The capture mechanism of micron-sized magnetic particles from a surfactant-containing suspension is examined in an axial magnetic filter with a bounded flow field by using the particle trajectory model. The effects of change in the suspension viscosity to the suspension flow velocity are modeled. Analytical and numerical solutions for the particle trajectory model are obtained for a laminar flow regime. It is evaluated that the magnetic particles in the suspension, subjected to a viscosity change, are captured in a gradient magnetic field formed by a magnetized external ferromagnetic wire and the capturing of particles gets harder due to condensation of surfactant layers.