(272f) Computational Studies of the Metastability of Colloidal Squares with Offset Dipoles at High Densities

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 create 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 used Discontinuous Molecular Dynamics (DMD) to simulate large systems of square particles containing a magnetic dipole that is offset from the square’s center of mass. We found that squares with transversely offset dipoles formed a nematic state under certain conditions. At high densities and moderate temperatures close to their melting temperature, the squares sometimes assembled into a nematic state. When this happened, the overall potential energy was at a minimum. The transition from a non-nematic state to a nematic state mainly occurred at very long timescales. These observations suggest that the nematic state is a free-energy minimum for the system, and furthermore that the system only transitions to the nematic state once sufficient energy allowed the system to transition and overcome a free-energy barrier.

The goal of the research described in his presentation is to quantify the free-energy barrier that exists between a nematic and non-nematic state for systems of colloidal squares with offset dipoles. To do this, we have implemented the Markovian Milestoning (MM) method to explore the potential metastability in this colloidal system. We do this by measuring the free-energy along the reaction pathway that is described by the nematic order parameter. Independent DMD simulations are distributed along the reaction pathway and constrained to the upper and lower boundaries nematic order parameter values. The MM algorithm recreates the free energy path between the non-nematic and nematic states by comparing the likelihood that a MD simulation will transition to either of the neighboring states along the path. We use this technique to calculate the likelihood that a system of dipolar squares will transition to a nematic state and to discover if the nematic state is the free energy minimum for the system. Finally, we apply this technique to explore the conditions that favor or disfavor the formation of a nematic state for systems of squares with offset dipoles.