  We present a simple method to control the position of ellipsoidal magnetic
particles in microchannel Poiseuille flow using a static uniform magnetic
field. The magnetic field is utilized to pin the particle orientation, and the
hydrodynamic interactions between ellipsoids and channel walls allow control of
the transverse position of the particles. We employ a far-field hydrodynamic
theory and simulations using the boundary element method and Brownian dynamics
to show how magnetic particles can be focussed and segregated by size and
shape. This is of importance for particle manipulation in lab-on-a-chip
devices.
