Self-diffusion of a sphere in an effective medium of rods

Self-diffusion of a sphere in a network of rods is analyzed theoretically. Hydrodynamic interactions are taken into account according to the model of Dhont et al., under the assumption that $\ka << 1$ and $\bar{a}/L<<1$, where $1/\kappa$ is the network hydrodynamic screening length, $\bar{a}=a+D/2$, and $a$ is the sphere radius, while $D$ and $L$ are the diameter and length of a rod, respectively. Simple expressions for the diffusion coefficients are derived and shown to be independent of $L$.