Charging is typically not expected in nonpolar environments due to a high electrostatic barrier to charge dissociation. Nevertheless, charge effects are observed in such environments upon the addition of surfactants, which aggregate to form charge-stabilizing reverse micelles. Surfactants facilitate the charging and electrostatic stabilization of particles dispersed in nonpolar solvents. Suspensions of charged particles in nonpolar solvents are found in a variety of applications, such as electrophoretic displays, in which charged pigment particles are arranged with an external electric field to form an image. The ability to precisely control the locations and trajectories of the particles using an electric field is essential. However, the behavior of charged particles in a nonpolar solvent in response to an electric field is not fully understood. To investigate the behavior of charged particles in nonpolar solvents, we fabricate a novel microfluidic device that allows us to apply an electric field across a particle suspension and directly visualize the particles as they move across a channel. We image the particles, analyze the particle dynamics, and explore the relationship between the dynamics and the electrical properties of the suspension. We find that the presence of reverse micelles has a significant effect on particle motion. In a constant applied electric field, the particles initially move, but then unexpectedly slow down and stop. This behavior is due to screening of the applied field by the accumulation of charged reverse micelles at the channel walls. Consequently, the internal electric field within the channel decays exponentially. The decay time constant is dependent on the electrical conductivity of the suspension and the size of the channel. We model this behavior as an equivalent RC circuit. We also explore the behavior of charged particles in applied fields that are large enough to transport the particles completely across the channel. We find that the transport of particles is governed by a fingering instability. Furthermore, repeated switches of the direction of the field results in the localization of particles into a well-defined, periodic pattern. The wavelength of this pattern is dependent on the frequency of the applied field.Physic
