How to let a ParticleSystem only send collision messages, without performing the actual collision physics?

I want to build a weapon that shoots poisonous smoke. To do this I use Unity's particle system. In the particle system we can enable "collision". These are my settings;

• Collision Mode: World Collision, 3D

• Collides with: the layer all enemies and players are in

• Send Collision Messages: true

This allows me to use "OnParticleCollision" from other scripts.

The problem is that I don't want my smoke to bounce of objects: I want it to just pass through enemies, and activate the code I have in my OnParticleCollision function. The feature I want is sort-of like having a Collider component and setting "is trigger" to true.

Is this a missing feature in Unity? Did anyone else encounter this problem? What would be a good solution?

Thanks, Max