Magnetic molds

Bartosz Grzybowski and colleagues at Northwestern University have demonstrated a new technique for precisely assembling colloidal particles into complex two- and three-dimensional arrangements. The process begins with a template: The researchers used photolithography to fabricate a pattern of nickel islands, 1–10 microns in diameter and 200 nanometers thick, and embedded the islands in a layer of silicone to create a flat surface. They then placed the template on a 0.4-tesla permanent magnet and underneath a solution of colloidal particles in a paramagnetic holmium nitrate solution. Because nickel has a large magnetic susceptibility, the template modulates and concentrates the magnet’s otherwise uniform field. The resulting microgradients in the fluid form a magnetic mold that draws paramagnetic particles onto the islands and pushes diamagnetic particles into the gaps between the islands. If desired, the particles can be configured to bond together, so that the structure remains even after the magnet is removed....