Particle interactions near the contact line in liquid drops

Micrometer-sized particles distributed in a drop of water are drawn toward the contact line upon evaporation, where they arrange themselves into a crystal structure. Here we report experiments with paramagnetic particles, which make it possible to measure the particle interactions by tuning an external magnetic field. By aligning the magnetic dipoles, we are also able to redistribute the particles back into the bulk of the droplet. This method allows us to assemble and disassemble colloidal crystals simply by applying a magnetic field.