calculation of surface curvature, surface tension

Hello every body,

I've a 2-D model in Comsol 4.2a where I use PDE and a moving mesh interface. I need to calculate the surface curvature on a boundary. I don't know how to do this accurately in Comsol. I've find out that you have access to the boundary normal which are the variable c.nx and c.ny for example for coefficient form pde.

But for the surface curvature I need to calculate div(n) so I need to have the derivatives. I read that I don't have access to those. So I try using divergence theorem and weak expressions. If I write divergence theorem in weak form on my boundary i have:

integral(test(phi)*div(n))= - integral(test(Grad(phi))*n)+[nx*test(phi)]+[ny*test(phi)]

I can evaluate the integral on the right side using weak contributions. What can I do to evaluate the expressions between [ ]?
In the documentation and in this forum I read several time that it's possible to calculate surface curvature using weak expressions but I don't succeed. Do someone know exactly what I should do to get the surface curvature on the boundaries? Thanks for your answers!

Karl