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

Colloidal particles with directional 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 colloid particles tend to align end-to-end and self-assemble into variety of structures ranging from co-crystals of novel symmetry, to open networks (gels) of cross-linked chains of particles. Quadrupolar colloid particles also tend to self-assemble in a wide variety of structural motifs including sheets, tubes and shells depending upon external conditions. We use molecular dynamics computer simulation to explore the self-assembly, structure, crystallization and/or gelation of systems of colloid particles with permanent dipole moments or quadrupole moments immersed in a high-dielectric solvent.

Phase diagrams for monodisperse and bidisperse system of dipolar colloid particles in the temperature-packing fraction plane are calculated using discontinuous molecular dynamics simulations. An interesting aspect of these phase diagrams is the appearance of co-crystals containing large and small dipolar colloid particles. Even more interesting is the appearance of two unique bicontinuous gel structures: the first consisting of two single-species interpenetrating networks of cross-linked chains and the second consisting of a network of cross-linked chains of one species coated by particles of the other species.

We also explore the structure formation of systems of colloid particles with permanent quadrupole moment. We introduce a simple quadrupole-quadrupole discontinuous potential model that gives rise to the self-organization of surface and tubular structures. A new type of anisotropic colloid particle is introduced having a displaced quadrupole moment with a unique symmetry; systems comprised of these particles form tubular structures. Our simulations predict optimal conditions for making tubes of specific diameter and length out of quadrupolar colloidal particles; this might eventually be a route to the formation of high quality tubes.