In this study, the Brownian diffusion of elongated nano-fibers was investigated numerically. Motions of nano-fibers of different sizes and aspect ratios were resolved by solving the corresponding system of equations governing their coupled translational and rotational motions. The study allowed a close examination of the Brownian diffusion of nano-fibers with respect to the coupling of their translational and rotational motions and how the rotation affects the fibrous particle macroscopic diffusion properties. Particular attention was given to the rotational relaxation time in determining the isotropic and anisotropic diffusive properties of elongated particles. Theoretical and semi-empirical equations were developed to quantify the diffusion coefficients of nano-fibers. Orientation averaged mobility of the nanofiber, nanorod and non-spherical nano-agglomerates was examined by the newly developed Brownian diffusion model. The result was compared with the particle mobility by collision limited reaction rate theory, empirical models and the experimental measurements. The study led to the discovery of a surface dominated mobility for nanoparticles with Knudsen number greater than 5 (Kn > 5). For particles in this range, all morphologies can be viewed as point collision and therefore the use of the equivalent surface mobility diameter was reasonably justified. Beyond this range when Kn < 0.1, however, the study showed that the particle morphology starts to play an important role, and accuracy of the surface mobility approximation reduces as the Knudsen number decreases. The study opened up new approaches for studying fundamental diffusive processes of nano-particles of complex shapes which are numerous in natural environment.

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