Listen to your tempura!

From tempura, schnitzel, samosa to french fries, deep-fried foods are gourmet favorites across cultures and times. The perfect delicious crunch is made by cooks who carefully manage the cooking time and oil temperature. A common household technique to estimate the temperature is to insert moisturized chopsticks into the heated oil. The result is dozens of bubbles as well as a crackling sound used to estimate temperature. In this study, we investigate this idea by dipping water-moisturized chopsticks and water-wetted paper into oil at different temperatures. High-speed imagery reveals bubbles of water vapor are formed when the wet objects are submerged. The bubbles' oscillations generates distinct acoustic signatures, which are recorded through a synchronized microphone. The magnitudes of the sound signals are quantified in terms of oil temperature. The visualization also reveals complex bubble dynamics including oil surface deformation, where varying the dimensionless depth H/R of a bubble (bubble depth H and radius R) results in different mechanisms of liquid ligaments expulsions at the oil surface. We use an analogy to cavitation dynamics in the vicinity of the free surface to classify typical expulsion behaviors of the heated liquid.