Effect of a Cone Shape on the Motion of Active Janus Colloids
The propulsion of active colloids is governed by their symmetry and shape, yet a systematic investigation of how small geometric effects influence the locomotion of anisotropic active colloids is lacking. In this paper, we study the effect of a cone shape by combining high-resolution two-photon polymerization 3D printing with AC electric field experiments to study anisotropic Janus micro swimmers. We fabricate a series of Janus colloids with different shapes, ranging from a sphere to a hemisphere with a cone-protrusion, but containing similar hemispherical gold-coatings. Under an applied alternating current (AC) electric field, all particles exhibit active, ballistic motion. We find a shape and size dependence on the propulsion velocity, with the spherical Janus particle moving fastest, followed by a small cone and a large cone protrusion, while surprisingly a printed sphere with a flat side moves slowest. In addition, a reversal in the direction of motion for all shapes was triggered, a phenomenon governed by a transition from induced-charge electrophoresis (ICEP) at low frequencies to self-dielectrophoresis (sDEP) at high frequencies. We reveal a distinct shape influence, with the large cone-protrusion increasing their velocity in the sDEP regime. Our results provide insight into the link between active particle geometry and their propulsion velocity that are important for understanding biological micro swimmers and designing optimized microrobots.