Highly ordered three-dimensional colloidal crystals are grown from a dilute colloidal suspension by AC electrical fields with frequencies near a few kilohertz. This process is advantageous over previously published methods because a template is not needed and assembly occurs within seconds. This technique exploits dielectrophoretic forces induced in the double layers surrounding the particles by the applied field. Using the standard electrokinetic model, the induced DEP force is predicted to depend on the particle size, charge and electrolyte concentration. This was studied on a model colloidal dispersion and the results compared to theoretical predictions (Hill et al., J Colloid Interface Sci., 258, 56 (2003)) through a predictive model that incorporates double layer polarization. Modeling identifies appropriate ranges of solution conditions favoring particle ordering.
Creating three-dimensional crystals with electric fields requires the use of multiple sets of electric fields. The first field creates the two-dimensional crystal while the other field directs the two dimensional layers together to form the full crystalline structure. These crystalline structures are observed visually and through light scattering techniques. Both methods are used to quantify the extent of order within the sample. Dielectrophoretic ordering is demonstrated to form both 2D and 3D structures with micron lengthscale features where ordering conditions are chosen to dictate final crystalline structure and strength.