Film-boiling droplets. (a)
Cartoon of a film-boiling droplet on a flat surface. The liquid
evaporates even before the droplet touches the hot surface,
and the droplet hovers on a vapour cushion about 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. In our experiments,
d ≈ 0.2 mm and s ≈ 1- 2 mm.
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. 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
such as a piece of brass with periodic, saw-tooth shaped ridges
movie). We propose that net vapour flow between droplet
and solid exerts a drag force onto the droplet.
We have observed this effect for all liquids we have investigated
(including nitrogen, acetone, methanol, ethanol, water, and hexadecane,
with boiling points ranging from - 196 °C to + 151 °C).
Possible applications include heat-actuated pumps that could eliminate
the need for additional power sources in cooling applications: the
image below shows water flowing through a closed, heated, ratcheted
Self-propelled Leidenfrost droplets beautifully demonstrate the
general principle of ratchet phenomena:
A combination of non-equilibrium (here: the vertical thermal gradient)
and spatial asymmetry generally results in transport.
for more info and high-speed movies.
Laura Melling, David Haskell, Zeb Schweickert, Kevin Young, Amanda
Stout, Benj Aleman,
Narayanan , Oregon State University
H. Linke, B. Aleman, L. Melling, M. Taormina, M. Francis, C.
Dow-Hygelund, R.P. Taylor, V. Narayanan, A. Stout:
Self-propelled Leidenfrost droplets..
Phys. Rev. Lett. 96, 154502
NSF Career Award (since 2003)
The Australian Research Council (2002)
Essay on the Leidenfrost effect (film boiling) by Jearl Walker.