When you hear "viscosity," you probably picture how thick a liquid feels as you pour it, that's shear viscosity. But when a wand pulls a film out to form a giant bubble, the liquid isn't flowing, it's stretching, like a sheet of plastic wrap thinning out as you pull it taut. That resistance to being stretched, not to flowing, is extensional viscosity. A team of physicists at Emory University (Frazier, Jiang, and Burton, 2020) used an extensional rheometer to show that this parameter, not shear viscosity, which barely differs between a great mix and a mediocre one, is what actually predicts whether a film can survive stretching without popping.
Why polymer drives it up, not soap
High extensional viscosity comes almost entirely from long polymer chains dissolved in the water: guar gum, J-Lube, or PEO. As the film stretches, those chains tangle with one another and resist, acting like an elastic net; soap alone barely contributes to this at all. The Emory study also found that blending polymers of different molecular weights (polydispersity) outperforms a single-size polymer at the same concentration, because the short and long chains complement each other as they tangle.
In KUANTIKA's engine, this effect shows up as the weight polymer contributes to the "giant bubbles" score: having enough soap isn't sufficient on its own (that's just an entry condition), the mix also needs that "elastic memory" that only well-hydrated long chains can provide.