Adhesion Gel

Adhesion Gel

The purple "sticky" or "adhesion" gel was found on accident in the Portal 2 authoring tools. It turns out that the purple gel was initially going to behave like the blue stick paint in Tag: The Power of Paint, the game off of which Portal 2's gel mechanics are based. The adhesion gel would have been in the game along with the repulsion, propulsion, and conversion gels in Portal 2 now. But some play testers got too disoriented when they had to think with portals while walking on walls and ceilings, so Valve ended up removing the gel before it was completed. As this video demonstrates however, the incomplete adhesion gel still exists deep in the game's files. It doesn't work properly, but I'm sure it wouldn't be too difficult for someone to make it work.