(678c) Behavior of Metallodielectric Janus Particles Under Direct Current Electric Fields: Electrokinetics and Assembly

Anisotropic colloids (e.g. ellipsoids, dumbbells, and dimers) have shown interesting, field-induced dynamics that can be harnessed in active systems and directed assembly. Janus particles (JPs), in particular, have been an important model system for active motion due to asymmetric flows caused by induced charge electroosmosis and/or electrohydrodynamics. To date, most studies have focused on the behavior of these particles in alternating current (AC) fields at high frequency. An alternative method is to apply direct current (DC) fields, which has the advantage of creating gradients in osmotic pressure that result in rapid concentration of colloids. In this work, we explore the behavior of metallodielectric JPs under DC electric fields. The fields are applied in cells that consist of two ITO slides separated by a dielectric spacer. The behavior of particles is strongly dependent on the magnitude of the electric field. When small positive potentials are applied at the bottom electrode, negatively charged JPs move downwards in agreement with electrophoretic theory. In contrast, when large potentials are applied, JPs rapidly move upwards, at several microns per second, until reaching levitation heights of tens of microns. In this regime, an upward force dominates over the electrophoretic force. The upward motion is observed regardless of field polarity, as long as the field strength is large. If the concentration of particles is large enough, collective dynamics emerge that result in particles forming levitated clusters. Such field-induced associations potentially offer new possibilities for directed assembly and active systems.