Shuriken Particle Collisions - Collider shape?

I'm trying to make acid particles that damage an enemy when either touches the other. The collision messages seem to be sending properly, however they only work when a particle is colliding with an enemy, not when the enemy collides with the particle. (i.e. the enemy only receives damage when the particle falls onto it, and not when the enemy moves into the stationary particle). I have tried calling OnParticleCollision() from the gameobject, from the particle system, and both simultaneously, but each condition yielded the same effect.

I'm beginning to think that it may have something to do with the shape of the collider on the particle system - I am using the Mesh (Cube) rendering method for the particles and it would be optimal if they each had their own little box collider, but that is obviously not the case with Shuriken. Can anyone shed some light on this/Is it possible to apply a custom physics-style collider to a particle system?