Ballistic thermophoresis of adsorbates on free-standing graphene [Applied Physical Sciences]
The textbook thermophoretic force which acts on a body in a fluid is proportional to the local temperature gradient. The same
is expected to hold for the macroscopic drift behavior of a diffusive cluster or molecule physisorbed on a solid surface.
The question we explore here is whether that is still valid on a 2D membrane such as graphene at short sheet length. By means
of a nonequilibrium molecular dynamics study of a test system—a gold nanocluster adsorbed on free-standing graphene clamped
between two temperatures
ΔT apart—we find a phoretic force which for submicron sheet lengths is parallel to, but basically independent of, the local
gradient magnitude. This identifies a thermophoretic regime that is ballistic rather than diffusive, persisting up to and
beyond a 100-nanometer sheet length. Analysis shows that the phoretic force is due to the flexural phonons, whose flow is
known to be ballistic and distance-independent up to relatively long mean-free paths. However, ordinary harmonic phonons should
only carry crystal momentum and, while impinging on the cluster, should not be able to impress real momentum. We show that
graphene and other membrane-like monolayers support a specific anharmonic connection between the flexural corrugation and
longitudinal phonons whose fast escape leaves behind a 2D-projected mass density increase endowing the flexural phonons, as
they move with their group velocity, with real momentum, part of which is transmitted to the adsorbate through scattering.
The resulting distance-independent ballistic thermophoretic force is not unlikely to possess practical applications.
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