Phonon Scattering Dynamics of Thermophoretic Motion in Carbon Nanotube Oscillators

Abstract

Using phonon wave packet molecular dynamics simulations, we find that anomalous longitudinal acoustic (LA) mode phonon scattering in low to moderate energy ranges is responsible for initiating thermophoretic motion in carbon nanotube oscillators. The repeated scattering of a single mode LA phonon wave packet near the ends of the inner nanotube provides a net unbalanced force that, if large enough, initiates thermophoresis. By applying a coherent phonon pulse on the outer tube, which generalizes the single mode phonon wave packet, we are able to achieve thermophoresis in a carbon nanotube oscillator. We also find the nature of the unbalanced force on end-atoms to be qualitatively similar to that under an imposed thermal gradient. The thermodiffusion coefficient obtained for a range of thermal gradients and core lengths suggest that LA phonon scattering is the dominant mechanism for thermophoresis in longer cores, whereas for shorter cores, it is the highly diffusive mechanism that provides the effective force.

Water thermophoresis in carbon nanotubes: the interplay between thermophoretic and friction forces

“…In the unconstrained MD simulations of thermophoresis 29,[33][34][35][36][37]40,41 al. 29 and then, in a similar context, reported by Zambrano et al 36 .…”

“…In fact, the practical use of thermophoresis in future CNT-based nanofluidic devices requires the feasibility of achieving predictable and controllable thermophoretic water transport inside CNTs. To date, despite the important efforts that have been devoted to investigate thermophoresis inside and outside CNTs [29][30][31][32][33][34][35][36][37][38][39][40][41] , the interplay between applied thermal gradient, thermophoretic velocity, solid-liquid friction [42][43][44][45] and thermophoretic force 30,33,36 in a water/CNT system, has not been studied. In this letter, we present an atomistic study of the kinetics associated with thermophoresis of water nanodroplets confined inside single wall carbon nanotubes and its interplay with the solid-liquid friction force.…”

Ballistic thermophoresis of adsorbates on free-standing graphene

“…Very recently, it was shown via phonon wave packet molecular dynamics (MD) simulations in CNTs that thermophoretic motion is driven by the scattering of LA phonons between the external and internal CNTs (12). In all of theoretical studies so far, the temperature gradient ∆T was nonetheless considered as the relevant quantity which controls thermophoresis as in Eq.…”

“…The temperature difference gives rise to a net imbalance of phonon population between the two sides, n hot and n cold . In coaxial nanotubes, Prasad et al (12) showed how a phonon wave packet traveling in the outer (longer) tube scatters at the edges of the inner (shorter) tube, exchanging energy and momentum. This "push" from a single wave is felt identically whether it comes from the hot reservoir or from the cold reservoir.…”

“…Computational studies have highlighted the possibility to drive thermally gold nanoparticles, water clusters, graphene nanoflakes, C60 clusters, and small CNTs over graphene layers or inside CNTs. Despite that, there appears to be so far insufficient intimate understanding of that phenomenon, besides the obvious consensus that the driving force stems from the spatial nonuniformity of the phonon population (3,8,12). Our scope will be to deepen that understanding.…”