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Adhesion Gel was an unnamed and fifth type of Mobility Gel in Portal 2. While it existed in the game files, this gel was incomplete and went unused in the main game. It was eventually replaced by the Reflection Gel in the Peer Review update.
Based on the blue paint appearing in Tag: The Power of Paint, Adhesion Gel was to allow players to walk on walls or ceilings covered with the gel. However, while it added a new twist to the game, it did not add complexity to the puzzles and playtesters tended to get nauseous after using it as its effect was very disorientating, hence its removal from the game.
Even though it didn't appear in any official levels, Adhesion Gel still existed within the game files for a time. Its sign texture to be used in Test Chambers can still be found, and when implemented in-game, Adhesion Gel had the appearance of a purple gel that caused any object covered in Propulsion Gel to ignore its speed enhancing abilities, effectively canceling it out and making the object to come to an immediate stop. Any object coated in Adhesion Gel also did not slide around in a puddle of Propulsion Gel. This version of the gel, however, did not allow players to walk on walls and ceilings as originally designed. As the gel was not finished at the time of Portal 2's release, its blob texture was missing, thus giving it the purple/black checkerboard appearance unless it already painted a surface. This also meant any object coated in Adhesion Gel would not appear to be; it had the appearance of an unpainted object.
Its replacement in the Peer Review update, the similarly unused Reflection Gel, still exhibits all of the properties of Adhesion Gel.
 List of appearances
- Portal 2 (First appearance) (Game files only)