Directed drift and fluid pumping of nanoswimmers by periodic rectification-diffusion

The steady ratchet transport of run-and-tumble nanoswimmers in a 3D microfluidic channel constructed by periodic chambers separated by half-cylinder funnels is explored by dissipative particle dynamics. Two regions in a chamber are identified: rectification and active diffusion. While the concentration gradient is driven by the concentration jump in the rectification region, the ratchet current is dominated by the diffusion rate in the active diffusion region, which is classified into normal and Knudsen types. The former obeys Fick's law and is proportional to va2τ, where v_a is the self-propulsion velocity and τ the run time. In addition, autonomous pumping of fluids is induced by aligned force dipoles associated with nanoswimmers accumulated near funnels, similar to the mechanism of bacteria carpet. The direction of fluid flow is the same as that of the ratchet current but the former is one order of magnitude smaller than the latter. Thus, the fluid velocity depends on the characteristics of nanoswimmers.