Tuning photoacoustics with nanotransducers via thermal boundary resistance and laser pulse duration

Abstract

The photoacoustic effect in liquids, generated by metal nanoparticles excited with short laser pulses, offers high contrast imaging and promising medical treatment techniques. Understanding the role of the thermal boundary resistance (TBR) and the laser pulse duration in the generation mechanism of acoustic waves is essential to implement efficient photoacoustic nanotransducers. This work theoretically investigates, for the paradigmatic case of water-immersed gold nanocylinders, the role of the TBR and laser pulse duration in the competition between the launching mechanisms: the thermophone and the mechanophone. In the thermophone, the nanoparticle acts as a nanoheater and the wave is launched by water thermal expansion. In the mechanophone, the nanoparticle directly acts as a nanopiston. Specifically, for a gold–water interface, the thermophone prevails under ns light pulse irradiation, while the mechanophone dominates shortening the pulse to the 10 ps regime. For a graphene-functionalized gold–water interface, instead, the mechanophone dominates over the entire range of explored laser pulse durations. The results point to high-TBR, liquid-immersed nanoparticles as potentially efficient photoacoustic nanogenerators, with the advantage of keeping the liquid environment temperature unaltered.

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

“…This is the so-called thermophone mechanism. On the other hand, the power density fed to the system also generates an oscillatory strain and stress behavior within the solid, which can thus act as a piston on the fluid, finally generating an acoustic wave in the fluid region, addressed as mechanophone mechanism. , In order to clarify the interplay and magnitude of these two effects across the frequency spectrum, we calculate the acoustic pressure generated in the two cases by alternatively turning off the thermophone and the mechanophone effects. To turn off the thermophone, we set the water thermal expansion coefficient α 0 = 0, and conversely, to turn off the mechanophone we set the solid thermal expansion coefficient α 1 = 0.…”

“…We analyzed, in the case of the mechanophone, an approximation for the pressure generated at low frequencies since this mechanism is less effective in that portion of the spectrum, see eq or eq for b = +1 and eq or eq for b = −1. Assuming that, conversely, the thermophone is less effective at high frequencies, , we now develop, for the pure thermophone case, a valid approximation for intermediate and/or high frequencies. The two approximations will allow us to determine a threshold frequency beyond which the mechanophone mechanism will be more effective than the thermophone.…”

“…The present results rationalize, on a formal basis, the physics behind recent numerical findings on impulsive acoustic excitation in nanofluids. 54,55,78,79 Furthermore, they allow revisiting, under a broader perspective, impulsive hypersonic acoustic wave generation in polymers, 103 and solid nanostructures, 104 triggered by heated solid nano-transducers. From an applicative stand-point, the mechanophone effect is of interest in bio-related applications requiring short acoustic wave generation while avoiding overheating the fluid.…”

“…However, propitiously, it was recently suggested that, in this high-frequency regime, a second generation mechanism intervenes, addressed as mechanophone . , In the thermophone, the heat transferred from the solid to the fluid generates compression and expansion processes in the fluid itself, thus launching acoustic waves in the latter. Differently, in the mechanophone, it is the heat retained in the solid that causes internal oscillations of stress and strain that are transmitted to the fluid by pure mechanical transduction, ultimately launching acoustic waves in the fluid.…”

“…Differently, in the mechanophone, it is the heat retained in the solid that causes internal oscillations of stress and strain that are transmitted to the fluid by pure mechanical transduction, ultimately launching acoustic waves in the fluid. These aspects were numerically investigated in the time domain, , yet lack an exhaustive physical comprehension and a formulation in analytical terms. The latter is a major impediment towards uncovering the parameters combination ruling the transition from the thermophone to the mechanophone regime.…”