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. Gel cannot be applied to transparent surfaces (such as glass), and will pass through grating (also without painting it). Most test elements also cannot be coated in gel, with the exception of Cubes and Turrets.

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. While not technically a gel, it does have some properties similar to 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. It has no emitter, is not used to solve chambers, and it cannot be "painted" onto surfaces like the gels introduced in Portal 2.

Adhesion Gel
Adhesion Gel is a cut gel which was originally meant to allow the player to walk on walls, similarly to the blue Stick paint in Tag: The Power of Paint, which Portal 2's gel mechanics are based on. The gel was made purple as a placeholder; it was cut before the developers decided on a final color, so it remains purple in the final game. The gel was cut because many play testers experienced motion sickness and disorientation, having to think with portals while walking on walls and ceilings. Most of the gel's effects were removed in the final game, except one: cubes coated in Propulsion Gel will not slide across Adhesion Gel-coated floors, and vice-versa.

The gel was replaced by Reflection Gel in the Peer Review update.

Reflection Gel
Reflection Gel replaced Adhesion Gel in the Peer Review DLC. It appears the same color as Conversion Gel when painted on surfaces, and the color of Propulsion Gel when splatted on the screen. The Thermal Discouragement Beam will reflect off of it like a mirror, similar to the Discouragement Redirection Cube. It also retains Adhesion Gel's effect of counteracting 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.