We show that DNA-linked anisotropic doublets composed of paramagnetic colloidal particles can be endowed with controlled propulsion when floating above a flat plate and subjected to a magnetic field precessing around an axis parallel to the plate. The propulsion mechanism for this artificial swimmer does not involve deformations, and it makes use of the minimal two degrees of freedom needed to propel it at low Reynolds numbers. We combine experimental observations with a theoretical analysis that fully characterizes the propulsion velocity in terms of the strength and frequency of the actuating magnetic field.

This perspective article will give an overview of recent experimental results related to the realization and use of anisotropic magnetic colloids, i.e. microscopic particles having the shape, composition or structured surface which set a preferred magnetization direction. The possibility of remotely controlling these soft matter building blocks via external fields makes them ideal for a wide range of applications, from their use as active microrheological probes to infer the viscoelastic properties of complex fluids to their implementation as active systems in microfluidic devices. This article will also review the recent use of these anisotropic units as field driven or guided propellers, which can be magnetically moved or guided in a fluid medium, thus being potentially useful in precise single-particle drug delivery operations.

To achieve permanent propulsion of micro-objects in confined fluids is an elusive but challenging goal that will foster future development of microfluidics and biotechnology. Recent attempts based on a wide variety of strategies are still far from being able to design simple, versatile, and fully controllable swimming engines on the microscale. Here we show that DNA-linked anisotropic colloidal rotors, composed of paramagnetic colloidal particles with different or similar size, achieve controlled propulsion when subjected to a magnetic field precessing around an axis parallel to the plane of motion. During cycling motion, stronger viscous friction at the bounding plate, as compared to fluid resistance in the bulk, creates an asymmetry in dissipation that rectifies rotation into a net translation of the suspended objects. The potentiality of the method, applicable to any externally rotated micro/nano-object, is finally demonstrated in a microfluidic platform by guiding the colloidal rotors through microscopic-size channels connected in a simple geometry.