Watching rain drops hit a puddle or lake is remarkably fascinating. Each drop creates a little cavity in the water surface when it impacts. Large, energetic drops will create a crown-shaped splash, like the ones in the upper animation. When the cavity below the surface collapses, the water rebounds into a pillar known as a Worthington jet. Look carefully and you’ll see some of those jets are energetic enough to produce a little satellite droplet that falls back and coalesces. Altogether it’s a beautifully complex process to watch happen over and over again. (Image credit: K. Weiner, source)
Crown Splash Like the Worthington jet, crown splashes often follow a drop’s impact into another liquid. But they can also show up when slicing or stomping through puddles!
Free Surface Dynamics Anytime you have a body of water in contact with a body of air, fluid dynamicists call that a free surface. How the interface between the two fluids shifts and transforms is fascinating and complicated. Waterfalls are a great example of this, but so are ocean waves or even the ripples from tossing a rock into a pond.
Hydrophobic Surfaces Water-repellent surfaces are called hydrophobic. Water will bead up on the surface and roll off easily. While many manmade surfaces are hydrophobic, like the teflon in your skillet, so are many natural surfaces. Many leaves are hydrophobic because plants want that water to fall to the ground where their roots can soak it up. Keep an eye out as you wash different vegetables and fruits and see which ones are hydrophobic!
Check out all of this week’s posts more examples of fluid dynamics in daily life. (Image credit: S. Reckinger et al., source)
Droplet collisions captured instantaneously create beautiful fluid sculptures that, though common, are too fast for the human eye. Here a bubble was blown onto the surface of the fluid, then a droplet was released to fall into the center of the bubble, bursting it. As that droplet rebounded in a Worthington jet, a second droplet was released and impacted the jet, creating the umbrella-like shape in the center. See Liquid Droplet Art for more photos. (Photo credit: Corrie White and Igor Kliakhandler) #
I’ve often noticed that, when water splashes (especially as with raindrops or other forms of spray), often it appears that small droplets of water skitter off on top of the larger surface before rejoining the main body. Is this an actual phenomenon, or an optical illusion? What causes it?
That’s a great observation, and it’s a real-world example of some of the physics we’ve talked about before. When a drop hits a pool, it rebounds in a little pillar called a Worthington jet and often ejects a smaller droplet. This droplet, thanks to its lower inertia, can bounce off the surface. If we slow things way down and look closely at that drop, we’ll see that it can even sit briefly on the surface before all the air beneath it drains away and it coalesces with the pool below. But that kind of coalescence cascade typically happens in microseconds, far too fast for the human eye.
But it is possible outside the lab to find instances where this effect lasts long enough for the eye to catch. Take a look at this video. Here Destin of Smarter Every Day captures some great footage of water droplets skittering across a pool. They last long enough to be visible to the naked eye. What’s happening here is the same as the situation we described before, except that the water surface is essentially vibrating! The impacts of all the multitude of droplets create ripples that undulate the water’s surface continuously. As a result, air gets injected beneath the droplets and they skate along above the surface for longer than they would if the water were still. (Video credit: SuperSloMoVideos)
Diving is a popular event for spectators, but it can also be rather confusing. We know that divers are rewarded for minimizing their splash, but what exactly does that mean and how do they do it?
The ideal water entry, called a rip entry by divers, requires a diver to hit the water in a vertical orientation with their arms braced and palms held flat over their head. Striking the water tears open a cavity for the athlete’s body to enter. To minimize splash, the diver wants to fall into this expanding cavity without striking the sides, which would throw up an additional splash. This is the reason for vertical entry. Hand position is also important. If the athlete were to point their fingers, they would create a narrower cavity and larger splash.
After the athlete enters the water, the cavity closes off under the surface and the water rebounds in a splashy Worthington jet. For the speed and size of human divers, this later splash is essentially unavoidable. What the commentators don’t really tell you, though, is that diving judges are only supposed to judge a diver’s entry up to the point that their feet go under the surface. They’re instructed to ignore everything that happens underwater and after entry. So that big rebound splash we all see isn’t meant to count! (Image credits: A. Pretty/GettyImages; kaorigoto, source)
As a dropletimpacts a pool, it deforms the surface before rebounding in a Worthington jet and releasing secondary droplets as ejecta. Although we witness this act dozens of times a day, seeing it at 5,000 fps drastically alters one’s perspective.