Experienced chefs can determine when the cooking oil reaches a temperature suitable for frying by inserting wet chopsticks into the hot oil. Bubbles form on the chopsticks, and their hissing sound can tell the chef when to add food to the pot. In physics of fluids published by AIP, researchers from the United States, Canada and Saudi Arabia carefully studied the bubbles formed when water droplets contact heated edible oil.
When researchers inserted wet chopsticks into hot oil, they found that the type and number of bubbles formed depended on the amount of water absorbed by chopsticks and the material of chopsticks.
They experimented with water drops and dough drops suspended on the top of chopsticks. It is understood that water drops will explode when they touch hot oil, while batter drops will produce bubbles on their surface.
To further investigate 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, in this case, the number and type of bubbles depended on the amount and temperature of the water. They observed that when water meets hot oil, different types of vaporization chambers are formed in the hot oil.
These initial experiments led to a series of more controlled studies using devices that allowed water droplets to be added to hot oil from overhead lines on a movable stage. A high-speed camera and a sensitive microphone were used to collect detailed data on the shape of bubbles formed when water droplets touched hot oil and the hissing sound they made when they burst.
Tadd Truscott, the author of the research paper, said: "we found three types of bubble events in the experiment: explosive cavity, elongated cavity and oscillating cavity."
The explosion cavity is formed when water drops enter the hot oil and micro explosion occurs due to the sudden rise of temperature. The steam bubble formed will rupture the surface. The elongated cavity involves a water drop, which will not rupture the surface when it explodes.
The oscillating cavity occurs when water drops fall from the wire and are quickly submerged. It experienced a multi-step explosion process and began to oscillate. Then it broke down into countless small bubbles.
The audio signal from the microphone shows that the three cavity types produce different acoustic or sound characteristics.
"We can distinguish the different acoustic signal characteristics of each type of cavity. Deciphering the acoustic signal may lead to future applications, such as acoustic sensing of aerosol generation," Truscott said.