  Ionic and molecular selectivity is considered unique for the nanoscale and
not realizable in microfluidics. This is due to the scale-matching problem -- a
difficulty to match the dimensions of ions and electrostatic potential
screening lengths with the micron-sized confinements. Here, we demonstrate a
microscale realization of the ionic transport processes closely resembling
those specific to ionic channels or in nanofluidic junctions, including
selectivity, guidance through complex geometries and flow focusing. As a model
system, we explore electrokinetic spherical Janus micromotors moving over
charged surfaces with a complex spatial charge distribution and without any
topographical wall. We discuss peculiarities of the long-range electrostatic
interaction on the behavior of the system including interface crossing and
reflection of positively charged particles from negatively charged interfaces.
These results are crucial for understanding the electrokinetic transport of
biochemical species under confinement, have the potential to increase the
precision of lab-on-chip-based assays, as well as broadening use cases and
control strategies of nano-/micromachinery.
