The sound of deep-frying

Home cooks don’t need to rely on a thermometer to tell them whether the oil in a pan is hot enough to cook with. Rather, they can test how it responds to a tiny bit of water: Sufficiently hot oil quickly produces telltale bubbles and audible crackling. Through audio and video recordings of bubbles formed by water in hot oil, Akihito Kiyama (then at Utah State University) and his collaborators sought to better understand and quantify the bubble-formation process by investigating the relationship between fluid temperature, bubble dynamics, and the sound that the bubbles generate.

In their initial experiments, the researchers observed that three types of vaporous cavities, or bubbles, form when wet bamboo chopsticks or paper are dipped in hot oil (see the first figure). Then they investigated when and how each cavity type would form by lowering a 4 µm droplet of water suspended from a wire into a beaker of oil. The hot oil evaporated the water and formed steam-filled bubbles. A microphone held above the oil’s surface recorded the ensuing sounds.

In the range of temperatures tested, 170–210 °C, the type of cavity that formed was largely independent of temperature. Rather, it depended most strongly on the ratio h/R, where h is the depth of the water droplet in the oil before bubble formation and R is the bubble’s maximum radius. The three formation mechanisms are illustrated in the second figure. So-called explosion cavities formed when h/R was small, typically less than 0.5, and the rupturing bubble formed a hemispheric indentation in the oil’s surface. When h/R was larger, between 0.5 and 1.5, elongated cavities formed below the surface; as the tops of the bubbles collapsed inward, they formed a downward jet that pushed the air bubble down and away from the oil–air interface. A similar upward jet formed at the oil’s surface.

Droplets that slid off the wire during their descents into the oil reached larger values of h/R. The resulting bubbles remained relatively spherical, and their surface vibrations induced oscillations in the oil–air interface that generated small jets of oil. Unlike the other cavity types, which were formed by droplets held at fixed heights, oscillating cavities originated from moving droplets, and the droplets’ falling speeds may play a role in producing such behavior.

The audio signals for the explosion and elongation cavities had similar fundamental frequencies of around 1.4 Hz. The timing of the peaks suggests that the sounds are primarily produced during cavity expansion. The two processes can be differentiated by the signal amplitude. The oscillating bubble had a lower peak frequency, of 1.1 Hz, and the sound generation was dominated by the bubble’s surface oscillations. Because the different bubble types produce distinct jets and ejected droplets at the oil–water interface, audio recordings could be used in the future to distinguish between them and thereby track aerosol generation. (A. Kiyama et al., Phys. Fluids 34, 062107, 2022 .)