The hissing sound of deep frying reveals the complex physics of steam cavities

Wet chopsticks, drops of water in hot oil reveal the complex dynamics of bubbles and sound characteristics.

As the name implies, fluid mechanics is a branch of physics that deals with the mechanics of liquids (liquids, gases, and plasma) and the forces acting on them. It can be divided into two areas of study: fluid statics and hydrodynamics.

While many hydrodynamics researchers are concerned with fluid flow, a group of scientists recently looked at something more explosive: what happens when a drop of water or a drop of dough gets on hot oil?

Experienced cooks can determine when the oil has reached the desired temperature for frying by inserting dipped sticks into the hot oil. Bubbles form on the chopsticks, and the hissing sound they make can tell the chef when it’s time to add food to the pan.

In the magazine Fluid physics, from AIP Publishing, researchers from the US, Canada and Saudi Arabia have carefully studied the bubbles that form when water droplets come in contact with heated oil. When investigators inserted dipped chopsticks into the hot oil, they found that the type and number of bubbles formed depended on the amount of water absorbed by the chopsticks as well as the material of the chopsticks.

They experimented with drops of water and drops of dough hanging on the tip of a stick. A drop of water exploded when it hit the hot oil, where a drop of dough formed bubbles on its surface.

To further study what happens when food is placed in hot oil, the researchers used a small piece of paper soaked in water as a model. They found that the number and type of bubbles in this case depend on both the amount of water and the temperature. They noticed that different types of vapor cavities are formed in hot oil when water encounters it.

These initial experiments led to a series of more controlled studies using an installation that allowed water drops to be added to hot oil from a suspended wire on a moving stage. A high-speed camera and a sensitive microphone were used to gather details about the shape of the bubbles that form when a drop of water gets on hot oil, and the hissing sound they make when they burst.

Video of an explosion cavity formed when a drop of water hits hot oil. Written by Ted T. Trascot

“In our experiments, we found three types of bubbles: an explosive cavity, an elongated cavity, and an oscillating cavity,” said author Ted Trascot.

An explosion cavity is formed when a drop of water gets into a hot oil and is exposed to a micro-explosion due to a sharp rise in temperature, forming a vapor bubble that can rupture the surface. The elongated cavity includes a water droplet that explodes without tearing the surface.

The oscillating cavity occurs when the drop slips off the wire and quickly sinks into the water. It goes through a multi-stage explosion process and begins to oscillate before breaking up into many small bubbles.

Audio signals from the microphone showed that the three types of caverns produce different acoustic, or sound, characteristics.

“We can distinguish different characteristics of the acoustic signal for each type of cavity,” Trascot said. “Deciphering audio signals could lead to future applications such as acoustic sensing of aerosol formation.”

Reference: “Morphology of bubble dynamics and sound in heated oil” by Akihito Kiyama, Rafson Rabi, Zhao Pan, Som Dutta, John S. Allen and Ted T. Trascot, June 7, 2022, Fluid physics.
DOI: 10.1063 / 5.0088065