Surfaces

Each level within the Portal series consists of surfaces. Typically, a portal can be placed upon any light-colored flat surface, whereas an uneven or dark surface cannot be used in this way. Similarly, the dark metal panels present in Portal 's Test Chambers are present for the express purpose of requiring the player to consider other techniques in order to solve the puzzle(s). In this way, the placement and location of portal-conductive surfaces is a fundamental design choice, and determines the way in which the puzzle(s) need to be solved.

In some Test Chambers, surfaces are blocked by doors or other obstacles, or must be activated before a portal can be placed upon them. In Portal 2, the only way to overcome some puzzles that lack portal-conductive surfaces is through the use of Gels.

Gels
Introduced in Portal 2, Gels can be applied on any surface in order to modify its attributes.


 * Repulsion Gel is a blue gel that bounces a player or object away from the surface.


 * Propulsion Gel is an orange gel that accelerates objects touching it, allowing players to run forward at high speeds.


 * Conversion Gel is a white gel that allows portals to be placed on surfaces that portals normally cannot be placed.