Dynamic self-assembly and directed flow of rotating colloids in microchannels

Abstract

Nonequilibrium structure formation and dynamics in suspensions of superparamagnetic colloids driven by an external rotating magnetic field are studied by particle-based mesoscale hydrodynamics simulations in confined geometry. We address the fundamental question how the rotation of the colloids about their own axes can be converted into a translational motion by breaking the symmetry of the confining geometry. We study a two-dimensional system of colloids with short-range repulsive interactions, which mimics flow in shallow microchannels. In straight channels, we observe a two-way traffic but-for symmetry reasons-no net transport. However, by keeping some colloids fixed near one of the two walls, net transport can be achieved. This approach allows the control and switchability of the flow in complex microchannel networks. A minimal geometry that fulfills the requirement of broken symmetry are ring channels. We determine the translational velocity of the spinning colloids and study its dependence on the channel width for various median radii. We conclude that spinning colloids present a promising alternative for flow generation and control in microfluidic devices.

Purely hydrodynamic ordering of rotating disks at a finite Reynolds number

“…The effects of confinement on such a system are also quite interesting not only from a fundamental viewpoint, but also from a viewpoint of applications to microfluidics 42,43 . Such effects on rotors in a fluid have recently been numerically studied by Götze and Gompper 22,23 . It is quite interesting to study how such spatial confinements affect all the dynamical phases we reported and the transition between them.…”

Conveyor belt effect in the flow through a planar duct of a viscous fluid with spinning particles

“…11,12 In experiment the couples can be applied only by acting with an electric field on a permanent electric dipole moment, or with a magnetic field on a permanent magnetic dipole moment. Since the fields generated by polarization or magnetization are spatially non-uniform, they also exert a force on a dipole.…”

Conveyor belt effect in the flow through a tube of a viscous fluid with spinning particles

“…Recent numerical simulations have exhibited the effect of spinning particles on fluid flow. 15,16 It would be of interest to pursue such simulations in a comparison with the above theoretical calculation of the conveyor belt effect in a tube.…”