By placing moistened chopsticks into the heated oil, experienced cooks can identify when the cooking oil has achieved the proper temperature for deep-frying.

The sizzling sound produced by the bubbles on the sticks might alert the cook when it’s time to add food to the pan.

In Physics of Fluids, published by AIP Publishing, scientists from the U.S., Canada, and Saudi Arabia carefully looked at the bubbles that form when water droplets touch hot cooking oil. When the researchers put wet chopsticks into hot oil, the type and number of bubbles that formed depended on how much water the chopsticks had soaked up and what they were made of.

They tried putting drops of water and batter on the end of a chopstick and seeing what would happen. When the water droplet struck the hot oil, it exploded, and the batter droplet created bubbles on its surface.

The researchers utilized a little piece of moistened paper as a model to further study what occurs when food is placed in hot oil. The amount and type of bubbling in this situation was found to be dependent on both the amount of water and the temperature. When water was added to the hot oil, they saw that different types of vapor holes appeared.

Following these preliminary tests, a series of more controlled research were conducted utilizing a system that allowed water droplets to be dropped into heated oil from an overhead wire on a moveable platform. The shape of the bubbles that develop when a water droplet strikes the hot oil and the sizzling sound they create when they pop were studied using a high-speed camera and a sensitive microphone.

“We found three types of bubble events in our experiments: an explosion cavity, an elongated cavity, and an oscillating cavity,” says author Tadd Truscott.

When a water droplet enters heated oil and experiences a microexplosion as a result of the abrupt temperature increase, a vapor bubble emerges that can tear the surface. A water droplet explodes in the elongated cavity without rupturing the surface.

When a droplet falls off the wire and is instantly submerged, it creates an oscillating hollow. It goes through a multistep eruption and starts oscillating before breaking up into many little bubbles.

The microphone’s audio signals showed that the three types of cavities had different acoustic, or sound, properties.

“We can distinguish different acoustic signal characteristics for each type of cavity,” Truscott adds. “Deciphering the sound signals could lead to future applications, such as acoustic sensing of aerosol generation,” says the researcher.

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