Here's a fun little bubble popping shader that works on particles. Good for mud, lava, or other bubble forming liquids.
The bubble popping part is all done with opacity clip and vertex offset - I've made a quick toon-lite version as an example, but you can create your own colour variations depending on the bubble material you're after (or let me know what you'd like and I'll make it up for you - tis but the work of a moment).
I've used a sub emitting particle system for the 'popping' version of the particles (controlled via Vertex Color alpha in 'Color over Lifetime') so it's easier to keep the bubbles around for however long you want before you pop them, but if you're using a consistent bubble lifetime you could just keep it all part of the same single particle system.