Tuesday, October 15, 2013

Fluid Juggling

Fluid dynamics often produces some of the most beautiful, albeit under-appreciated, physics images and videos. To celebrate this field's striking images, the American Physical Society's Division of Fluid Dynamics holds a contest every year for related posters and videos that best display "artistic value, scientific content, and originality."

One promising entry this year features a high-speed capture of fluid jets juggling ping pong balls. It's a gorgeous video, and you can watch it below.

As researchers Roberto Zenit and Enrique Soto (National Autonomous University of Mexico) explain in the video, streams of water can suspend the balls due to a Bernoulli-like effect similar to the one responsible for lift in an aircraft. To levitate the ball for long periods of time, the researchers needed two things: a balance of vertical forces (we'll call this levitation) and horizontal forces (we'll call this stability).

For an explanation of the ball's stability, the researchers relied on Bernoulli's principle. Upon impacting the ball, the water creates a thin film around it as it creeps toward the top of the ball. Upon reaching the top, it starts to form back into a jet before sliding off the other side and plummeting back toward the ground.

If the jet starts hitting more on the right side of the ball, for instance, the velocity of the film will increase slightly on that side, leading to a decrease in pressure on this side. This pressure imbalance will push the ball back toward the left so that the jet hits the center of the ball again. Regardless of which side the ball drifts toward, it'll remain "trapped at a stable point," as Zenit and Soto point out in the video.

This sort of pressure imbalance also explains how airplanes achieve lift: higher-pressure air underneath a plane's wing (with lower-pressure air above) gives the plane the necessary push to lift off the ground.

To maintain levitation, the researchers had to balance the weight of the ball with the upward force of the stream. Problems arise if the water film accumulates too much on top of the ball, causing the ball to gain extra weight and fall back toward the ground. That's why researchers had to keep the ball spinning.

The ball's spinning motion continuously recycles the water in the film off the other side, maintaining the balance between the ball's weight and the force of the upward stream. To induce this spinning, the researchers had to shoot the water up at a slight angle, as you can see in the video.

Roberto Zenit excels at explaining fundamental physics principles through his videos. As you may remember, we've covered his videos in the past, including his research into the "Accidental Painting" technique of artist David Siqueiros.

Winners for the annual Gallery of Fluid Dynamics will be announced on November 26 at the Division of Fluid Dynamic's annual meeting in Pittsburgh.


  1. Awesome! Love the background music also. What artist & sing is that? Sounds like the laser dance music from Ocean's 12.

    1. The artist's name is "Jaspertine" and the song is titled "Books v Cigarettes." If you look at the video description on Youtube, there's a download link for the song.