the pipe's width, the pipe's length and the honey's viscosity, which depends on its temperature.

Predict: you double the pipe's width and change nothing else. The flow rate becomes…

Try: run the pipe, change one thing, run it again. Your runs are logged below.

Pump push

Pipe width

Pipe length

Temperature

Flow rate

500 mL jars per minute

Run	Push	Width	Length	Temp	Flow

The order: a grocery chain wants 12 jars a minute, so at least 100 mL/s. Your limits: the line's pipe is 1.5 m long, the pump tops out at 30 kPa, pipes come no wider than 30 mm, and raw honey must stay at or below 40 °C or it loses its flavour.

Putting it back together

What makes a fluid flow

ViscosityA fluid's resistance to flowing: its particles cling to each other and have to keep sliding past. Honey is about 10,000 times as viscous as water at room temperature.
Not densityThick and dense are different properties. Oil is thicker than water but floats on it; mercury is 13.5 times as dense as water but almost as runny.
TemperatureWarm a liquid and it gets runnier, a lot: honey at 40 °C pours about ten times faster than at 20 °C.
Flow rateMeasured in mL per second. More push, a shorter pipe, a runnier fluid: more flow. Width matters most of all: double it and the flow goes up 16 times.

Next in Fluids: why you can squeeze air but not water, and how that lets a small push lift a car.