Watch This Water Droplet Pulse And Form A "Star"

You'll have heard of this effect in high school, but you probably haven't seen this manifestation of it. It makes an ordinary water droplet pulse like it has a heartbeat. And sometimes, it makes the drop form into a pulsing star.


If you were to know that the water droplet in the clip above is being pipetted onto an incredibly hot metal surface, would you be able to guess what's going on here? It's our old friend, the Leidenfrost Effect. When a drop of fluid hits a surface a great deal hotter than itself, the portion of the droplet that makes contact with the surface vaporizes instantly. This vapor temporarily provides a cushion which protects the rest of the droplet from the heat. The droplet forms a bead that rolls around the surface until it eventually succumbs to the heat and evaporates.

When you add enough water, something different happens. The liquid collects together to form a whole — if it's broken apart it will come back together again —and pulses. These pulses can make an oscillating oval, as they do above, or form a star shape, as they do in the video below.

The oscillations of the water are driven by the evaporating liquid on the surface below it. The newly-formed vapor will try to make its way upwards any way it can. A "chimney" forms within the water droplet, allowing gas to escape but drawing water upwards and drawing the edges of the water inwards. When the chimney closes again, the water collapses back down. When the water forms a shape three or more "points," it's called a Leidenfrost Star.

[Source: Leidenfrost Dynamics]



A three pointed shape? Maybe they should call it the "Leidenfrost Triskelion?"

And I wonder at the role the circular barrier plays in this. I mean I know its there to keep the Leidenfrost drop from skittering all over the place so we can watch its pulsations carefully. I also see that the Leidenfrost Star seems pretty stable and not touching the barrier at all in the second video.

But it's hot too, right? I have to wonder how it affects the droplet. Does changing the shape that barrier, does changing its size change the behavior of Leidenfrost Star?