[quote]
particles which stick to each other on contact
[/quote]

For those particles which would stick to each other, what need to be modeled when they stick together?
It is a inelastic collision: momentum is conserved but energy is not conserved.

How about angular momentum? Will it rotate around their center of mass?
Or two particles become one bigger particles (with twice the mass and larger radius)?
What will happened when the third particle contact with those two particles?

I need a model in order to create simulation (which is suit to what you really want).
Please provide more detail information so that I can understand what you might want.

I have another question: suppose there are two kind of particles A and B.
Do you mean if A collide with A or B collide with B , they will stick together;
Particle will not stick with each other if A collide with B?