When surface charges present at the wall of a channel of micro-scaled width, counterions are attracted and co-ions are repelled by electrostatic force. The counterions form a shielding area near the wall. This region penetrate into solution to a certain distance called Debye length until the electric potential decays to the bulk value of neutrality. The Debye length is ranging typically from 1 nm to 100 nm for aqueous solutions.
In nano-channels, the Debye length is usually comparable with the channel width, therefore solution within the channel is charged. Ions inside the fluid is no longer shielded from surface charge. Instead, surface charge affect the dynamics of ions within a nano-channel.