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Dust pucks

Question: I have a coal-fired Aga cooker. It is refuelled by tipping coal
into a small hole in the middle of the hotplate. During refuelling with wet
coal, water droplets fall onto the hotplate and bounce around violently,
scattering as they boil and superheat on contact with the plate. But some larger
droplets gather a layer of dust on their surfaces (which I suspect comes from
coal that has also landed on the plate).

The surprising thing about the coated droplets is that they stop bouncing
around and form neat, squashed-sphere shapes, 3 to 4 millimetres in diameter,
that float gently around like an ice hockey puck on the plate. They can last for
10 to 15 seconds, and even then they usually die by falling off the plate,
rather than boiling and disintegrating. When they do disintegrate on the plate,
they leave behind a small pile of grey powder (coal dust presumably).

How does the dust have such a dramatic effect on the water droplet? What
forces are holding the drop and the dust together? What is the critical size for
a stable drop to form?

Answer: These observations are examples of the Leidenfrost phenomenon, so
called because it was first reported by the German physicist Johann Gottlob
Leidenfrost in 1756. His treatise has been called the first substantial study of
boiling heat transfer, although Leidenfrost himself did not consider that he was
observing a boiling process. When he saw the small deposits that were left in
the hot spoons that he used to study the process, he thought he might be
observing a reaction between Aristotle’s elements, namely Fire + Water =
Earth

When water is dropped onto a surface heated to a temperature greater than 100
°C, it obviously boils away. This boiling gets more vigorous, and eventually
even explosive, as the surface gets hotter. However, somewhere between 230
°C and 300 °C (the Leidenfrost point) water is boiled off at such a high
rate as the drop approaches the surface that the vapour produced stops the
liquid coming into contact with the plate. The vapour then insulates the drop
from the plate and it silently floats above the hotplate. A drop of clean water
4 millimetres across will take up to 2 minutes to evaporate.

The exact temperature at which this phenomenon starts to occur depends on the
physical properties of both the hot surface and the liquid. The coal particles
in your correspondent’s droplets will absorb radiant energy from the hot plate
allowing the phenomenon to occur at lower temperatures.

Bob Carpenter

Nuneaton, Warwickshire

Answer: The Leidenfrost effect can be seen in clean systems when there is no
dust present. So what does the dust do in the instance described above? The coal
dust is likely to be hydrophobic—it is not easily wet by water, except
perhaps at certain points on each particle. Therefore it will tend to stay on
the water surface and be spread by surface tension, so as to cover the surface
uniformly. This may allow the drop to deform more easily, so that a dusty drop
will probably be flatter than a dust-free drop. This could help to support the
drop on the steam bed, because then it provides a larger area for support.

The insulating properties of a rapidly boiling layer of moisture also appear
to explain how people can walk on red-hot coals without suffering ill effects.
There is a fascinating paper by Jearl Walker of Cleveland State University at
www.wiley.com/college/phy/halliday320005/pdf/leidenfrost_essay.pdf which
describes some scientific measurements of the Leidenfrost effect.

Richard Hann

Ipswich, Suffolk

Answer: Some older books of laboratory demonstrations mention an experiment
where the lecturer puts his wet hand into a container of molten lead—a
thin film of steam insulates the hand, provided the moisture level is just
right. I have never heard of anybody who has actually seen this done.

David Simmons

Bottisham, Cambridgeshire

In the fascinating paper mentioned above, Jearl Walker of Cleveland State
University not only explains how it is possible to walk on red-hot coals and to
plunge your hand into a container of molten lead—as related by David
Simmons—but also describes Walker’s experiences when he performed these
and other tricks himself. They do appear to work, but as Walker’s injuries
included burnt feet, a scarred face and ruptured tooth enamel we cannot repeat
strongly enough his advice that you should never try these experiments
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Hail capsules

Question: During a recent hailstorm in Wales, I was surprised to see that all
the hailstones were conical. Each one had an apex angle of about 75°, with a
flat or slightly curved base about 4 millimetres across. What caused this? Is
this shape common, and is its similarity to an Apollo re-entry capsule
coincidental?

Answer: True hailstones are roughly spherical pellets of snow and ice. They
are formed in the turbulent airflow that is a characteristic of thunderstorms,
where they are repeatedly swept vertically upwards and grow due to collisions
with other particles and supercooled water—water that is way below its
freezing temperature but still in liquid form.

True hailstones never occur when the ground temperature is below freezing
because thunderstorms cannot form without warm, moist air. I don’t know what the
temperature was when these conical hailstones formed, but their shape suggests
they were simply frozen raindrops. The unusually flat bottom was probably caused
by wind resistance as the raindrop fell to the ground before freezing solid. I
am not sure how common such hailstones are.

The shape is indeed very similar to an Apollo re-entry capsule. These
capsules were shaped to protect the astronauts inside from the heat of re-entry.
The bottom heated up but diverted hot air away from the astronauts’ compartment.
It also served to increase air resistance and slow the craft down.

Casper Marciniak

Sydney

This week’s question

Jaws of note: I notice that when I thrust my lower jaw forward, so that my
bottom front teeth move out past the top ones, I hear a highpitched noise in
both ears. The noise persists as long as my teeth are sticking out. Assuming
this is a common experience, what causes it?

David Bernstein

Pasadena, California