(615i) Computational Studies of the Directed Assembly of Dipolar Rods in the Presence of an External Field

In fabricating new colloid-based materials via bottom-up design, particle-particle interactions are engineered to encourage the formation of the desired assemblies. One way to do this is to apply an external field, which orients magnetically-polarized particles in the field direction. External fields have the advantage that they can be programmed to change in time (e.g. field rotation or toggling), shifting the system out of equilibrium. In this project, we use molecular simulations to explore the phase behavior of dipolar rods exposed to a constant external field, as well as to external fields that change with respect to time.

Previous computational studies have shown that dipolar rods have diverse phase behaviors, which depend on the length of the dipole embedded within the rod. Rods containing a dipole whose length is approximately equal to the particle length prefer head-to-tail assemblies, while rods containing a dipole whose length is significantly less than the particle length prefer side-to-side assemblies. Head-to-tail assemblies are associated with the formation of gel-like, percolated phases, whereas side-to-side assemblies are not.

We recently developed a new computational technique applicable to discontinuous molecular dynamics simulations that models the effect of an applied, external field on a dipole embedded within a colloid. This new technique is an extension of the Anderson thermostat, where ghost collisions selected from a Maxwell-Boltzmann distribution create behavior statistically representative of the constant temperature, canonical ensemble. We apply this new method of modeling the effect of external fields to coarse-grained simulations of systems of dipolar rods, the goal being to study how an applied external field shifts the assemblies out-of-equilibrium.

First, we explore how an external field with constant strength and direction shifts the assemblies of dipolar rods from preference for side-to-side assemblies (anti-parallel configurations) to preference for head-to-tail assemblies (parallel configurations). The nuanced phase behaviors of these systems are illustrated through constant density phase diagrams as a function of the field strength (H) and reduced temperature (T), and through three-dimensional phase diagrams. Second, we explore how a rotating external field affects the clustering and assembling behavior of the dipolar rods. The results from our simulations demonstrate how external fields can be used to direct and control the types of assemblies that dipolar colloids form.