Portal 2 'Design'

Work on Portal 2 began almost immediately after the release of Portal in The Orange Box. The intent to work on a sequel came from both the game's critical reception, as well as internally at Valve, as other teams within the company who were working on other games in The Orange Box finally had the opportunity to play Portal and expressed desires to help create its sequel. Internal ideas for the sequel were put together by a small team of about four using minimal art assets, and then passed to an "Overwatch" team, made up of previous team leaders and marketing executives, who would comment on the prototypes, allowing the developers to refocus their ideas. Once the core ideas were in place, full development with the larger team continued alongside the Left 4 Dead games; at times, some of Portal 2's developers would assist the Left 4 Dead team to meet milestones, while once Left 4 Dead 2 was completed, several of its team members joined to help complete Portal 2.

Initial ideas for Portal 2 considered dropping the portal aspect from the game; instead, the player would still participate in Aperture Science Laboratories testing a device centered around a new type of physics-based puzzle. For six months, Valve focused on a mechanic called "F-Stop"—the specifics not yet known outside of Valve as they have considered using the mechanic for a new game. The story was envisioned to be a prequel for the first game, set in the 1950s before GLaDOS's takeover of the Aperture Science facility, set in motion when Aperture CEO Cave Johnson becomes trapped within a computer. Eventually, Cave would have led an army of robots against the player to rise to power within Aperture. When Valve presented these ideas to others for comment, the lack of portals was criticized. Valve therefore returned to the portal mechanic and sought what other physics mechanics could be incorporated with portals into the larger game. The return to portals was also pushed forward by an unintentional leak regarding the F-Stop prototype. On June 10, 2008, based on information from a casting call website, Kotaku reported that Valve was seeking voice actors for the character role of Aperture Science CEO Cave Johnson, identifying the prequel aspects and naming Johnson as another AI. Valve attributed this leak to an "overeager agent", and shortly after Kotaku's report, Newell directed the team to reconsider their plans for Portal 2, including the lack of portals in the game.

Portal 2 introduces paint-like gels that affect the physical nature of surfaces they coat. Here, the blue Repulsion Gel, carried over the turret guns by the faint blue Excursion Funnel beam, causes the painted turrets to bounce off any other surface.

To complement portals, other game mechanics were introduced to Portal 2. One of these was a gel mechanic that can alter the physics of surfaces coated with that gel. Valve found that this addition gave players more control over the game world, but as a result, required the chamber designer to be more devious with their solutions to account for the various possibilities of the gel mechanics. The gel mechanic comes from Tag Team's Tag: The Power of Paint, a DigiPen student-developed game that won the 2009 Independent Games Festival Student Competition prize. Valve's vice president of marketing, Doug Lombardi, said that upon viewing the student game then, "the decision to combine their tech with Portal 2 came naturally". Wolpaw recalled that they had already considered the nature of surfaces in Portal in a binary fashion, whether or not it would allow for a portal to be created on it, and the ability to modify surface properties in the manner that Tag did was an obvious extension on that. Subsequently, Tag Team was hired by Valve, though initially their work was to "develop Tag in an interesting way", according to Wolpaw, only some time later being brought into the Portal 2 team.The Tag Team members were able to work out the interaction of the paint mechanics with portals, leading to new types of puzzle features in the game. Journalists compared this to the evolution of Narbacular Drop, another DigiPen student project, into the basis of Portal. While Tag features three paint types, only two of them were included with Portal 2; the third, one that allowed the player to walk on any surface coated with the paint, was excluded after playtesters found it induced motion sickness. Instead, new gel types were designed for inclusion in the game, according to Wolpaw, ultimately leading to the white Conversion Gel. The gels themselves are rendered using new fluid dynamics routines included in the engine to simulate their blob-like nature; the technology for this as well as the delivery of the blobs were from a small internal Valve project called the "Blobulator", discovered by the Tag team once they were working at Valve. Portal 2 also contains advanced rendering techniques for liquids that were developed from Left 4 Dead 2. Portal 2 combines the concepts of "flowing" surface maps to mimic the motion of water in a setting, along with "debris flow" maps and random noise to create realistic real-time rendering of water effects.