songhan.. by liquid.. do you mean the Liquify function..?
if you wanna create custom brush..
- create canvas
- new layer
- create drip effect
- turn off the background (so that it'll be transparent)
- Edit > Define Brush Preset > Name the brush and press OK
to create that drip effect..
- use the brush then open the brush options, play with the Shape Dynamics for Pressure.

- perfect the lines using the brush.
- draw out the drips like your example.
- save the brush with the steps above.
or wasn't that what you were looking for..?
just forgot the name. I remembered that i did it in channel or sumthing..crap..forgot it