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In fabricating new colloid-based materials via bottom-up design, particle–particle interactions are engineered to encourage the formation of the desired assemblies. One way to do this is to apply an external field, which orients magnetically polarized particles in the field direction. External fields have the advantage that they can be programmed to change in time (e.g., field rotation or toggling), tunably shifting the system away from equilibrium. Here, we apply a model for ferromagnetic colloidal rods that simulates their phase behavior in the presence of an external magnetic field with constant strength and direction. An annealing process slowly reduces the temperature during molecular dynamics simulations to estimate the system’s equilibrium configuration in the ground state when the magnetic interactions between colloidal rods dominate the thermal forces. Numerous annealing simulations are performed at various particle densities and external field strengths. In the absence of an external field, the magnetic rods assemble into antiparallel configurations. When the strength of the external field is sufficiently strong, the magnetic rods are forced to orient in the direction of the field and therefore form head-to-tail structures. The formation of a head-to-tail state is associated with a net magnetic moment that results from the collective alignment of all magnetic particles in the field direction. Furthermore, when systems of magnetic rods assemble into a head-to-tail state, they occupy more space than they do in a phase in which most rods are assembled into antiparallel configurations. Phase diagrams predict that the magnetic properties of systems of rod-like magnetic particles can switch between magnetic and nonmagnetic states by tuning not only the external field strength but also the particle density. 

Microplastics (MPs) and nanoplastics (NPs) in water pose a global threat to human health and the environment. To develop efficient removal strategies, it is crucial to understand how these particles behave as they aggregate. However, our knowledge of the process of aggregate formation from primary particles of different sizes is limited. In this study, we analyzed the growth kinetics and structures of aggregates formed by polystyrene MPs in mono- and bidisperse systems using in situ microscopy and image analysis. Our findings show that the scaling behavior of aggregate growth remains unaffected by the primary particle size distribution, but it does delay the onset of rapid aggregation. We also performed a structural analysis that reveals the power law dependence of aggregate fractal dimension (df) in both mono- and bidisperse systems, with mean df consistent with diffusion-limited cluster aggregation (DLCA) aggregates. Our results also suggest that the df of aggregates is insensitive to the shape anisotropy. We simulated molecular forces driving aggregation of polystyrene NPs of different sizes under high ionic strength conditions. These conditions represent salt concentration in ocean water and wastewater, where the DLVO theory does not apply. Our simulation results show that the aggregation tendency of the NPs increases with the ionic strength. The increase in the aggregation is caused by the depletion of clusters of ions from the NPs surface.