The brain-frying physics of the oil sizzle

My personal favourite onomatopoeia is “sizzle”. The word invokes the greasy goodness of a good fry-up and artery clogging joy in food form.

If you’ve fried anything, you’ve experienced the satisfying sizzling sound, which means the hot oil is working its fatty magic.

But what causes this auditory splendour which heralds deep-fried gaiety?

New research published in the Physics of Fluids journal demonstrates the complex physics behind the sizzle. And the results show that it is largely about moisture content.

More on physics: What makes champagne pop?

Scientists from the United States, Canada and Saudi Arabia collaborated to look into the phenomenon. They studied the bubbles that form in hot oil when a droplet of water is added – and they repeated the same experiment with a droplet of batter.

Video of the explosion cavity which forms when a water droplet hits hot oil. Credit: Tadd T. Truscott.

They also mimicked the old chef’s trick of dipping a moistened chopstick into the oil to test for the sizzling sound which indicates the oil is ready for frying. They found that the type and number of bubbles formed in the oil depended on the amount of water absorbed by the chopstick and the material from which the chopstick was made.

The droplets were also suspended on the end of a chopstick. The water droplet exploded upon hitting the hot oil, while bubbles developed over the surface of the batter droplet.

Explosion cavity which forms when a water droplet hits hot oil. Credit: Tadd T. Truscott.

Taking things a step further, the researchers placed moistened paper into the frying pan. As different types of vapour cavities formed, the team found that the amount and type of bubbling depended on the amount of moisture on the paper and temperature of the oil.

But these researchers didn’t stop there. Out of the frying pan, into the fire they went … and then back into the frying pan.

As sonically gratifying as I am sure the initial experiments were, the team followed up with more controlled studies. Water droplets were added to the hot oil from an overhead wire, and a high-speed camera and microphone were used to gather detailed data on bubble shape, type, number and pop sound.

“We found three types of bubble events in our experiments: an explosion cavity, an elongated cavity, and an oscillating cavity,” says study author Tadd Truscott, mechanical engineering associate professor at the King Abdullah University of Science and Technology in Saudi Arabia.

The explosion cavity forms when a water droplet enters the hot oil and experiences a sudden temperature increase leading to a micro explosion. The vapour bubble that is produced, breaks through the surface of the oil.

The elongated cavity involves the same process, but the vapour bubble does not break the surface.

The oscillating cavity occurs when the water droplet is quickly submerged. It undergoes an explosion process in multiple steps before oscillating and breaking up into smaller bubbles.

Audio signals from the microphone show that the three cavity types produced different sounds.

“We can distinguish different acoustic signal characteristics for each type of cavity,” says Truscott.

“Deciphering the sound signals could lead to future applications, such as acoustic sensing of aerosol generation.”

The only way one could ever hope to enjoy fried foods more is by understanding the complex physics behind that sizzle. And, with this deep dive into the deep fryer, we have achieved it!

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