The Electrohydrodynamic Magnus Effect: Coupling between Electrophoresis and Quincke Rotation Propels Colloids Orthogonal to a Driving Electric Field

Colloids dispersed in electrolytes polarize the surrounding ion cloud when exposed to an electric field. For sufficiently strong fields, an instability occurs that causes spherical colloids to break symmetry and spontaneously rotate about an axis orthogonal to the applied field, a phenomenon named Quincke rotation. If the colloids also have a net charge, the electrophoretic motion couples to Quincke rotation and propels particles in a direction orthogonal to both the driving field and the axis of rotation, an electrohydrodynamic analogue to the Magnus effect. The orthogonal Magnus velocity is of a comparable magnitude to the electrophoretic velocity. Typically, motion orthogonal to the field requires anisotropy in particle shape, dielectric properties, or geometry of boundaries. Here, the electrohydrodynamic Magnus effect occurs for bulk, isotropic spherical particles, with the Quincke rotation instability providing broken symmetry driving orthogonal motion. The direction of the Magnus velocity is not changed by flipping the sign of the field, so net orthogonal motion persists in AC electric fields. This orthogonal motion acts as a type of “self-propulsion”, and colloids with an electrohydrodynamic Magnus velocity can be used to create a new type of active matter.