ALE+ PDE form

hi all.

I have a PDE coefficient form for evaporation of fluid in a fluids mixture. The governing equation has the general form of:

d(X)/dt=d{D(X,T)*d(X)/dz}/dz

in above X is the evaporating fluid volume fraction, T is temperature, z is the direction at which fluid leaves the mixture, and D(X,T) is diffusivity coefficient, and as can be seen, it depends on X and therefore the whole equation is strongly nonlinear.

As a result of this equation, I will get X, and therefore, I know the rate at which fluid leaves the mixture, this rate gives me a velocity which is the displacement rate of the mixture's free surface. To consider this latter, I am using ALE.

Now, the problem is that if I want to solve these aforesaid models individually, everything is fine. But, as I couple them in one study, then COMSOL freezes. I have noticed that the problem occurs because of diffusivity coefficient and its nonlinear nature. If I have a constant diffusivity, then I can run both modules in one study at the same time.

Is there anyway that I can play with solvers to circumvent this problem? any hint would be highly appreciated.

Thanks,

Shawn