that between two parallel plates with $h=
15$ $\mu$m. Details of the electrical characteristics of the water used in the experiment were not reported, but the water was deionized; we will therefore assume that the ion concentration was small and will take it to be that of pure water $n_0\approx
10^{-6}$ mol l$^{-1}$ for which $\kappa^{-1}\approx 300$ nm, so that $\kappa h \approx 50$. Particles with radius $a=$150 nm were used in the P.I.V. system, so that $\kappa a \approx 1/2$, for which we will approximate $f(\kappa a)\approx 2/3$. If we assume $|\zeta_p|=10$ mV, we obtain that $\lambda^*$ is essentially zero. If however $|\zeta_p|=50$ mV, we get $\lambda^*\approx 1$ nm and $|\zeta_p|=200$ mV leads to $\lambda^*\approx 18$ nm. Although beyond molecular size, these values are much too small to explain the data reported in [@Meinhart] where $\lambda \approx
1$ $\mu$m. As a consequence, we can conclude that the effect reported here is probably not responsible for the large slip length observed in [@Meinhart]. Alternative mechanisms would have to be invoked to explain the data, such as the presence of surface attached bubbles [@LaugaStone].

Conclusion
==========

We have reported in this paper the following new mechanism. When small charged colloidal particles 