Surface tension effects on a free boundary

Dear all,

In COMSOL website, there is a model for "Square Droplet Oscillations under Surface Tension". In this model, instead of two phase flow modeling, ALE and single phase flow have been combined, and the effects of surface tension has been introduced in a weak form. the model has been developed in COMSOL3.5 in 2D axi-symmetric.

The surface tension effects are introduced over the free surfaces of the droplet for "u" and "v" components of the velocity in weak form as:

for "u" (r direction) => -(surface_tension)*(r*test(uTr)+test(u))
for "v" (z direction) => -(surface_tension)*r*test(vTz)

Now, I would like to do the same thing in COMSOL4.3, But the problem is that I have no idea how to define directional weak forms. When I add a boundary weak contribution it asks for just one expression regardless of any direction, and unfortunately I am not expert in weak form equations.

Could anyone give me any hint about this, it would be highly appreciated.

Regards,

Shawn