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Directed Assembly of Particle Suspensions with Electrical Fields

Jason McMullan, Chemical Engineering, University of Delaware, 150 Academy Street, Newark, DE 19711 and Norman J. Wagner, Department of Chemical Engineering, University of Delaware, 150 Academy St., Newark, DE 19716.

Directed Self Assembly (DSA) methods can assemble particles in suspension into useful structures and devices using particles ranging in size from nanoparticles to micron sized particles. In this work, electrical fields are being explored as methods for DSA focusing on how this method 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.

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.

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