Two-dimensional patterns of magnetic particles at air-water or glass-water interfaces induced by an external magnetic field: Experimental observation and dependencies

We report a formation of hexagonally aligned patterns of paramagnetic polystyrene particles attached at air-water or glass-water interfaces. Contrary to the closely packed 2D particle arrays reported up to now, the patterns described here showed interparticle distances from 6 to 10 particle diameters. The particles were gathered due to the gradient in the external magnetic field, which was applied across the interface. The source of the magnetic field was a permanent magnetic dipole. The created magnetic field attracted the particles toward the interface and the magnetic dipole axis of symmetry. The external field induced magnetic dipole moments in the particles and they repulsed each other due to the dipole-dipole magnetic interactions. The particles were negatively charged and they repulsed each other also electrostatically, which was proven by adding electrolyte in the water subphase. Finally, we suggest that the interparticle distances were determined by the action among three forces: magnetic attraction toward the axis of symmetry, dipole-dipole magnetic repulsion, and electrostatic repulsion. This statement is clarified in the complementary theoretical paper.