Acoustic Wave Generation in Nanofluids: Effect of the Kapitza Resistance on the Thermophone to Mechanophone Generation Mechanism Transition

Abstract

High frequency, short wavelength acoustic wave generation in fluids, by means of immersed nanotransducers, is key to applications ranging from the biomedical to the ICT sector, all the way to investigating fundamental aspects of the mechanics of fluids. In this context, the commonly adopted generation mechanism is the thermophone, where the nanotransducer serves as a nanoheater for the surrounding fluid and the acoustic wave is launched by water thermal expansion. Its performance, however, is severely degraded when reaching up to hypersonic frequencies. We analytically investigate the thermoacoustic response in the frequency domain for the case of a gold nanofilm transducer in contact with water. We prove that another generation mechanism, the mechanophone, sets in this critical frequency range, widening the acoustic generation bandwidth. In the mechanophone, it is the nanotransducer thermal expansion that launches pressure waves in water. We find a threshold frequency discriminating between these two regimes, which can be tuned by acting on the Kapitza resistance at the solid–liquid interface. We then show how the mechanophone mechanism can be activated in the frame of photoacoustic generation with pulsed laser sources. The unveiled physics bears generality beyond the specific system and explains the acoustic generation mechanisms in nanofluids.