Scientists Uncover Bubble-Trapping Vortex Rings With Environmental Applications

Watching ocean waves crashing on the beach is a relaxing, almost restful experience for many people. But for others, the oceanfront is a better place to study climate change than have a lazy getaway. The air-water interface is teeming with interesting physics—vortex rings forming, gas bubbles bursting, gas bubbles being trapped, and drops bouncing, floating, and splashing. All of this activity has a direct impact on the climate.

Bursting bubbles on the ocean’s surface can turn sea salt and organic matter into aerosols in the atmosphere, affecting cloud formation, precipitation, and how much light from the sun reaches the Earth. On the other hand, bubbles trapped underwater by falling ocean spray can dissolve and transfer gas from the atmosphere into the water. Our ability to model and study climate change accurately depends on understanding the physics of where water meets air.

Research published last week in Physical Review E unveils the physics behind one type of interaction—when a drop hits a pool of liquid and, in doing so, traps an air bubble even larger than itself. This is called large bubble entrainment.

The culprit? A strong vortex ring that forms when the drop hits the water. A vortex ring is a spinning donut-shaped area of fluid. You may be familiar with vortex rings created by air cannons, smokers (“smoke rings”), or dolphins.

Image Credit: US National Oceanic and Atmospheric Administration

The scientists used high-speed video to capture large bubble entrainment in action in the lab. The frames below break down the story of this bubble-trapping ring vortex. For scale, the small black bar in the bottom corner of the first image is 1 mm long.

Image Credit:  Yangfan Li and Sigurdur T. Thoroddsen

1. Time before impact: 1.3 miliseconds (ms)
The process starts with a drop approaching the air-liquid interface (the horizontal line running across the image).

2. Time after impact: 5.4 ms
The dark area underneath the line is air. The drop hit the surface and pulled the air-liquid interface in that region downward. The drop now looks like a bowl with its edges coming part way up and around the interface. This is the vortex ring.

3. Time after impact: 7 ms
The air-liquid interface is being pulled radially outward, growing in diameter.

4. Time after impact: 11 ms
The bottom of the air cavity is being pulled downward. The vortex ring continues to pull the interface outward and slightly upward. This creates a tongue of liquid above the air cavity, and a neck in the shape of the air cavity.

5. Time after impact: 20 ms
The liquid tongue has now closed above the air cavity, trapping a large bubble.

In order to really get at the physics behind these observations, the researchers simulated large bubble entrainment using supercomputers. They reproduced the experimental observations, as show below, and then systematically studied how the process was affected by different factors like drop size, shape, and the presence of gravity.

In these simulation results the drop is red, the pool is blue, and the air is green. The liquid that composed the drop stays red after the impact so that you can differentiate it from the pool.
Image Credit: Marie-Jean Thoraval and Sigurdur T. Thoroddsen 

Want to trap a large bubble? It turns out that the critical factor is the shape of the drop. The researchers found that only the longest drops, shaped kind of like little footballs, create a vortex ring strong and stable enough to trap a large bubble. The drop also needs to be large and fall slowly. These are the kind of conditions you can find when waves break in the ocean, making this work especially relevant to climate studies.

This finding was several years in the making. Yangfan Li explored large bubble entrainment experimentally and identified the vortex while earning her PhD at National University of Singapore under Sigurdur Thoroddsen, now a professor at King Abdullah University of Science and Technology (KAUST). The simulations were done by Marie-Jean Thoraval during his PhD at KAUST, and he continued the analysis while a postdoc at University of Twente and at his current institution, Xi’an Jiaotong University. You can read the paper and find videos here.

—Kendra Redmond

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