Arraying nonmagnetic colloids by magnetic nanoparticle assemblers

We review our recent work on the manipulation and assembly of nonmagnetic colloidal materials above magnetically programmable surface templates. The nonmagnetic materials are manipulated by a fluid dispersion of magnetic nanoparticles, known as ferrofluid. Particle motion is guided by a program of magnetic information stored in a substrate in the form of a lithographically patterned template of micromagnets. We show how dynamic control over the motion of nonmagnetic particles can be accomplished by applying rotating external magnetic field. This unexpectedly large degree of control over particle motion can be used to manipulate large ensembles of particles in parallel, potentially with local control over particle trajectory. Index Terms--Assembly, colloid, inverse ferrofluid, patterned magnets.