unit normal vector

Hello,

I am solving a PDE using coefficient mode. I have a scalar filed, variable C, which has arbitrary boundary inside the domain (picture 1) that evolves with time. I want COMSOL to find the unit normal vector for each point along the BOUNDARY at each time step (n=gradient(C) / abs (gradient(C))=(Cx ex+Cy ey)/sqrt(Cx^2+Cy^2)). But it does not give any reasonable answer (ny is shown in picture 2).

So can anyone help me to find unit normal vector to an arbitrary evolving inner boundary?

Thank you,
Mehrdad