Particle trapping and undulation of a liquid surface using a microscopically modulated magnetic field.

An aluminum/iron-layered block (periodicity of 300 microm) was placed in a homogeneous magnetic field (ca. 1 T) to produce a periodic modulation of the magnetic field over the block surface. This modulation caused an undulation of the surface of a thin liquid layer spread over the block. The same modulated field was used to trap polystyrene spheres (20 microm in diameter suspended in a liquid) in a periodic line pattern. The spheres were trapped above the iron layers of the block where the field strength is lower in the present experimental setup. Upon drying, the trapped spheres formed self-organized packing.