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(438f) 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 in vitro. The search for potentially useful structures formed by this colloidal material can be enhanced by simulations of colloidal particle assembly.

In previous work, we have simulated large systems of cubic microparticles with one magnetic dipole using Discontinuous Molecular Dynamics (DMD). DMD is a fast variant of standard molecular dynamics that is applicable to systems of molecules interacting via discontinuous potentials (e.g., hard sphere, square well potentials). DMD is best suited for exploring phenomena that occur at long time scales. Microcubes were represented in quasi-2D as groupings of hard circles bonded together to create a rigid square geometry. Magnetic dipoles were mimicked in silico by embedding opposite electrostatic charges along one cubic face. A modified Anderson thermostat was employed to simulate the force that an external magnetic field exerts on a magnetically-polarized microcube while keeping the system’s temperature constant.

We have extended our previous studies to further explore how the location of the magnetic dipole embedded within the square colloid affects the assembling structures of the system. Annealing DMD simulations performed using the model described above have revealed that highly percolated structures form at high surface densities as the temperature is reduced and the strength of the interparticle, dipolar interactions overcome thermal fluctuations. Order parameters quantifying the different types of clusters that can form were used to characterize the phase behavior (fluid, string fluid, gel, etc.) of these colloidal systems.

Our simulations of these unique, anisotropic colloids show that the way the internal magnetic dipole is shifted from the square’s center plays a large role in determining the system’s phase behavior. The conformation preference of the squares, either head-to-tail or anti-parallel, results in single- or double-stranded assemblies, each with unique structures and phase diagrams in the temperature-density plane. Our results should be useful to colloidal scientists as they highlight the unique capabilities of particles with anisotropic features.