Magnetically Powered Active Colloids

Active colloids can be powered by external magnetic fields when they contain or are made of magnetic material. The general strategy to achieve propulsion is based on the application of uniform, time-dependent magnetic fields which induce periodic displacements or deformations, rectified into a net translation. This rectification can be produced by a close surface which breaks the spatial symmetry of the fluid flow, by the presence of flexibility in the particle structure, by friction anisotropy, or may result from cooperative interactions. In contrast to other types of actuation schemes, such as electrical or optical ones, magnetic fields neither affect the dispersing media, nor alter biological media. These features make active magnetic colloids appealing for their use in small microfluidic or biological networks, to transport or release cargos upon magnetic command. In this chapter, an overview of the main actuation mechanisms, physical properties, and interactions between active magnetic colloids will be given, with an eye on recent results in this growing research area.