Motion and Orientation of Janus Spherical Particles at a Liquid-Air Interface Governed by the Moses Effect

We investigated the motion of spherical polystyrene/polypyrrole-coated polystyrene Janus particles placed at an air/saline interface and driven by a permanent magnetic field of ca. 0.5 T. For the sake of comparison, the motion of pure floating polystyrene particles was studied. Both kinds of the studied particles moved toward the magnet and stopped at the boundary of the near-surface well produced by the magnetic field. The Moses effect-driven motion of floating Janus particles was analyzed and investigated under different strengths of the magnetic field and salt concentrations. The study of the Janus particle displacement led to the development of a unified theoretical framework explaining the mechanism of the motion. This framework predicts that the motion of particles placed at an air-salt solution interface is not only dictated by magnetic energy but also intricately influenced by the interplay of factors, including the curvature of the interface caused by the static magnetic field, gravitational potential, and capillary forces. The orientation of the particles was observed. A qualitative explanation of the observed phenomena is suggested. The investigated process has potential for the self-assembly of particles placed at the liquid/air interface.