first thing you need to do is open your favorite editor and make a small room with nothing in it. Decide where you want the splash of particles and create a trigger_splash entity.

Now you need to give the trigger_splash a targetname. For this example lets give the entity a target_name of splash1

Now we must specify what kind of spark you want. There are 6 different kinds.

1) sparks
2) blue water
3) brown water
4) slime
5) lava
6) blood

to set this use the keyname sounds and set it to whichever you want from above. For this example we are going to set sounds=4 for slime.

The next step is to set the number of sparks you want. For this example we are going to make it 2. so set count=2 for the trigger_splash entity.

We arent done yet though. To get the sparks to actually appear and work right you must create a func_timer. This is what actually spawns the sparks. It doesnt matter where you make this entity so just make it anywhere on the level and set the target of this func_timer to the trigger_splash. So in this example we would set target=splash1. If you want the sparks to start on then be sure to set the spawnflag start_on. Below is my some shots from my example using multiple trigger_splashes to make the acid look like it is bubbling, etc. first 2 shots are from the editor(bsp), last one is from the game. You can download the map file [here].