I have searched far & wide for a solution to this problem with no luck and it seems very simple (I'm using Blender 2.8).
I'm having two problems:
I can't get the particles to appear only on the face of the cup, they end up all floating in the center of it as well (bad for file size, and I actually need the cup to be transparent in my final render / different program). In order to fix this, I want to convert them all to mesh and delete all of the extra drops on the inside / floating in the cup.
I have obviously found the particle system > render > object workflow that is tried and true, so I can select a single object as particle and then convert to mesh BUT - I am using a COLLECTION of objects and want to maintain the variety of size/shape/etc. If you select an object in that menu it only uses one shape to create the array, unfortunately blender will not let you hit "convert" in the particle options menu when the source is a collection rather than a single object.
Created 7 raindrops with smooth shading and grouped them in a collection
Added particles using Hair, and emitting from "faces" (settings below)
I figured this out and it was quite simple - I did not have the correct piece of the layer (top layer) selected when attempting to convert the particles, I clicked the triangle shape symbol for the cup and it converted the collection for me.
I still cannot figure out how to apply the particles only to the exterior face of the cup, so they end up either floating in the volume, or on the interior face of the cup as well which is a problem for me.