(634a) Directed Self-Assembly of Colloidal Crystals with Electric Fields

Authors: 
McMullan, J. M. - Presenter, University of Delaware
Wagner, N. J. - Presenter, University of Delaware

Directed Self Assembly (DSA) methods can create useful structures and devices using particles ranging in size from nanoparticles to micron sized particles. In this work, we explore the fundamental mechanics of electrical field driven (dielectrophoretic) DSA with the goal of enabling the engineering of methods for generating two and three-dimensional ordered structures of high fidelity and controlled architecture.

Highly ordered three-dimensional colloidal crystals are assembled from a dilute colloidal suspension by AC electrical fields with frequencies on the order tens of kilohertz. This technique exploits dielectrophoretic forces induced in the double layers surrounding the particles by the applied electric field. These crystalline structures are observed and the extent of order quantified both optically and with scattering techniques. In situ light scattering characterizes the degree of ordering and crystal structure for micron-sized particles. A new sample environment developed for Small Angle Neutron Scattering (SANS) is used to study the ordering and crystal structure of sub-micron crystalline assemblies. Using the order-disorder transition data obtained from these experiments and overlaying them with previous data from Lumsdon et al., (Appl. Phys. Lett., 82, 6 (2003)) rescaled by Mittal et al., (J. Chem. Phys., 129, 6 (2008)) validates the parameter scaling over a large range of experimental conditions. 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.

Measurement of fundamental colloidal properties, such as the electrophoretic mobility, are validated with the standard electrokinetic model (Hill et al., J Colloid Interface Sci., 258, 56 (2003)). Modeling identifies appropriate ranges of solution conditions favoring particle ordering. Using the standard electrokinetic model, the induced DEP force is predicted to depend on the particle size, charge and electrolyte concentration. DEP studies on a model colloidal dispersion are compared to theoretical predictions through a predictive model that incorporates double layer polarization. Standard model predictions agree with the observed order-disorder transition observed experimentally with SANS and light scattering.