(322f) Active Particle Propulsion Due to 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 their environment to perform directional motion. The simplest way to achieve propulsion is to use particles with an asymmetric polarization pattern. The original report by our group demonstrates that 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 lately, 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 AC electrohydrodynamic effect in which spatially homogenous, temporally non-uniform signals drive field-colinear motion of symmetric particles. A systematic voltage sweep revealed an exponential 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 speed. Further, the particle velocity drops steeply with increasing frequency (U ∝ 1/ω) and increasing particle size. For a temporally nonuniform signal, we propose that particle (homogeneous sphere) propulsion is size-dependent and results 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. The newly reported temporally asymmetric AC-driven electromigration effect opens various possibilities for controlling particle propulsion, as it may be combined with induced- charge electrophoresis to induce field-driven motion in two directions independently controlled by signal shape, amplitude, and frequency.