Energy Dissipation in Fluid Coupled Nanoresonators: The Effect of Phonon-Fluid Coupling

Resonant nanomechanical systems find numerous sensing applications both in the vacuum and in the fluid environment but their performance is degraded by different dissipation mechanisms. In this work, we study dissipation mechanisms associated with high frequency axial excitation of a single-walled carbon nanotube (CNT) filled with argon, which is a representative fluid coupled resonator system. By performing molecular dynamics simulations, we identify two dissipative processes associated with the axial excitation of the resonator: (i) perturbation of the resonator phonons and their relaxation and (ii) oscillatory fluid flow developed by the resonator motion. Dissipation due to the first process, a form of "intrinsic" dissipation, is found to be governed by the Akhiezer mechanism and is verified for an empty CNT in vacuum. To estimate the dissipation due to the second process, which is the conventional "fluid" dissipation, we formulate an approach based on the response of the hydrodynamic force on the resonator. Our analysis of the coupled system reveals that phonon relaxations associated with the Akhiezer dissipation are significantly modified in the presence of fluidic interactions, which have been ignored in all previous dissipation studies of fluid-resonator systems. We show that an important consequence of this phonon-fluid interaction is inverse scaling of dissipation with density at low excitation frequencies.