Formation of Two-Dimensional Magnetically Responsive Clusters Using Hematite Particles Self-Assembled via Particle-Induced Heating at an Interface.

Hematite particles, which exhibit a high magnetic moment, are used to apply large forces on physical and biological systems under magnetic fields to investigate various phenomena, such as those of rheology and micromanipulation. However, the magnetic confinement of these particles requires complicated field configurations. On the other hand, laser-assisted optical confinement of single hematite particles results in thermophoresis and subsequent ejection of the particle from the laser spot. Herein, we explore an alternative strategy to induce the self-assembly of hematite. In this strategy, with indirect influence from an optically confined and heated upconverting particle (UCP) at an air-water interface, there is the generation of convection currents that facilitate assembly. We also show that the assembly remains at the interface even after removal of the laser light. The hematite particle assemblies can then be moved using magnetic fields and employed to perform interfacial rheology.