Ultrafast nano generation of acoustic waves in water via a single carbon nanotube

TL;DR

This paper theoretically investigates how laser-excited water-immersed carbon nanotubes generate ultra high frequency acoustic waves, revealing mechanisms that depend on nanotube size and laser pulse duration, with potential applications in nanoscale sensing and imaging.

Contribution

It introduces a multi-scale theoretical model showing CNTs can act as thermophones or mechanophones, enabling control over high-frequency acoustic wave generation in water.

Findings

CNTs can act as thermophones or mechanophones depending on conditions.

Activation of mechanophone effect can produce nanometer wavelength sound waves.

Findings differ from metallic nano-objects, highlighting unique CNT capabilities.

Abstract

Generation of ultra high frequency acoustic waves in water is key to nano resolution sensing, acoustic imaging and theranostics. In this context water immersed carbon nanotubes (CNTs) may act as an ideal optoacoustic source, due to their nanometric radial dimensions, peculiar thermal properties and broad band optical absorption. The generation mechanism of acoustic waves in water, upon excitation of both a single-wall (SW) and a multi-wall (MW) CNT with laser pulses of temporal width ranging from 5 ns down to ps, is theoretically investigated via a multi-scale approach. We show that, depending on the combination of CNT size and laser pulse duration, the CNT can act as a thermophone or a mechanophone. As a thermophone, the CNT acts as a nanoheater for the surrounding water, which, upon thermal expansion, launches the pressure wave. As a mechanophone, the CNT acts as a nanopiston, its thermal expansion directly triggering the pressure wave in water. Activation of the mechanophone effect is sought to trigger few nanometers wavelength sound waves in water, matching the CNT acoustic frequencies. This is at variance with respect to the commonly addressed case of water-immersed single metallic nano-objects excited with ns laser pulses, where only the thermophone effect significantly contributes. The present findings might be of impact in fields ranging from nanoscale non-destructive testing to water dynamics at the meso- to nano-scale.