How bulk nanobubbles are stable over a wide range of temperatures.

Hypothesis: Bulk nanobubbles are nanoscopic gaseous domains in an aqueous solution. Their surprising long-term stability remains controversial due to the widespread assumption that spherical bubbles cannot achieve stable equilibrium. To uncover the intrinsic mechanisms underlying stabilization, the thermodynamic behavior of nanobubbles in water over a wide range of temperatures is explored.

Experiments: Bulk nanobubbles with a typical radius of 50 - 200 nm are generated using acoustic cavitation. Increasing temperature significantly narrows the bubble-size distribution and their mean radius shrinks to a minimum of approx. 50 nm at 45 °C. For higher temperatures a slight increase is observed. The thermal induced shrinkage is reversible: upon cooling they return to the original state.

Findings: The observation can be explained with a charge-stabilization mechanism. The intricate balance of competing interactions between water self-ionization and mobility of ions on the surface gives rise to this non-monotonic dependency. Nanobubbles consequently undergo charge loss at lower temperatures and charge conservation at higher temperatures, corresponding to their shrinkage and slight expansion. With theoretical calculations, we further quantity the equilibrium properties of nanobubbles and their zeta potential under various initial conditions. The temperature-sensitive nature of bulk nanobubbles offers a vital step forward exploring and industrializing their stability.