Chiral electrophoresis: a new steering wheel

Even rigid colloidal objects in conventional fluids in general respond chirally to external gradients such as electric fields. They twist around the gradient direction in a sense dictated by the object. Do real objects have enough chiral response to make a difference? Any chiral response requires either a chiral (non-inversion-symmetric) shape or a chiral charge distribution. But no shape can give chiral electrophoresis if it is uniformly charged. Nor can a sphere respond chirally even with the most chiral charge distribution. Here we demonstrate a new approach for calculating chiral responses of asymmetric objects. It represents the sheath of electrically-driven flow over the object as a superposition of stokeslet sources. Insights from this approach show how symmetric shapes can show strongly chiral response. We illustrate using a cube-shaped object. Chiral response is a handle by which a dispersion can be manipulated in powerful ways using time-varying electric fields.