Directed Self Assembly (DSA) methods can assemble particles in suspension into useful structures and devices using particles ranging in size from nanoparticles to non-Brownian particles. In this work, electrical and oscillatory shear fields are being explored as methods for DSA focusing on how these methods can be harnessed to generate three-dimensional ordered structures. Potential applications of these highly ordered structures are for photonic devices or for materials with consistent mechanical properties dictated by an ordered structure. DSA methods can create highly ordered three-dimensional structures under predictable experimental conditions.

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. Dielectrophoretic ordering is demonstrated to form both 2D and 3D structures with micron lengthscale features.

Colloidal dispersions can be ordered by oscillatory shear fields within specific amplitudes and frequencies. However, creating ordered structures with non-Brownian particles in the ~10-100μm range is challenging. This work shows that Large Amplitude Oscillatory Shear (LAOS) imparts particle mobility through shear-induced diffusion that can direct ordering. LAOS is demonstrated on monodisperse particle suspensions between 1 and 65 microns by exploring the degree of order by varying particle loading and shear amplitude and frequency. A Rheo-LS (Light Scattering) device is used to monitor the degree of order, the type of crystalline order and rheokinetics of the ordering process. The Rheo-LS device gives direct observations of the degree and type of order, either a three-fold twinning or a six spot Bragg pattern. The order parameter is determined from the light scattering results and compared with the kinetic rheological data. Further, use of polymer glass forming matrices enables direct examination of the ordered structure through scanning electron microscopy. The rate of ordering is found to scale directly with the frequency and inversely with the applied strain rate.