The transition from self-propelled particles to multifunctional motile microdevices requires the design of active particles with complex shape, structure, and programmed and switchable motility direction. Our group has investigated a broad range of principles for assembly and propulsion of responsive and active colloidal structures driven by electric and magnetic fields. Here, we will specifically discuss how AC electric fields can endow engineered asymmetric metallo-dielectric particles with directional propulsion energy and means of responding in complex ways to AC field parameters. The rich variety of mechanisms of motility in such systems include induced charge electrophoresis, electrohydrodynamic flows, reversed electrohydrodynamic flows and self-dielectrophoresis. The analysis of the origins, length-scales, and time-scales of these mechanisms and their combined use and control by the frequency of the AC field enabled the design of new classes of microspinners and particles with complex engineered shapes that move in steerable trajectories. These particles can change direction of motility, and can also be steered and reversibly assembled by the AC field. We will present examples of particles such as: (1) Supracolloidal spinners that display up to three frequency-dependent switching of direction of rotation; (2) Microswimmers that can move multi-directionally and can reversibly assemble in trains; (3) Asymmetric microcubes and shaped particles that controllably switch between individual multi-directional active motility and massively assembled states. Finally, we will discuss how the directional motility of these particles can be affected by the time-dependent frequency responses of the counterionic layers and how new nonlinear frequency-dependent effects can be used in very simple and highly tunable schemes to control the complex particle translation and rotation in 2D and 3D space.

AC field powered propulsion and frequency-controlled steering of engineered metallo-dielectric microparticles

ACS Spring 2022

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