are used in a pressure-driven flow experiment to probe the profile of the velocity field of an electrolyte solution (e.g. P.I.V. in water), their velocities may include an “apparent slip” component even though the velocity field in the fluid does not violate the no-slip boundary condition. This apparent slip is in fact an electrophoretic velocity for the particles which are subject to the streaming potential, [*i.e.*]{}, the flow-induced potential difference that builds up along the channel due to the advection of free screening charges by the flow. A similar effect is expected to occur in shear-driven flows.

The expected maximum orders of magnitude for the apparent slip lengths were given under normal conditions in water. Although the effect was found to be too small to explain the data reported in [@Meinhart], its magnitude is consistent with other indirect investigations of fluid slip in pressure-driven flow experiments. As a consequence, the analysis presented here could be a useful tool for experimentalists by allowing them to estimate quantitatively the importance of this apparent slip in their experiments.

The idea that free passive particles could go faster than the surrounding flowing liquid, although counter-intuitive at first, is in fact not unnatural: a similar 