Programmable rotation of levitating patchy microtiles via electric and acoustic fields

In this work, we designed polymeric microtiles with precise gold surface patches to investigate how combined electric and acoustic fields interact to drive particle motion. The particles were fabricated using maskless lithography, allowing control over both shape and patch placement. When placed in an acoustic standing wave at megahertz frequencies, the particles levitated to the nodal plane by acoustic radiation forces and exhibited steady rotation. The addition of an AC electric field at kilohertz frequencies introduced induced-charge electroosmotic (ICEO) flows at the metal patches, generating a second source of rotational torque. By varying the number and position of the gold patches, we can tailor how the torques from the electric and acoustic fields combine, allowing them to either reinforce each other and increase the rotation rate or oppose each other and reduce it. The rotation speed can also be tuned by varying the amplitude and frequency of the applied fields. Comparisons between symmetric and asymmetric particle geometries further reveal how shape and patch configuration govern the torque balance between ICEP and acoustic effects.