Adhesion Gel

The Adhesion Gel or sticky gel, was a gel type scrapped from the final release of Portal 2 and can only be found in the Portal 2 authoring tools.

The sticky gel was initially meant to function the same manner as the blue stick paint in Tag: The Power of Paint, the game off of which Portal 2's gel mechanics are based on. According to the Portal 2 developer commentaries, play testers were disoriented when they had to think with portals while walking on walls and ceilings. Valve had ultimately scrapped the gel as a result.

Since textures of the adhesion gel still exists along with its indicator signage, various community map makers have attempted to incorporate the gel into their maps from scratch as the entity itself no longer exists.