Fluid rarefaction and surface roughness modulate the thermal response of mass-asymmetric rods

Thermophoresis refers to the translation of particles under a temperature gradient. While the thermophoresis of spherical particles has been well studied, recent advances have shifted focus to anisotropic particles, which exhibit complex phenomena such as thermo-orientation and anomalous rotational diffusion with non-Gaussian angular displacement distributions. However, studies on how fluid rarefaction affects both translational and rotational thermal responses remain limited. In this work, nonequilibrium molecular dynamics simulations were employed to investigate the thermophoresis and thermo-orientation characteristics of mass-asymmetric rods with different surface roughnesses under various fluid rarefaction conditions. We propose that fluid rarefaction, in addition to temperature gradient, is a key parameter regulating thermal responses. Increasing fluid density enhances both thermophoretic force and thermo-orientation. Specifically, rough rods, due to their grooved structure that captures more fluid atoms, exhibit higher thermophoretic torque, stronger thermo-orientation, and lower rotational diffusion, while smooth rods, with a larger fluid-particle contact area, display greater thermophoretic force. By analyzing rotational diffusion dynamics, we reveal a strong link between thermo-orientation and rotational diffusion, offering insight into anisotropic thermophoresis. Our study further reveals that reducing the Knudsen number simultaneously increases both the Soret coefficient and thermo-orientation, a finding contrasting with previous studies based on temperature gradient and mass asymmetry variations.