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Self-Propelled Droplets
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 surfaces (movie), such as a piece of brass with periodic, saw-tooth shaped ridges (see highspeed 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 channel. 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.
The
Self-Propelled Droplet Resource Site Key Researcher at UO Project Alumni Collaborator
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