Gels



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

Repulsion Gel
This gel is a light blue substance that provides repelling abilities. Any objects like the Storage Cubes and Turrets that hits a surface coated with the Repulsion Gel, will either bounce away from the gel splat or bounce erratically on its own upon being coated with it. Due to the effect, the height from which a test subject falls before landing on a gel-covered surface affects the height of a jump. Test subjects can also use the Gel to move back and forth between two parallel surfaces rather than merely be propelled up and down.



Propulsion Gel
The Propulsion Gel is a light orange substance that greatly increases object velocity and reduces the friction of objects moving on the surface(s) it is applied to. Any objects covered in this gel have their friction reduced, causing them to slide around across the floor easily. In other words it just makes anything that touches it, and runs on it, fast.



Conversion Gel
A portal-conducting white substance. It enables test subjects to fire portals onto surfaces previously incapable of conducting portals.



Cleansing Gel
This gel may appear to be regular water with an ability to wash away other types of gels off of surfaces and any objects that have been coated. It has no special properties in regard of the gameplay, and as such, does not affect any surfaces other than ones covered in other gels.

Slime
Slime or Sewage was found only in Portal during Chell's escape, and is not often considered a gel due to the fact it has no emitter, is not used to solve chambers, and Chell cannot "paint" with it like with the official gels introduced in Portal 2. Regardless, it does have some properties similar to some gels in Portal 2, such as the slippery effect of running on Propulsion Gel and an adhesion effect that keeps Chell somewhat stuck on the surface.

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.

Reflection Gel
The Reflection Gel replaced the Adhesion Gel in the authoring tools after Portal 2 major update of their first DLC. The gel uses the Conversion Gel texture when hits the floors, and causes the Thermal Discouragement Beam to reflect, similar to the Discouragement Redirection Cube. When this gel hits the player it shows the screen effect of the Propulsion Gel.

Trivia

 * The gel mechanic is a borrowed concept from TAG: The Power of Paint.


 * A fourth gel, "Adhesion 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 dietetic pudding substitute.