Australian_Science_Illustrated_Issue_52_2017

scienceillustrated.com.au | 79

UP TO 0.2 MM

WATER DROP

SURFACEHOT

HEAT

STEAM

Heat produces

protective vapour

An insulating water vapour cushion makes small water

drops hover above the pot bottom at a temperature of

200° – without evaporating.

E

very child knows that water will boil and

evaporate at a temperature of 100° C (or

slightly less if you live at high altitude). But

what happens, at extreme temperatures?

Then you will prevent the water from

evaporating, and instead, the water drops

remain intact, skittering about.

This is due to the Leidenfrost effect,

which can be observed quite simply, using

just water, a hotplate, and a pot.

At a temperature of just over 200°C, the

Leidenfrost point is reached. The point is

the optimum tempe rature above the

boiling point of water

to keep drops intact. At

this temperature, a cushion

of vapour will be produced in

the contact area between the

hotplate and the water drop. The cushion

will make the drop hover immediately

above the bottom of the pot, suspended

like a steampunk hoverboard..

It works, because steam is a poorer

conductor of heat than liquid water, so the

water of the drop is not affected by the

intense heat from the hotplate.

Water vapour is a shield against heat

1 The intense heat from the hotplate makes the water

touching the hotplate evaporate,

but rather than dispersing, it is

captured under the water drop.

3

The steam

under the

drop functions as a

cushion between

the hotplate and

the water drop.

2 The drop hovers above a layer of water vapour. It

will not remain intact forever,

as the steam under the drop is

gradually replaced by water

from the drop.

Due to the

Leidenfrost effect,

water drops will not immediately

evaporate at temperature of 200°.

An extremely high temperature

produces a protective cushion of

vapour under water drops, allowing

them to remain intact and dance.

By Kristian Filrup