How to define a variable for a range of values?

Hi,

I am trying to simulate a spontaneous fluid uptake process in a rock where capillary pressure is the driving force. I am using Coefficient Form PDE physics. In such a case, only damping coefficient and source term are non-zero. The source term (f) is -d(d(Pc,x),x). Here, I would like to define Pc as a variable under Global Definitions wherein for its expression I want to write 2*sigma/r.

However, Pc is different for different values of r. My question is how can I incorporate this in COMSOL? How do I define Pc as something that varies for a range of 'r' values. At different Pc values, subsequent variables are also calculated such as Se, krw, etc.

I would appreciate your help in setting up this problem in COMSOL.

Thanks