The physics of moving wetting lines

Scientists tend to think in terms of their most familiar models. It is not accidental that the earliest descriptions of the moving wetting line and its associated dynamic contact angle were in terms of displaced equilibria (chemists), friction (physicists) and viscous bending of the liquid-vapour interface (engineers and mathematicians). Each of these approaches has progressed since its inception, but, while each reflects a different facet of the underlying physical mechanism, and each offers at least a semi-empirical route to its description, none is complete. There is, as yet, no fully agreed treatment that is consistent with all three viewpoints and provides an effective basis for prediction -- though at least one new hydrodynamic approach has emerged that goes some way in this direction. This paper seeks to offer a status report: to briefly review each of the current approaches, to illustrate their successes and limitations as revealed by experiment and simulation, and to suggest ways in which the different aspects of wetting dynamics might be investigated in the future.