Engineering the phase behavior and morphology of colloids suspended in a magnetic nanoparticle dispersion




Sreenivasan, Adithya N.

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Determining and implementing strategies to control self-assembling colloidal systems is a problem of long-standing interest in soft matter physics. Recent experimental work has demonstrated that the assembly of colloids suspended in an aqueous magnetic nanoparticle dispersion can be precisely tuned by adjusting the strength of an externally applied magnetic field. Furthermore, this system is driven by simultaneous attractive and repulsive interactions, which implies the presence of a number of accessible metastable states with varying morphologies. To leverage this behavior, we develop a physically-motivated model for the pair interaction energies, with contributions from depletion attractions and magnetic repulsions. Molecular simulations are utilized to analyze the phase behavior of the system as well as the impact of the model parametrization on morphology. Our physical model is able to adequately replicate the experimental system's phase behavior. In doing so, we confirm that the size and shape of the clusters formed appear to be driven by the competitive interplay between attractive and repulsive interactions. This analysis is supplemented by the computation of theoretical phase boundaries, to which the simulated phases are compared. Structural data including the cluster size distribution, radius of gyration, and radial distribution function also provide signals as to which areas of the phase diagram contain the widest variety (sizes, shapes, or isotropy) of cluster phases. Our results demonstrate trends which can be used to work towards precise dynamic control of the system as well as for targeted design of systems with specified properties.


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