.
is independent of . So, for convenience, we evaluate it at the free surface.
If , there would be instability due to the forces.
is the ratio of interaction among the molecules of the liquid to that between the liquid molecules and the surface.
If , it would cause beading. But on the other hand, if , then the surface would be wettable.
In the present problem, and . So the dispersion relation can be written as
is independent of 
. So, for convenience, we evaluate it at the free surface. 
, there would be instability due to the 
forces.
is the ratio of interaction among the molecules of the liquid to that between the liquid molecules and the surface.
, it would cause beading. But on the other hand, if 
. So the dispersion relation can be written as
