Propulsion of Homonuclear Colloidal Chains Based on Orientation Control under Combined Electric and Magnetic Fields.

The remarkable efficiency and dynamics of micromachines in living organisms have inspired researchers to make artificial microrobots for targeted drug delivery, chemical sensing, cargo transport, and waste remediation applications. While several self- and directed-propulsion mechanisms have been discovered, the phoretic force has to be generated via either asymmetric surface functionalization or sophisticated geometric design of microrobots. As a result, many symmetric structures assembled from isotropic colloids are ruled out as viable microrobot possibilities. Here, we propose to utilize orientation control to actuate axially symmetric micro-objects with homogeneous surface properties, such as linear chains assembled from superparamagnetic microspheres. We demonstrate that the fore-and-aft symmetry of a horizontal chain can be broken by tilting it with an angle relative to the substrate under a two-dimensional magnetic field. A superimposed alternating current electric field propels the tilted chains. Our experiments and numerical simulation confirm that the electrohydrodynamic flow along the electrode is unbalanced surrounding the tilted chain, generating hydrodynamic stresses that both propel the chain and reorient it slightly toward the substrate. Our work takes advantage of external fields, where the magnetic field, as a driving wheel and brake, controls chain orientation and direction, while the electric field, as an engine, provides power for locomotion. Without the need to create complex-shaped micromotors with intricate building blocks, our work reveals a propulsion mechanism that breaks the symmetry of hydrodynamic flow by manipulating the orientation of a microscopic object.