Observation of particle trajectories near a magnetized fiber

Abstract

The motions of 20–30‐μm paramagnetic particles have been observed in the neighborhood of a 250‐μm ferromagnetic fiber. The particles are entrained in nitrogen flowing down over a horizontal fiber. The particles and fiber are magnetized by a vertical magnetic field of strength up to 0.73 T. The free‐stream velocities range from nearly zero to 1 m/sec. A Fastax movie camera is used to record the positions of the particles as they pass by or collide with the fiber. The particle trajectories thus observed determine the collision cross section as a function of field strength and free‐stream velocity. Cross sections of over five diameters are observed. The cross sections and trejectories are compared and agree with the Newtonian theory of the particle motion. The theorey assumes potential flow over the fiber and accounts for the magnetic, viscous, and gravitational forces and particle inertia, all of these being significant in the range of conditions considered. The observed trajectories show details of the motion which are clear manifestations of particle inertia. The sharp particle shadows cast by the isolated fiber are a striking feature of the trajectory patterns. Such shadows should be the source of an intereference effect in multiple‐fibr filters.