Phase diagram and aggregation dynamics of a monolayer of paramagnetic colloids

TL;DR

This study investigates the phase behavior and aggregation dynamics of a monolayer of paramagnetic colloids under tunable magnetic fields, revealing complex interactions and assembly patterns that challenge existing models.

Contribution

The paper introduces a tunable colloidal system and simulation model to explore magnetic field-induced phase behavior, highlighting discrepancies between two-body predictions and experimental observations.

Findings

Attractive interactions at $ heta=90^{\u00b0}$ match equilibrium models.

SALR interactions predicted for $50^{\u00b0} extless heta extless 55^{\u00b0}$.

Experimental assemblies differ from two-body interaction predictions.

Abstract

We have developed a tunable colloidal system and a corresponding simulation model for studying the phase behavior of particles assembling under the influence of long-range magnetic interactions. A monolayer of paramagnetic particles is subjected to a spatially uniform magnetic field with a static perpendicular component and rapidly rotating in-plane component. The sign and strength of the interactions vary with the tilt angle $\theta$ of the rotating magnetic field. For a purely in-plane field, $\theta=90^{\circ}$, interactions are attractive and the experimental results agree well with both equilibrium and out-of-equilibrium predictions based on a two-body interaction model. For tilt angles $50^{\circ}\lesssim \theta\lesssim 55^{\circ}$, the two-body interaction gives a short-range attractive and long-range repulsive (SALR) interaction, which predicts the formation of equilibrium microphases. In experiments, however, a different type of assembly is observed. Inclusion of three-body (and higher-order) terms in the model does not resolve the discrepancy. We thus further characterize the anomalous behavior by measuring the time-dependent cluster size distribution.