, , Michael Bevan
We report a feedback method for controlling the sizes of hexagonally close packed two-dimensional colloidal crystals in a microfluidic quadrupole electrode device. Crystal size, measured by the number of colloidal particles comprising the crystal, is controlled over an order of magnitude range with a variation of less than 10% for colloidal crystals containing 20 to 250 micron-sized silica spheres. The total number of particles in the control region is tracked in real time and alternating applications of electrokinetic mechanisms either electro-osmosis or negative dielectrophoresis are actuated to guide the formation of the crystals of the targeted size. Subtle changes in the bounding geometry of the crystal compare well with scaling models of the number of particles on the crystal edge and a geometrical parameter reflecting the degree of hexagonal close packing. This model system gives us the opportunity to explore concepts relevant to the controlled assembly of nanoscale objects for nanomanufacturing.
feedback control, crystal assembly, dielectrophoresis, electroosmotic flow