Microfluidic Assembly of Colloidal Armor

Assembly of colloidal particles on fluid interfaces is a promising technique for synthesizing two-dimensional microstructured materials useful in fields as diverse as biomedicine, materials science, mineral flotation and food processing. Current approaches have succeeded using bulk emulsification methods, require further chemical and thermal treatments, and are restrictive with respect to the materials employed. Microfluidic techniques provide a new method for assembling these shells. They allow direct visualization of the dynamics of colloid ordering on curved interfaces. The image illustrates the approach for directing a suspension of micron-size particles onto an inner gas core so as to form a gas bubble completely covered with colloidal particles. The bubbles are periodically ejected to form stable jammed shells, with colloidal armor. As one example of the control possible, a "janus shell" is shown on the right; it is made from yellow 4.9 mm diameter polystyrene particles dyed with rhodamine and green 4.0 mm particles dyed with fluoresceine.