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(369g) NSEF Poster Session: Computational Studies on the Structural Properties of Square Colloids with Offset Magnetic Dipoles

Velev, O. D., North Carolina State University
In recent work, Velev and coworkers have developed a new class of engineered materials that interact, assemble, reconfigure, and propel in response to external magnetic and electric fields. Cubic microparticles with a ferromagnetic-metallic coating on one or two opposing faces retain residual polarization when exposed to an external magnetic field, even after the field is turned off. The many different interactions that exist when anisotropic, magnetically-polarized colloidal particles are placed in tunable, external fields creates numerous design variables that are challenging to fully explore experimentally. The search for potentially useful structures formed by this colloidal material can be enhanced by computer simulations of colloidal assembly. In this work, we used Discontinuous Molecular Dynamics (DMD) to simulate the behavior of large systems of dipolar squares in the absence of a magnetic field. DMD is a fast variant of standard molecular dynamics that is applicable to systems of molecules interacting via discontinuous potentials, and is best suited for exploring phenomena that occur at long time scales. Microcubes were represented in quasi-2D as groupings of hard discs bonded together to create a rigid square geometry. Magnetic dipoles were mimicked in silico by embedding opposite electrostatic charges along one cubic face. Annealing, or “slow-cooling”, simulations were performed in the absence of a magnetic field using the model described above to discover the equilibrium conformations of the dipolar squares.

We find that, as the strength of the dipolar interactions between squares overcomes the system’s thermal energy, the dipolar squares assemble into single- or double-stranded assemblies, each with unique structures and phase diagrams in the temperature-density plane. Single-stranded assemblies of dipolar squares are associated with the formation of a percolated, or gel-like, state, while double stranded assemblies are associated with the formation of a nematic state. Using pair-wise potential energy calculations between dipolar squares, we show that whether a system of dipolar squares assembles into single- or double-stranded conformations depends on how the dipole is embedded within the square. Furthermore, by parameterizing the location of the dipole within the square, we predict that certain dipolar squares can transition between percolated and nematic states, depending on the system’s density.

Our results highlight how colloidal particles with several degrees of anisotropy exhibit a rich phase behavior that sensitively depends on particle geometry and directional interactions. Our theoretical predictions are useful to colloidal scientists that are attempting to rationally synthesize particles with controllable properties. Finally, our studies of dipolar squares in the absence of a magnetic field provide a useful background as we extend our simulations to dipolar squares in the presence of an external magnetic field.