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(471g) Networks of Anisotropic, Magnetically-Polarized Colloidal Particles Reversibly Reconfigure Under the Influence of an External Magnetic Field

Velev, O., 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.

We have simulated large systems of cubic microparticles with one magnetic dipole under the influence of an external magnetic field 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.

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 particle clusters that can form were used to characterize the phase behavior (fluid, string fluid, gel, etc.) of these colloidal systems. Our studies differentiate between structures that the colloidal aggregates form in the presence of, and in the absence of, an external magnetic field. The formation of chain-like structures was seen to a larger degree in networks formed under the influence of an external magnetic field. Additionally, diverse field-on and field-off time evolution patterns revealed the ability of networks formed under the influence of an external magnetic field to reversibly reconfigure, whereas irreversible jamming was seen in networks formed in the absence of an external magnetic field. Our results highlight the conditions under which reversible, reconfigurable networks form, adding to the fundamental knowledge base for directed assembly of colloidal particles.