Gels



Gels are a gameplay mechanic introduced in Portal 2. They are paint-like liquids that impart different effects when applied to surfaces and objects.

Repulsion Gel
Repulsion Gel is a light blue substance that repels objects. A player or an object that hits a surface coated with the Gel bounces off. Due to the effect, the height from which a player falls before landing on a Gel-covered surface affects the height of a jump. Players can also use the Gel to move back and forth between two parallel surface rather than merely be propelled up and down. Objects such as Storage Cubes and Turrets can be covered in Repulsion Gel which causes them to bounce around erratically.

Propulsion Gel
Propulsion Gel is an orange substance that greatly increases object velocity and reduces the friction of objects moving on the surface(s) it is applied to. Objects covered in Propulsion Gel similarly have their friction reduced, causing them to slide around.

Conversion Gel
Conversion Gel is a portal-conducting white substance. It enables the player to place portals on surfaces that are not normally portal-conductive.

Unused 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.

Water
While not technically a gel, water washes off other gels from surfaces and objects. It has no special properties in regards to gameplay, and does not stick to surfaces.

Trivia

 * The gel mechanic is a borrowed concept from TAG: The Power of Paint.
 * A fourth gel, "Sticky Gel", was also borrowed from TAG and was tested within Portal 2. It would have allowed players to "stick to a surface so you can walk up a wall like Fred Astaire in Royal Wedding" ; however, it was removed early on during development due to making play-testers motion-sick. Its emitter is still in the game's files, however, but does not have any sticky properties or a texture.
 * Repulsion Gel was Aperture Science's first attempt at creating a dietetic pudding substitute.