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Computer Simulation of Self-Assembly of Dipolar Colloid Particles for the Design of Stimuli-Responsive Materials

Amit Goyal, Carol K. Hall, and Orlin D. Velev. Department of Chemical and Biomolecular Engineering, North Carolina State University, College of Engineering 1, Box 7905, 911 Partners Way, Raleigh, NC 27695

Colloid particles with dipolar interactions that self-assemble into pre-defined microstructures have the potential to serve as the foundation for a new generation of micro- and nano-structures of remarkable complexity and precision. Dipolar colloidal particles self-assemble into a variety of interesting microstructures ranging from nematic or smectic liquid crystals, to co-crystals of novel symmetry, to open networks (gels) of cross-linked chains of particles. A technique for embedding either temporary or permanent dipoles in colloidal particles has been proposed by Velev et al [1]. However, the multitude of possible structures that colloid particles can form makes experimental study of all possible variations infeasible. Therefore, we use theory and computer simulation to explore the phase behavior including formation, structure, crystallization and/or gelation of systems of dipolar colloid particles so as to guide the discovery of advanced materials in the laboratory.

Our goal has been to predict the formation of gel and crystal structures in systems of dipolar particles. We have developed a discontinuous colloid-colloid potential model to simulate the self assembly of dipolar colloid particles. We applied discontinuous molecular dynamics (DMD) to the discontinuous colloid-colloid potential model to investigate the self-assembly of dipolar colloid particles. Several different types of phases are found in our simulations. At high packing fractions we find ordered phases including face centered cubic (FCC), hexagonal close packed (HCP) and body centered tetragonal (BCT). At low packing fractions we find fluid, string fluid and gel phases. At very high temperatures, hard-sphere phase behavior (a transition from fluid at low packing fraction to FCC solid at high packing fraction) is recovered. . We study how the kinetics and thermodynamics of the assembly process is affected by particle size, concentration, particle size ratio (for mixtures), electric field strength, dipolar interaction strength and location of the dipole within the particle. Our study should help us to advise our experimental colleagues in their quest to design and engineer “smart” gels and materials.

[1] O. Cayre, V. N. Paunov and O. D. Velev, Chem. Comm., 2296-2297 (2003). J. R. Millman, K. H. Bhatt, B. G. Prevo and O. D. Velev, Nature Mater., 4, 98-102 (2005).