Accessing Ultrafast Photothermal-Acoustic of Carbon Nanotubes in Water
Résumé
The photoacoustic effect in carbon nanotubes (CNTs) provides a new technique for biomedical diagnostics and treatment in living systems [1, 2]. However, a thorough understanding of the thermal-acoustics properties of a matrix embedded CNT is still lacking. In this context, we theoretically and computationally investigate here the transient photothermal-acoustic response of CNTs immersed in water, triggered by an ultrafast laser pulse. Given the time and length scales involved, the system requires a multi-physics, multi- scale approach [3]. First, the laser pulse triggers an impulsive temperature increase of the CNT (sub-ps time scale). Heat is then dissipated to the proximal water portion surrounding the CNT (ns time-scale). The thermal exchange at the CNR/water interface is governed by the thermal boundary resistance [4], which we retrieve from dedicated molecular dynamics (MD) simulations [5]. The CNT and water temperature increase leads to their thermal expansion, finally launching a pressure wave in water. Our simulations follow step-by-step the photothermal-acoustic steps involved, thus combining the optical, heat transfer and thermo- acoustic phenomena. The problem is here tackled solving, via Finite Element Methods, the macro-physics equations upon insertion of the microscopic thermal parameters calculated from MD. Different combinations of CNT sizes and laser pulse characteristics are discussed for different generation mechanisms of pressure waves in water. [1] A. De La Zerda et al.; Nat. Nanotechnol. 3, 557 (2008) [2] B. Kang et al.; Small 5, 1292 (2009) [3] C. Caddeo et al.; Phys. Rev. B 95, 085306 (2017) [4] M. Gandolfi et al.; Photoacoustics 20, 100199 (2020) [5] S. M. Nejad et al.; Int. J. Therm. Sci 159, 106588 (2021)