Anomalous orientations of a rigid carbon nanotube in a sheared fluid

The nanoparticle orientation in fluid systems can be correlated with the rotational diffusion and is widely used to tune the physical properties of functional materials. In the current work, the controllability of the orientation of a single rigid carbon nanotube in a fluid is investigated by imposing a linear shear flow. Molecular dynamics simulations reveal three forms of anomalous behavior: (i) "Aligned orientation" when the nanotube oscillates around a particular direction which is close to the flow direction at a small angle of about 10° in the velocity-gradient plane; (ii) "Interrupted orientation" when the oscillation is interrupted by a 360° rotation now and then; (iii) "Random orientation" when 360° rotations dominate with the rotational direction coinciding with the local fluid flow direction. The orientation order is a function of the Peclet number (Pe). The results show that the correlation between Pe and the orientation order from the two-dimensional model does not apply to the three-dimensional cases, perhaps due to some anomalous behavior and cross-section effects. This work provides clear pictures of the nanoparticle movement that can be used to guide particle manipulation techniques.