Why Do Cooks Flick a Drop of Water into a Hot Pan to Check Its Temperature?
― When It's Too Hot, Water Beads Ride on a Cushion of Vapor
Drop water into a well-heated frying pan and it doesn't vanish with a hiss. It forms round beads that glide around. In fact, the hotter the pan, the harder it is for the water to disappear. The reason is an invisible layer of vapor under each bead.
A cook puts a stainless steel pan on the heat and flicks a drop of water from their fingertips. At first it hisses and vanishes at once.
After a short wait, another drop. This time the water forms a round bead and races around the pan. It barely shrinks.
The cook nods, "Good," and adds the meat. You'd think hotter means faster evaporation. So why did the bead last longer?
Hotter water lasts longer, for just two reasons
On a very hot surface, only the underside of the bead flashes into vapor. Before that vapor can escape, more is made, and the bead floats on a thin layer of steam.
Water vapor conducts only about one-twentieth as much heat as liquid water. Heat struggles to reach a floating bead, so it evaporates more slowly.
The effect is named the "Leidenfrost effect" after Leidenfrost, an 18th-century German doctor who described it in detail. Cooks use the moment the beads change behavior as a thermometer.
There is a threshold between "hiss and vanish" and "roll around"
When the pan is only a little over 100°C, the water spreads out and touches the metal, then boils with bubbles from below. The contact area is large, so heat flows into the water quickly. That is why it vanishes with a hiss in a few seconds.
But as the temperature rises, things change suddenly at one point. The water's underside evaporates so fast that the vapor layer lifts the bead. For water on metal, this threshold is put at around 200°C. It shifts a lot, though, with scratches, dirt or a film of oil on the surface.
Look at Figure 1. The graph on the right is a rough sketch of how long a bead lasts at each pan temperature. Move the slider to change the temperature, and the bead on the left changes shape.
Why does a layer of vapor make water last longer?
Materials differ greatly in how easily heat passes through them. Water conducts heat reasonably well, but a gas such as water vapor hardly conducts it at all. A down jacket is warm in winter for the same reason: the air between the feathers does not pass heat.
Between a floating bead and the pan lies a layer of vapor thinner than a sheet of paper. Heat must get through this layer to reach the water. So even if the pan gets hotter, the heat going into the water can actually drop.
The same reason explains why the beads skate around. On a cushion of vapor, there is nothing to rub against. Like a hockey puck on ice, the bead moves with a slight tilt or a puff of escaping vapor.
Stainless steel and iron pans are said to be less sticky if you heat them well before adding food. Even without a thermometer, whether the beads roll tells you whether you've crossed the threshold. However, many manufacturers advise against strongly heating an empty pan with a coated surface. Follow the instructions.
This test is for an empty pan, before any oil goes in. When water gets into hot oil, it turns to vapor all at once under the oil and throws oil in every direction. Never drop water into a pan that has oil in it.
Summary
Beads roll on a hot pan because their undersides turn to vapor and they float on that layer. Vapor carries heat poorly, so the water lasts even though the pan is hot. Cooks use the switch from "hiss and vanish" to "roll around" as a marker of temperature.
When it's too hot, water is protected by its own vapor.
A rolling bead is a signal that you've passed about 200°C.
For what happens when water meets hot oil, see Why does pouring water on a burning tempura oil fire send flames up to the ceiling? For what the sound of searing meat really is, see Does searing meat over high heat really seal in the juices? And for another craftsperson who reads heat with the eyes, try How can a blacksmith tell the temperature of iron without a thermometer?
- Put an empty stainless steel or iron pan with no coating on medium heat.
- Every 30 seconds, flick just one or two drops of water from your fingertips, from a little distance. At first they should hiss and vanish.
- After a while, the water forms round beads and starts rolling. Compare how long that bead lasts with the first drop.
Both the pan and the flying droplets get very hot. Keep your face back and touch nothing but the handle. Do not do this in a pan with oil in it.
For those who want more ― terms, formulas and links to textbooksFrom middle-school science to university courses, each part is labeled by level
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school physics
- High school+Advanced high-school material, or a textbook sidebar
- UniversityUniversity-level courses not taught in high school (heat transfer, fluid mechanics)
- ResearchNot taught even at university as settled fact; researchers are studying it now
Middle schoolTerms: this phenomenon has a name
- Leidenfrost effect: On a very hot surface, a liquid floats on a layer of its own vapor and lasts a long time.
- Boiling and evaporation: Both turn a liquid into a gas. When bubbles form inside the liquid, it is boiling; when the change happens at the surface, it is evaporation.
- Heat conduction: Heat moving through a material. Metals conduct it well; gases hardly at all.
Middle schoolHigh schoolChecking with a formula: how many seconds does a floating bead last?
Divide the heat needed to turn the whole bead into vapor by the heat arriving each second through the vapor layer. That gives an estimate of how long the bead lasts.
| In symbols | t ＝ ( m × L ) ÷ ( q × A )、 q ＝ k × ΔT ÷ δ |
| In words | Lifetime = heat needed to evaporate ÷ heat arriving per second. The heat flux = conductivity × temperature difference ÷ layer thickness |
| Where it comes from | Conservation of energy (all the heat that arrives turns water into vapor) and the law for heat flowing through a thin layer (Fourier's law) |
| Symbol | Meaning and unit |
|---|---|
| t | Time the bead lasts (s) |
| m | Mass of the bead (kg) |
| L | Heat needed to turn 1 kg of water into vapor, the latent heat of vaporization (J/kg) |
| q | Heat arriving per second per square meter (W/m²) |
| A | Area of the bead's underside (m²) |
| k | How easily water vapor conducts heat, thermal conductivity (W/(m·K)) |
| ΔT | Temperature difference between pan and bead (K) |
| δ | Thickness of the vapor layer (m) |
| Thermal conductivity of water vapor (approx.) | 0.03 W/(m·K) |
| Temperature difference (pan 200°C, bead 100°C) | 100 K |
| Vapor layer thickness (assumed 0.05 mm) | 0.00005 m |
| Mass of the bead (a drop about 6 mm across) | 0.0001 kg (0.1 g) |
| Latent heat of vaporization of water | 2260000 J/kg |
| Area of the bead's underside (approx.) | 0.00002 m² |
| Heat arriving during vigorous boiling in contact (approx. upper limit) | 1000000 W/m² |
| Conductivity × temperature difference | 0.03 × 100 ＝ 3 |
| Divide by layer thickness (heat flux) | 3 ÷ 0.00005 ＝ 60000 W/m² |
| Multiply by area (heat per second) | 60000 × 0.00002 ＝ 1.2 W |
| Heat needed to evaporate | 0.0001 × 2260000 ＝ 226 J |
| Lifetime of a floating bead | 226 ÷ 1.2 ≒ 188 s |
| For comparison: heat per second when boiling in contact | 1000000 × 0.00002 ＝ 20 W |
| For comparison: lifetime when boiling in contact | 226 ÷ 20 ≒ 11 s |
The estimate is about 3 minutes for a floating bead and about 10 seconds for one boiling in contact. The vapor layer alone stretches the lifetime more than tenfold. In reality heat also enters from the sides and top, so a floating bead lasts less than this. Even so, a single drop is said to last around a minute at times, and the difference from a few seconds of hissing is easy to see.
High schoolHigh school+How heat travels, and the balance that keeps the bead afloat
High schoolTurning water into vapor takes a large amount of heat, called latent heat, on top of the heat that raises temperature. Turning 1 g of water at 100°C into vapor takes more than five times the heat needed to warm the same water from 0°C to 100°C. Whether a bead lasts depends on how fast this heat can arrive.
High school+The bead floats because its weight balances the pressure made by vapor trying to escape sideways underneath. If the layer gets thinner, the vapor has a harder time escaping, the pressure rises, and it pushes the bead back up. So the layer thickness settles naturally at a steady value.
UniversityThe boiling curve and the Leidenfrost point
In heat-transfer engineering, the relation between surface temperature and the heat flux is drawn as the "boiling curve" (also called the "Nukiyama curve," after Shiro Nukiyama, who presented it in 1934). The stage with bubbles is nucleate boiling, the point where the most heat arrives is the critical heat flux, and the stage where a vapor film covers the surface is film boiling. The temperature where film boiling begins is the Leidenfrost point. The vapor flow under the bead is treated with the same ideas as lubrication theory.
📖 Derivations and further reading: Leidenfrost effect (Wikipedia, in Japanese) ／ Boiling (Wikipedia, in Japanese)
ResearchWhat is still not well understood
- Why does the threshold temperature vary so much? The Leidenfrost point is said to shift by tens of degrees or more with tiny bumps and dirt on the surface, and how to predict it accurately is still being studied.
- Can surface treatment move the threshold at will? There is research on surfaces with fine patterns that make beads run in one direction, or raise and lower the threshold.
- Cooling in nuclear reactors and steelmaking. When hot metal is cooled with water, a vapor film gets in the way of cooling. When that film collapses is a research question tied to safety and quality.
So this article, too, is "an explanation of what we know now." In one drop of water in a kitchen pan lies a problem that is still being studied.
Links to textbooks (by level)
| Level | Subject and unit | Where in this article |
|---|---|---|
| Middle school | Science Field 1: "Changes of state," "How heat travels" | Boiling and evaporation; gases pass heat poorly |
| High school | Basic Physics: "Heat and energy" | Latent heat; estimate from conservation of energy |
| High school+ | Physics: extension of "Gas pressure" | Balance between vapor pressure and the bead's weight |
| University | Heat-transfer engineering, fluid mechanics | Boiling curve, nucleate and film boiling, Leidenfrost point |
| Research | Surface science, boiling heat transfer | Scatter in the threshold temperature; control by surface treatment |
| ― | Links to daily life | Guide for preheating a pan; why not to add water to hot oil |
- Wikipedia (Japanese), "Leidenfrost effect" (ライデンフロスト効果)
- Wikipedia (Japanese), "Boiling" (沸騰)
- Wikipedia (Japanese), "Latent heat of vaporization" (蒸発熱)
- Biance, A.-L., Clanet, C., Quéré, D. "Leidenfrost drops", Physics of Fluids 15 (2003)
- Japan Society of Mechanical Engineers (日本機械学会), Heat Transfer Engineering Data Book (『伝熱工学資料』), on the boiling curve and critical heat flux
※This article is a popular-science explanation for general readers. The figures given are rough guides and estimates meant to help you understand the mechanism. The threshold temperature varies a great deal with the pan's material and surface condition. Use cookware according to each product's instructions.