Rheology of highly concentrated bulk nanobubbles in water

Bulk nanobubbles (BNBs), unlike macro bubbles, exhibit extraordinary longevity in water. While their super-stability is well-established, the rheological properties of BNB-water systems remain poorly understood. To address this, we employed molecular dynamics simulations to model nitrogen BNBs with a radius of ∼1.9 nm, systematically varying the volume fraction and evaluating the zero-shear viscosity. Our simulations reveal that BNBs exhibit rheological behaviour akin to that of microbubbles, but with a markedly stronger effect. Specifically, the zero-shear viscosity of BNB-water systems exceeds that of pure water and increases significantly with volume fraction-far surpassing the trends observed in microbubble systems. This pronounced increase resembles the behaviour of dilute charged colloidal dispersions, likely due to the high-density gas and surface charges within the nanobubbles. We derived a relative viscosity model grounded in classical theories to describe this behaviour. Under Couette flow, BNBs become unstable at high shear rates, coalescing into larger bubbles. Prior to coalescence, shear viscosity also rises with increasing volume fraction. To validate our findings, we conducted experiments using nanobubbles with a radius of ∼70 nm at a high concentration of 4.0 × 10¹³ bubbles per mL. The experimental data closely matched our simulations: at low shear rates, BNBs-water viscosity exceeds that of pure water, while at high shear rates, it drops to baseline levels-likely due to bubble coalescence and gas release. Our experimentally validated model predicts zero-shear viscosity within 8.93% of measured values, underscoring the complex, shear- and concentration-dependent rheology of BNB systems and suggesting current technologies may underestimate actual BNB concentrations.