This perspective reviews the historical explanations for specific ion effects, and explores the frontiers of the field before summarising its challenges and opportunities.

We report direct measurements of hydrodynamic drainage forces, which show clear evidence of boundary slip in a Newtonian liquid. The degree of boundary slip is found to be a function of the liquid viscosity and the shear rate, as characterized by the slip length, and is up to $\sim 20\mathrm{nm}$. This has implications for confined biological systems, the permeability of microporous media, and for the lubrication of nanomachines, and will be important in the microcontrol of liquid flow. We also show that current theories of slip do not adequately describe the experimental data.