New Mechanisms of Active Particle Propulsion Powered by Temporally Asymmetric AC Fields

AC electrokinetics offers convenient means of powering the motion of active or “self-propelling” microparticles that draw energy from the external field to perform precisely controlled motion. Our group reported earlier how an AC-field induced charge electrophoretic effect, ICEP, emerges with Janus metallodielectric particles (PRL 100:058302, 2008) or in particles with asymmetric conductance (Nat. Mater. 6:235, 2007). We recently found that there is a third basic effect of symmetry breakdown and particle propulsion. We will present a new electrohydrodynamic effect termed asymmetric field electrophoresis (AFEP) in which spatially homogenous, temporally non-uniform AC signals drive field-collinear motion of any types of particles. AFEP has unusual power law dependence of particle velocity on field strength. The experimental velocity data as a function of field strength, frequency, and signal asymmetry are supported by models of asymmetric ionic concentration-polarization. The direction of particle migration exhibits a size-dependent crossover in the low frequency domain, which can be harnessed for efficient on-chip sorting. The AFEP can be combined with other AC electrohydrodynamic effects, such as ICEP) enabling multiaxially controlled active particle systems.