Binary Colloidal Structures Assembled through Ising Interactions
Assembling microscopic particles into macroscopic structures can open new pathways for producing complex materials that cannot be produced by lithographic methods. Although several methods for assembling colloidal particles exist, including controlled drying, ionic interactions, and dipolar interactions, a general pathway for producing a wider variety of structures remains a fundamental challenge. Here a versatile colloidal assembly system is demonstrated in which the design rules can be tuned to yield over 20 different pre-programmed structures, including kagome, honeycomb, square tiles, and various chain and ring configurations. The assembled structures are tuned by controlling the relative concentrations and interaction strengths between spherical magnetic and non-magnetic beads, which behave as paramagnetic or diamagnetic dipoles when immersed in a ferrofluid. A comparison of our experimental observations with potential energy calculations suggest that the lowest energy configuration within binary mixtures is determined entirely by two parameters, namely the relative dipole strengths and their relative concentrations. The enhanced flexibility enabled by tunable Ising interactions within colloidal particle systems makes this work relevant not only to the exploration of fundamental problems in materials science and soft matter, but also to the bottom-up fabrication of photonic, phononic, and magnetic devices.
