I'd recommend using Meta-Balls and a particle system in this case.
Step 1: We need a few Objects in the Scene to get started.
- a Plane (our Emitter)
- a Metaball (our Particle Object, this will be duplicated)
- a Force Field (to get some turbulences going)
Step 2: We then add a particle system to our Plane, and set it to render an object, there we put in our meta-ball.
Step 3: We adjust some settings to our liking, i also turned off the gravity influence, then the particles will start rising. Mainly i'd change:
- random particle scale
- random velocity
We then get particles that emit at different speeds and sizes, once they are close to each other, they will merge, as metaballs do.
I've added my file for you, in order to get it looking nice i had to change the scale of the metaball object and particle, otherwise the divisions of them won't seem to work well and i got just some weird triangles floating around.
I'd recommend to take a look at the wiki regarding metaballs too (mainly resolution and threshold): https://docs.blender.org/manual/en/latest/modeling/metas/index.html?highlight=metaball