(188g) Magnetic Janus Particles: Preparation, Self-Assembly, Simulation, and Application

We report the preparation of different shaped and sized magnetic Janus particles (MJPs) using electrohydrodynamic (EHD) co-jetting. Spherical MJPs with 20 µm sizes in diameter are directly prepared by the EHD co-jetting. The MJPs are composed of poly(lactic-co-glycolic acid) (PLGA) in both compartments, where one compartment has magnetite (28nm in diameter) and the other has not. The EHD co-jetting also produces bicompartmental PLGA fibers with much thicker diameters (up to 450 µm in a diameter), and subsequent cryosectioning of the fibers using a microtome fabricates cylinder-shaped MJPs. A newly developed method to change a particle shape from cylinder to sphere is introduced, allowing us to easily manufacture large-sized spherical MJPs.

This study also demonstrates a simple approach to harness the rotational freedom of the MJPs using a magnetic field gradient. Two-dimensionally ordered assemblies of a high concentration of the MJPs are also directed within a boundary system. Various assembled structures having from low to high particle density including hexagonally close-packed one are achieved by changing the field gradient strength. By altering the field direction from a perpendicular to a parallel orientation, the assembled MJPs orthogonally rotated to simultaneously reconfigure their assembled structures. All assemblies are reversible and applicable to the opposite orientation. Furthermore, a simulation developed in this study, which is well matched with the experimental results, reveals that the functions of the gradient field strength and the induced dipole interactions of the particles determine the degree of alignment and the particle packing density in the assembled structures.

Finally, the large-sized spherical MJPs, where the magnetite compartment has a coloring additive (carbon black) and the none-magnetic compartment contains a yellow-colored dye (Yellow 14), are utilized to demonstrate a color switchable device. The simple magnet operation assembles the MJPs to show a black color or yellow color at a time. This process induced by the permanent magnet is energy efficient and reproducible.