1. If you slow down water droplets or milk splashes, you see strange and fascinating things going on, says Philip Ball
  2. Chances are, you have made something stunning today. You won’t have known it as you stood in the shower, poured milk on your breakfast, dropped sugar into your coffee or turned the tap on and off. But if only you could have seen what was going on in those dribbling, splashing, dripping fluids!

    Now you can, thanks to a competition called the Gallery of Fluid Motion, which celebrates the incredible imagery and strange physics of liquids, close-up and in slow motion.

    Run by the American Physical Society’s Division of Fluid Mechanics, the entries come in the form of three-minute videos exploring some aspect of fluid flow, and are judged “for their combination of striking visual qualities and scientific interest” – in other words, they have got to be interesting but also should look stunning.

    Consider the seemingly familiar process of splashing. When a spherical object is dropped into a liquid, like in the image below, the impact pushes up a crown-shaped rim of liquid that spills tiny droplets from its edge: an image so familiar that it was adopted as the symbol of the British dairy farmers’ marketing cooperative Milk Marque in the 1990s.
  3. The winning entry in the competition this year, a slow-motion video submitted by researchers in the US and Saudi Arabia highlights new beauty and complexity in this everyday process. As their video reveals, the rim also folds inwards into a dome, and as it does so it buckles like a sheet of thin plastic into a series of ridges running around the circumference, looking just for an instant like an ornate vase, a weird daffodil-like flower head, or some bizarre sea creature.

    The process is perfectly mesmerising:
  4. Buckling Instability of Crown Sealing
  5. That this competition exists at all says something unique about fluid mechanics, the science of flow. It is a hugely important field, relevant to matters ranging from aeronautical engineering to meteorology to industrial chemistry. It is also hugely intellectually demanding: the literature of the field is stuffed with equations, and it has attracted some of the most brilliant scientists and mathematicians of modern times, including the Lords Rayleigh and Kelvin, and Werner Heisenberg. Some of its questions, such as the nature of turbulence, are considered to be among the great unsolved mysteries of science.
  6. And yet it is also so familiar, indeed ubiquitous. You don’t need a particle accelerator to study it; a glass of water will do. When a raindrop falls, when you run the bath or cook a meal, the flows involved are precisely those that are still leaving top scientists scratching their heads.
  7. Consider another entry this year ‒ “Hidden complexities of a simple match” ‒ which reveals the writhing, circulating gases produced when a match is struck to produce a flame. It is beautiful, but also extraordinarily complicated: a pattern shaped by randomness, never to be repeated, and yet with a tantalising logic that is hard to put into mathematical form:
  8. The Hidden Complexities of the Simple Match
  9. As many of the entries testify, it is the behaviour of jets, splashes and bubbles that produces some of the most striking results. Take a look at “Raindrop on a sandy surface”. It starts off predictably enough: a splash that sends out a crown of little grains. But then what is going on here? The droplet spreads into a many-tentacled mass before retracting like some alarmed creature on the seashore.
  10. Raindrop Impact on a Sandy Surface
  11. In “The inner world of a collapsing bubble” (there is an art-movie quality to these shorts that their titles reflect) by a Swiss team, the bubble executes a movement that I can barely begin to describe, seeming to fold elegantly on itself before disappearing in a double puff of turbulence.
  12. The Inner World of a Collapsing Bubble
  13. There are seemingly endless examples of this almost absurd elaboration in what you’d imagine to be just, well, a splash or pop. Take a droplet falling onto a surface of the same liquid. It will splash and mix, right? Yes, but typically the energy of the impact pushes up a narrow column of liquid in the centre, from the top of which a secondary droplet gets pinched off before it too falls onto the surface.
  14. In some cases, it can get really crazy. Droplets can actually bounce before coming to rest sitting on top of the liquid below, separated by the thinnest film of trapped air. Eventually the air flows away and the droplet merges. This, however, releases energy held in surface tension, and that energy can propel a smaller droplet up from the surface, until it too bounces and comes to rest… and the same process repeats, maybe a couple of times, with ever-smaller droplets.
  15. This video from 2009 demonstrates how:
  16. Watch the bouncing droplet
  17. That fluid mechanics is beautiful enough to prompt an annual art competition is a reflection of how visually driven the science is. Not only do the images tell their own story, but the story is almost impossible to tell without them. What’s more, close and careful observation is more important here than in most other fields: often it is only by watching that you know even which questions to ask, let alone what the answers are.

    Studying these processes can also speak to higher questions about the way nature works. In 1944, the Scottish zoologist D’Arcy Wentworth Thompson published the revised edition of his classic book on patterns in nature, On Growth and Form. And for its opening illustration he chose a high-speed photograph of a simple splash, created by the American electrical engineer Harold ‘Doc’ Edgerton:
  18. Here was the central mystery that Thompson was addressing: where does the order come from? Sure, the splash’s rim is easy to understand, but how come its perfectly circular symmetry breaks up into the crown of spikes with droplets at each tip, each equally spaced (roughly speaking) from the next? This question lies at the very heart of physics. Analogous processes of “symmetry breaking” produced our entire universe and differentiated the fundamental forces and particles it contains. There’s cosmology in a drop of milk or water.
  19. So the droplets and bubbles of the Gallery of Fluid Motion aren’t just froth. (Nor, for that matter, is froth.) They encode deep mysteries and endless surprise, an understanding of which might not only help us build better inkjet printers but also deduce how craters form on planets and moons, and how gases such as carbon dioxide are exchanged between the skies and the oceans as waves break and crash – an important process in climate science.

    And they don’t half look good, too.

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    (Images: Gallery of Fluid Motion, Getty Images, Science Photo Library)

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