Flower power

Waves are mesmerising. Thanks to high-speed photography specialist Phred Petersen at RMIT University, we can see the beautiful patterns they make in gases. Petersen’s work usually informs aerospace engineers about airflow around their designs. But occasionally he indulges himself – and us.

“These images were done just because I was curious to see what things look like, and understand how seemingly ordinary events actually unfold.” In this image, he aimed his camera down a smooth cylindrical tube, then detonated a small explosive cap (red) inside it. The billowing cloud and shockwaves are seen bouncing around inside.

“I find fluid flows fascinating, and some of the structures quite innately beautiful, but because they happen so fast, most of us never see them.”

Credit: Phred Petersen

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Ripple effect

The waves that ripple through gases are normally impossible for the naked eye to see.

Petersen uses “Schlieren photography” to capture them. He shines “collimated” light composed of parallel rays onto his gaseous subject. Wave peaks in the gas, such as the blast released by an explosive cap, bend the light slightly more than the surrounding air does, creating lines of light and dark that can be captured on camera. “In effect, the gas itself becomes a lens that bends light to form the image,” says Petersen.

In this series – over in less than a millisecond – Petersen captured the shock waves that race around inside a corrugated cylinder to create a tulip shape.

Credit: Phred Petersen

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While the images Petersen captures are often over in microseconds, setting up a shot can take days of work.
The sequence starts when, just out of shot, Petersen fires a bullet of air at a soap bubble. We see the tight vortex of fast moving air – visible as a faint vertical line travelling left to right across the frame – rupturing one side of the bubble and then the other.

Credit: Phred Petersen

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Bubble burst

Petersen’s high-speed images can capture unexpected details about the world around us.
When a soap bubble pops, it appears simply to vanish. In fact, this series shows that a bursting bubble rather gracefully unwraps. “I believe that well-crafted pictures can invite the non-scientist in; the first time someone actually sees how a soap bubble unwraps, they always say “Wow! I never would have guessed ...”

Credit: Phred Petersen

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Fall of the sycamore seed

Even the gentle descent of a sycamore seed leaves intricate swirls in the air. A dab of theatre smoke applied to the seed traces its helical path and the flow vortices generated in its wake.
“High-speed photography crosses the chasm that sometimes separates art and science, showing us the intrinsic beauty that exists in the order of nature. Is it science or is it art? I believe the answer is ‘yes’.” 

Credit: Phred Petersen