Velocity reinitialization in particle tracing module

Hi everyone,

I am trying to model a 2D particle particle collision using velocity reinitialization.
What I want is, the velocity should reinitialization into a set of specified x and y velocity components, whenever the r < 0.03. This is what I tried:

1. I have defined distance between particles using dest function. r= sqrt(qx-dest(qx)^2+(qy-dest(qy)^2+0.02)

2. Then I defined an analytic function as step1(-r) with argument r. (step function is already been defined under comp1 with location 0.03)

3. Then I put e = an1(r)

But I am getting an error message as "Internal error in expression evaluation.
- Undefined operator.: dest
- Feature: Time-Dependent Solver 1 (sol1/t1)"

When i put an arbitrary function of x and y instead of an1(r), it worked. It seems dest function can't be an entry for e. But I am not sure.
Pls help me....
Any other ways to do 2D collision is also greatly appreciated.

Thanks in advance :))))