Use a Hard Round Brush to paint your desired fluid shape (e.g., drips, splashes, or blobs) on a new layer.
To mimic thickness and slight opacity, add an Inner Glow set to Overlay or Screen using a light gray or off-white color. Cum Photoshop Layer Style
Use this to create subtle internal shading. Set the blend mode to Linear Burn with low opacity (around 20–40%) to ground the liquid. Use a Hard Round Brush to paint your desired fluid shape (e