Why does honey coil up in a spiral when you drizzle it?
― The thin thread jams at the bottom and bends
Drizzle honey from a spoon and the thread draws loops on your toast. Nobody is turning anything, so why does it spin? The answer is that the falling thread has nowhere to go at the bottom, gets pushed, and bends. The same thing happens when you lower a rope onto the floor.
In the morning, you lift a spoon of honey a little high above your toast. A golden thread stretches thin and reaches the bread.
Then the end of the thread starts to draw a small loop on the toast. Round and round it goes, like a machine winding up cord. The loops overlap bit by bit and grow into a tiny tower.
But bring the spoon close to the toast and the loops stop at once. Try the same with water and no loops form. What is the difference?
There are just two reasons
The falling thread hits the toast and stops. More thread keeps arriving from above. Pushed from above, the thin thread cannot stand straight and bends sideways.
Honey is a far stickier liquid than water. So when pushed it does not break up, and it behaves like a soft rope. Water breaks into drops and splashes.
When both are in place, the bent end of the thread keeps circling and draws loops. Let's take them one at a time.
Why does the thread "jam" at the bottom?
Honey leaving the spoon is pulled by gravity and speeds up as it falls. When it is faster, it needs less width to carry the same amount. So the thread gets thinner toward the bottom. Water from a tap thins out lower down in just the same way.
Look at Figure 1. The tip of the thinned thread is suddenly stopped where it hits the plate. But more thread keeps arriving from above at the same speed. Between the stopped part and the arriving part, the thread is squeezed lengthwise.
Push down on a thin rod and, past a certain force, it suddenly bends sideways. This is called "buckling". You can try it by pressing one dry spaghetti strand against a desk with your finger. A honey thread bends in the same way.
The bent tip lands at a spot shifted to the side. The next piece arrives, is pushed too, and shifts a little further. Repeat this and the tip traces a circle. Each time the thread makes one loop, it lays down one loop's length of thread.
Why don't loops form with water?
Honey is said to be thousands of times stickier than water. A sticky liquid changes shape only slowly. When pushed, it cannot spread sideways at once, and it keeps its thread shape for a while. In that time, the whole thread bends.
Water runs freely. A thin stream of it tends to pinch off partway and break into drops. Drops hit the plate and bounce back, so they cannot push on each other as one continuous thread. Compare the left and right of Figure 2.
The same idea explains why the loops stop when you lower the spoon. With a short drop, the thread does not speed up much or thin out. A thick, short thread does not bend easily when pushed, and just spreads out on the plate.
Any sticky liquid can coil, not just honey. Shampoo squeezed into your palm, or melted chocolate drizzled from a height, makes the same loops. Cold honey is stickier, so its loops tend to come out more neatly.
Summary
A honey thread gets thinner and faster as it falls, then is stopped and pushed on the plate. The pushed thin thread buckles and bends sideways, and its tip circles around. Water makes no loops because it is not sticky enough and the thread breaks into drops.
The honey is not spinning.
A thread with nowhere to go is being pushed and bent, again and again.
The thinning with height is explained in why tap water gets thinner as it falls, and the feel of stickiness in why water feels as sticky as honey to a water flea. For another puzzle of sudden flow, see the ketchup article.
- Scoop honey onto a spoon and drizzle it from about 2 cm above a plate. The thread is thick and hardly any loops form.
- Next, drizzle it from about 10–20 cm up. The thread thins and starts drawing loops on the plate.
- Try the same with water and with shampoo. Compare which ones make loops and which do not.
You could also lower a thin string or yarn onto a desk bit by bit. It bends when it hits the desk and piles up in loops. A honey thread behaves like this "soft string".
For those who want more ― terms, formulas and links to textbooksEach part is labelled with its level, from junior high science to university courses
- JHSCovered in junior high school science
- HSCovered in high school physics
- HS+Advanced high school material, or a textbook sidebar
- Univ.Not taught in high school; university courses (fluid mechanics, elasticity)
- ResearchNot settled even at university; what researchers are studying right now
JHSTerms: this phenomenon has a name
- Liquid rope coiling: a thin stream of sticky liquid draws loops and piles up on the surface it lands on. It is named for looking like a rope being coiled.
- Buckling: when a long thin object is pushed along its length, it suddenly bends sideways past a certain force. It is a key idea for the strength of columns and rods.
- Viscosity: how sticky a liquid is. Honey is said to be thousands of times more viscous than water.
JHSHSCheck with a formula: how many turns per second does honey make?
Each time the tip draws one loop, it lays down one loop's length of thread. So dividing the length of thread arriving per second by the loop's circumference gives the turns per second. The length arriving depends on the flow rate and the thread's thickness.
| In symbols | v ＝ Q ÷ A 、 f ＝ v ÷ ( π × D ) |
| In words | Thread speed ＝ volume flowing per second ÷ cross-section area of the thread. Turns per second ＝ thread speed ÷ circumference of the loop |
| Where it comes from | The first part is that liquid is neither created nor lost along the way (conservation of volume, the continuity equation). The second is a length count: each turn lays down one circumference of thread |
| Symbol | Meaning and unit |
|---|---|
| Q | Honey flowing per second (cubic millimetres per second) |
| A | Cross-section area of the thread just before it reaches the plate (square millimetres) |
| v | Speed at which the thread reaches the plate (millimetres per second) |
| D | Loop diameter (millimetres) |
| f | Turns per second |
| Flow per second | 0.5 mL ＝ 500 cubic mm |
| Thread thickness just before the plate | Diameter 2 mm (cross-section about 3.14 square mm) |
| Loop diameter | 5 mm |
| Speed of the arriving thread (mm/s) | 500 ÷ 3.14 ≒ 159 |
| Circumference of the loop (mm) | 3.14 × 5 ＝ 15.7 |
| Turns per second | 159 ÷ 15.7 ≒ 10.1 |
The thread arrives at about 16 cm per second and turns about 10 times per second. That is why the circling looks so fast to the eye. The real number varies a lot with the honey's temperature and the drop height. The slider in Figure 1 runs on this same formula.
HSHS+Why does it get thinner and faster lower down?
HSA falling thread speeds up under gravity. For the same flow, a faster part has a smaller cross-section. This is exactly the first half of the formula above.
HS+A honey thread does not fall freely: the upper part holds back the lower part. Stickiness creates pulling forces inside the thread. So it does not get as fast as water. The higher it drops, the thinner and faster it gets, and the easier loops form.
Univ.Buckling of a slender viscous jet
This phenomenon is called "liquid rope coiling", and is treated as the buckling of a slender viscous jet under compression. The flow inside the thread has a very small Reynolds number, a regime where stickiness beats inertia. Research sorts the coiling into regimes by which forces balance: a viscous regime, a gravitational regime, an inertial regime and so on. As the drop height rises, the regime switches, and the way the coiling speed is set changes too.
📖 Derivations and further reading: Buckling (Wikipedia, in Japanese) ／ Viscosity (Wikipedia, in Japanese)
ResearchWhat is still not clear
- Several ways to coil at the same height. Over a certain range of heights, it has been reported that a fast and a slow way of coiling switch back and forth under the same conditions. How one gets chosen is still being studied.
- Behaviour at the boundaries between regimes. Near where the viscous, gravitational and inertial regimes switch, the coiling speed can change abruptly or wobble. The full picture is still being worked out through experiments and computation.
- Uses in manufacturing. In 3D printing that stacks thin threads of melted material, and in food production, this coiling affects quality. How to make it coil into a target shape is still under research.
In other words, this article is also "an explanation of what is known so far". The phenomenon is common in the kitchen, but not everything about it has been worked out.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| JHS | Science: force and motion | A thread that speeds up as it falls, a rod that bends when pushed |
| HS | Physics: falling motion | A thread that speeds up under gravity and thins lower down |
| HS+ | Physics: advanced fluids | Conservation of volume (continuity equation), and the force of stickiness holding the thread back |
| Univ. | Fluid mechanics, elasticity | Buckling of a viscous jet, Reynolds number, classification of regimes |
| Research | Nonlinear fluid dynamics | Multiple ways of coiling, boundaries between regimes |
| ― | Link to daily life | Drizzling honey, shampoo or chocolate |
- Barnes, G. and Woodcock, R. (1958) "Liquid rope-coil effect", American Journal of Physics 26, 205.
- Mahadevan, L., Ryu, W. S. and Samuel, A. D. T. (1998) "Fluid 'rope trick' investigated", Nature 392, 140.
- Ribe, N. M. (2004) "Coiling of viscous jets", Proceedings of the Royal Society A 460, 3223–3239.
- Ribe, N. M., Habibi, M. and Bonn, D. (2012) "Liquid rope coiling", Annual Review of Fluid Mechanics 44, 249–266.
- Wikipedia (Japanese), "粘度" (Viscosity)
* This article is a popular-science explanation for general readers. The figures shown are rough guides to help you understand the mechanism. The number of turns and the loop size vary a lot with the type of liquid, its temperature and the drop height.