Magnetic Manipulation of Superparamagnetic Colloids in Droplet‐Based Optical Devices

Abstract Magnetically assembled superparamagnetic colloids are exploited as fluid mixers, swimmers, and delivery systems in several microscale applications. The encapsulation of such colloids in droplets may open new opportunities to build magnetically controlled displays and optical components. Here, the assembly of superparamagnetic colloids inside droplets under rotating magnetic fields is studied, and this phenomenon is exploited to create functional optical devices. Colloids are encapsulated in monodisperse droplets produced by microfluidics and magnetically assembled into dynamic 2D clusters. Using an optical microscope equipped with a magnetic control setup, the effect of the magnetic field strength and rotational frequency on the size, stability, and dynamics of 2D colloidal clusters inside droplets is investigated. The results show that cluster size and stability depend on the magnetic forces acting on the structure under the externally imposed field. By rotating the cluster in specific orientations, it is possible to magnetically control the effective refractive index and the transmission of light through the colloid‐laden droplets, thus demonstrating the potential of the encapsulated colloids in optical applications.