Open problems

Long lectures like these tend to be over-optimistic, giving the impression that most physical questions are under control. The reality is different: soft interfaces are far from a happy end. Let me give a few examples:

On the dynamics of wetting: the role of molecular processes is not fully appreciated.

1. (a) Following Blake's ideas, they may sometimes be dominant (at large dynamic contact angles). The difficulty is that they are critically dependent on the atomic structure of the surface.

2. (b) When the liquid induces a real chemical reaction on the supporting solid (e.g. a silanation on the OH groups of a silica surface), the exact nature of the driving force is subtle: what fraction of the reaction enthalpy is directly transformed into heat, and what fraction pulls the contact line?

3. (c) In the case of Aztec pyramids, for instance with one molecular layer spreading out from a thicker region, the description of this layer as a two-dimensional liquid is open to some doubt: we may, in some cases, be dealing with a two-dimensional gas rather than a two-dimensional liquid.

4. (d) The role of surface rugosity is important for these thin layers. It may be that the spreading molecules follow preferentially certain channels (or steps) on the surface: the percolation properties of the channel network may be essential.

5. […]