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When a liquid drop is placed on a surface that is held at
a temperature much higher than the liquid's boiling point
(such as a drop of water in a very hot pan) it hovers on its
own vapour cushion, without wetting the surface (figure a
below).This phenomenon is called the Leidenfrost effect (or
film boiling) and occurs beyond a surface temperature called
the Leidenfrost point (about 200 - 300 C for water on flat
surfaces, depending on surface quality).
We discovered that film-boiling droplets move at speeds of
several centimeters per second when placed on asymmetrically
structured surfaces (movie),
such as a piece of brass with periodic, saw-tooth shaped ridges
(see
highspeed movie).
We think that the reason this works is that the ratchet redirects
the vapor flow underneath the droplet, so that most of it
goes in one direction (figure b below. The droplet the rides
along ontop of the vapor, like a boat on a river - one that
can flow uphill!
Film-boiling droplets.
(a) Cartoon of a film-boiling droplet on a
flat surface. The vapour cushion separating liquid and
solid is typically 10 - 100 µm thick. (b) Film-boiling
droplets on saw-tooth shaped.surfaces (ratchets) are
found to accelerate to the right, perpendicular to the
vertical thermal gradient. The blue arrows indicate
vapour flow which, according to our tentative model,
exerts a viscous force on the droplet, pulling it along.
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This effect works for all liquids we have cared to try so
far (including nitrogen, acetone, methanol, ethanol, water,
and hexadecane, with boiling points ranging from - 196 °C
to + 151 °C).
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