How to determine when a Particle hit/touch a boundary in Particle Tracing Module

I am trying to model particle trajectory using Particle Tracing for Fluid Flow module of COMSOL 4.4. I am using Dielectrophoresis and Electric force. When the particle touch an electrode, I would like to give some charge to it. But I don't know how to find the precise moment when the particle hit the solid electrode. I have tried particle position but its not accurate. I would like to know how do I determine the precise moment when a particle hit or touch a specific boundary? Could anyone please help me with this?