Hydrodynamic Brownian motors

Microchannels of periodically varying diameter may serve as a ratchet device, rectyfying the movment of tiny colloidal particles immersed in the fluid. The fluid is pumped back and forth, the average flow being zero. Despite of that, the particles move in a specific direction determined by the mirror-asymmetric variation of the tube diameter. This was proved by several recent experiments. In our work we compute analytically the particle flow and show that the decisive mechanism behind the rectification is the inertial effect of the fluid flow, combined with the Brownian motion of the colloidal particle. The ratchet current depends very strongly on particle diameter, thus making such setup an ideal device for sorting of micron- and submicton-sized particles.

Left panel: Sketch of particle flowing in tube of periodically varying diameter. Right panel: streamlines of the flow of particles. Note the "half-vortices", responsible for the ratchet effect.

F. Slanina: Inertial hydrodynamic ratchet: Rectification of colloidal flow in tubes of variable diameter, Phys. Rev. E 94, 042610 (2016).