  Nonlinear field dependence of electrophoresis in high fields has been
investigated theoretically, yet experimental studies have failed to reach
consensus on the effect. In this work, we present a systematic study on the
nonlinear electrophoresis of highly charged submicron particles in applied
electric fields of up to several kV/cm. First, the particles are characterized
in the low-field regime at different salt concentrations and the surface charge
density is estimated. Subsequently, we use microfluidic channels and video
tracking to systematically characterize the nonlinear response over a range of
field strengths. Using velocity measurements on the single particle level, we
prove that nonlinear effects are present at electric fields and surface charge
densities that are accessible in practical conditions. Finally, we show that
nonlinear behavior leads to unexpected particle trapping in channels.
