(383e) New Modes of Active Particle Propulsion Powered By Temporally Asymmetric AC Fields

AC electrokinetics is one of the most convenient and efficient means of powering the motion of active or “self-propelling” microparticles that draw energy from the external field to perform directional motion. The simplest way to achieve such propulsion is to use particles with an asymmetric polarization pattern. Our group reported earlier how an AC-field induced charge electrophoretic effect emerges when the symmetry of the system is broken by having Janus metallodielectric particles (PRL 100, 058302, 2008). It has also been demonstrated that active motility can be imparted by having particles with asymmetric conductance (Nat. Mater. 6, 235, 2007) or by breaking the symmetry of the medium by using liquid crystals (Nature 467, 947, 2010). While these effects have been used in many active particle studies, we recently found that there is a third basic effect of symmetry breakdown and particle propulsion – driven by a time-asymmetric AC field. We will present results that reveal a new electrohydrodynamic effect in which spatially homogenous, temporally non-uniform AC signals drive field-collinear motion of symmetric particles. A systematic voltage sweep revealed an unusual power law dependence of particle velocity on field strength (U ∝ Eⁿ, 2 < n < 3). Reversing the signal asymmetry results in a reversal of particle direction without a change in velocity. Further, the particle velocity drops steeply with increasing frequency (U ∝ 1/ω) and increasing particle size. For a temporally nonuniform signal, we observe that the particle (homogeneous sphere) propulsion is size-dependent and propose that it is a result from the imbalance in counterion motion due to signal asymmetry. The experimental electrophoretic findings are supported by a theoretical model employing a sawtooth wave signal and considering a partial induced-charge screening effect on the powering electrodes. This newly reported temporally asymmetric AC-driven electromigration effect opens numerous possibilities for controlling particle propulsion, as it may be combined with induced-charge electrophoresis to drive active motion in two directions independently controlled by signal shape, amplitude, and frequency.