For analyzing whether this small level of contaminants is sufficient to stabilize the bulk nanobubbles, we take the radius of the nanobubble as 50nm and assume that the cross-sectional area a of the surfactant molecule is about 9 Å 2 . For a single nanobubble of 100 nm, the number of surfactants 𝑛 required for the compressed monolayer can be estimated as 𝑛 = ≈ 3.49 × 10 . The total number of surfactants needed for all bulk nanobubbles in 1mL solution is about 𝑁 = 𝑛 𝑐 ≈ 3.49 × 10 .