Deformed geometry at each time-step, if a condition is observed

Hi,

I am trying to model the growth of an oxide scale with COMSOL4.4.
I have an element B diffusing in the system and consumed at a boundary x=Xi (the pb is in 1D).
I want to built a deformed geometry, making Xi move in function of the concentration of B.

For the moment I used the 'deformed geometry' module and had a 'prescribed mesh displacement' with a 'if' condition

dx = if(Nb<Nblim,Depl,0) [m]

the geometry moves from Xi to Xi+Depl, but only one time.

I want it to be verified at each step-time, but I don't find how to build that.

If anyone have some ideas, I would be happy to heard them!
Thanks,
Bastien