The frying sound reveals the complex physics of steam cavities

Water hot oil explosion cavity

Experienced cooks can tell that the cooking oil is at the right temperature by the burning noise produced by inserting damp chopsticks.

Moistened chopsticks and water droplets of hot cooking oil reveal the dynamics and sound characteristics of complex bubbles.

As the name implies, fluid dynamics is a field of physics related to the dynamics of fluids (liquids, gases, and plasmas) and the forces exerted on them. This can be divided into two research areas: hydrostatics and fluid mechanics.

While many fluid mechanics researchers deal with fluid flow, a team of scientists has recently focused on something a bit more explosive. What happens if a drop of water or a batter drops hits hot oil?

Experienced cooks can put moistened chopsticks in hot oil to determine when the cooking oil has reached the proper temperature for frying. The burning sound of foaming on the sticks can tell the cook when to add food to the pan.

In the journal Fluid physicsAccording to AIP Publishing, researchers in the United States, Canada, and Saudi Arabia have carefully studied the bubbles that form when water droplets come into contact with heated cooking oil. When I put the moistened chopsticks in hot oil, I found that the type and number of bubbles depended on the amount of water absorbed by the chopsticks and the material of the chopsticks.

Water hot oil explosion cavity

An explosive cavity formed when a drop of water hits hot oil. Credit: Tadd T. Truscott

They experimented with water droplets and batter on the tip of chopsticks. When the water droplets hit the hot oil, they exploded, and the batter’s water droplets created bubbles on the surface.

To further investigate what happens when food is placed in hot oil, researchers used a small piece of water-moistened paper as a model. They found that the amount and type of foaming in this case depends on both the amount and temperature of the water. They observed that when water encountered hot oil, various types of steam cavities were formed in the hot oil.

These first experiments led to a more controlled series of studies using settings that allowed water droplets to be added to hot oil from overhead lines on movable stages. Using a high-speed camera and a high-sensitivity microphone, we collected detailed data on the shape of the bubbles formed when the water droplets hit the hot oil and the burning sound that occurs when the bubbles pop.

Video of an explosive cavity formed when a drop of water hits hot oil. Credit: Tadd T. Truscott

“In our experiments, we found three types of bubble events: explosive cavities, elongated cavities, and oscillating cavities,” said author Tadd Truscott.

When water droplets enter hot oil, an explosion cavity is formed, and a rapid temperature rise causes a micro-explosion, forming steam bubbles that can explode the surface. The elongated cavity contains water droplets that explode without breaking the surface.

Vibrating cavities occur when a droplet slides off a wire and is immediately submerged. It goes through a multi-step explosion process and begins to vibrate before breaking into numerous small bubbles.

The audio signal from the microphone revealed that the three cavity types produce different acoustic or sonic characteristics.

“We can distinguish different acoustic signal characteristics by type of cavity,” says Truscott. “Decoding audio signals has the potential to lead to future applications such as acoustic sensing of aerosol generation.”

See: “Dynamics and Sound Morphology of Bubbles in Heated Oil”, Akihito Kiyama, Rafsan Rabbi, Zhao Pan, Som Dutta, John S. Allen, Tadd T. Truscott, June 7, 2022. Fluid physics..
DOI: 10.1063 / 5.0088065

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