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Adhesion Gel can be "painted" onto surfaces like any other gel. Originally, the gel was supposed to allow the player to walk on the walls and ceiling of test chambers. However, this effect has been removed in the final game. Objects can be coated in Adhesion Gel, but there are no Adhesion Gel-coated textures for anything, so it will not render. The only remaining effect of Adhesion Gel is that objects coated in it will not slide along Propulsion Gel, and vice-versa. The gel was made purple as a placeholder; a final color was never decided on, so it remains purple in the final game.
According to the Portal 2 developer commentary and The Final Hours of Portal 2, play testers were too disoriented when they had to think with portals while walking on walls and ceilings. Valve had ultimately scrapped the gel as a result.
Adhesion Gel was later replaced by Reflection Gel in the Peer Review DLC, which uses the same color as Conversion Gel. The updated gel is still not used in the game, but it is functional to redirect lasers if placed using the Portal 2 Authoring Tools.
Reimplementation in community maps
Because Adhesion Gel still partially exists in the game along with a signage texture, several community map makers have attempted to reimplement the gel using standard entities and scripts. Most of these involve tricks with rotating the entire map around the player, while making sure physics objects like cubes still fall towards the actual floor. One example of such a map that has been publicly released is Prototype's Adhesion series. Several other mapmakers have also created their own Adhesion Gel implementations, though most of these have not been released or no longer function.
- The Adhesion Gel was based on the blue "stick" paint in Tag: The Power of Paint, the game which Portal 2's gel mechanics are based on.