Water Droplets
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Water Droplets
SciCast Workshop
Download film (right-click)Adult , Demonstration , Explanation , Physics
Droplets of coloured water skitter around on a hot plate. A simple and beautiful film.
Director's Notes:
Filming technical subjects like this is often tricky, and this certainly was. We moved the camera quite a long way back, then zoomed in to fill the screen with the hotplate. This reduces the effective ‘depth of field’ - which means the background behind the drops goes all blurry, helping them stand out.
Trouble is, unless you’re focussed in exactly the right spot, the drops go blurry too. If you look closely you can see the camera ‘hunting’ back and forth, as it tries to work out what to focus on. We should have turned autofocus off, and done it by hand - ah well, next time.
Be very careful repeating this experiment, of course: not only does the hotplate have to be extremely hot, but the fizzing droplets spit boiling water around. Eye protection and gloves, people!
SciCast Notes:
I love this film - the physics is quite subtle, the crackly sound is wonderful, and the pictures are fabulous - it looks beautiful!
To recap Glen’s explanation: when the hotplate is hot enough, water boils on contact. But the boiling is so rapid, the resulting water vapour (gas) doesn’t have time to move out of the way of the rest of the droplet. So the droplet ‘floats’ on a layer of gas. And it turns out that water vapour is quite a good insulator, in the circumstances, so the rest of the droplet only boils rather slowly, refreshing the gas layer as it does.
Since the droplets are supported by the gas layer, they also skitter around madly, floating on their low friction support and pushed about as they wobble and boil.
Be very careful repeating this experiment: not only does the hotplate have to be extremely hot, but the fizzing droplets spit boiling water around. Eye protection and gloves, people!
— Jonathan