Functional microparticles are of increasing interest for their potential applications in biological probes, microfluidic systems and microdisplay devices. Many have been prepared with various properties and structures, but it has remained difficult to control their behavior. Seung-Man Yang and colleagues from KAIST in Korea now report1 a method that allows the rotation and position of magnetoresponsive microparticles with nanoscopic surface structures to be controlled remotely.

Yang and his colleagues performed their manipulation experiment on ‘Janus’ particles — particles having at least two faces that are physically or chemically distinct — prepared from a colloidal suspension of a photocurable resin, ethoxylated trimethylpropane (ETPTA), iron oxide nanoparticles (hematite; <50 nm diameter) and silica particles (330 nm diameter). Under a magnetic field, the magnetic hematite particles became aligned parallel to the field and migrated to one side of the emulsion droplet, while the hydrophilic silica particles organized themselves at the ETPTA–water interface to form a two-dimensional arrangement. Photopolymerization of the ETPTA then gave the Janus particles, which had a net magnetic moment and arrays of silica ‘nanodomes’ on their surfaces.

The Janus particles could be aligned by applying a magnetic field, which caused the magnetic hemispheres of the particles to realign toward the field. By rotating the field, the researchers could control both the rotation and position of the particles on a planar substrate. The type of motion was determined by the relative position of the axis of magnet rotation: if the axis was in the plane of the substrate, the particles moved around by a rolling motion, whereas if the axis was perpendicular to the substrate, the particles rotated around the same axis in the opposite direction.

Fig. 1: Microscopy images showing rotation-induced translational motion under an external magnetic field. Images were taken at 0.16 s intervals (scale bar, 200 μm).

The particles were shown to respond to the magnetic field in less than a tenth of a second. The silica nanodome arrays on the surface provided further control over the particles’ movement by enhancing the coupling intensity between rotation and translation, owing to an increase in mechanical interference and solid–solid friction associated with the surface roughness of the microparticles. The team was able to guide the microparticles one-by-one through a microfluidic channel (Fig. 1), demonstrating the potential of magnetic Janus particles for application in a wide range of microparticle-based systems.