  This study examined the effects of surfactants on the motion and positioning
of microparticles in an inhomogeneous electric field. The microparticles were
suspended in oil with a surfactant and the electric field was generated using
sawtooth-patterned electrodes. The microparticles were trapped, oscillated, or
attached to the electrodes. The proportion of microparticles in each state was
defined by the concentration of surfactant and the voltage applied to the
electrodes. Based on the trajectory of the microparticles in the electric
field, a newly developed physical model in which the surfactant was adsorbed on
the microparticles allowed the microparticles to be charged by contact with the
electrodes, with either positive or negative charges, while the non-adsorbed
surfactant micellizing in the oil contributed to charge relaxation. A
simulation based on this model showed that the charging and charge relaxation,
as modulated by the surfactant concentration, can explain the trajectories and
proportion of the trapped, oscillating, and attached microparticles. These
results will be useful for the development of novel self-assembly and transport
technologies and colloids sensitive to electricity.
