Nanobubbles (NBs) in liquid exhibit many intriguing properties such as low buoyancy and high mass transfer efficiency and reactivity as compared to large bulk bubbles. However, it remains elusive why or how bulk NBs are stabilized in water, and particularly, the states of internal pressures of NBs are difficult to measure due to the lack of proper methodologies or instruments. This study employed the injection of high-pressure gases through a hydrophobized ceramic membrane to produce different gaseous NBs (e.g., N₂, O₂, H₂, and CO₂) in water, which is different from cavitation bubbles with potential internal low pressure and noncondensed gases. The results indicate that increasing the injection gas pressure (60–80 psi) and solution temperatures (6–40 °C) both reduced bubble sizes from approximately 400 to 200 nm, which are validated by two independent models developed from the Young–Laplace equation and contact mechanics. Particularly, the colloidal force model can explain the effects of surface tension and surface charge repulsion on bubble sizes and internal pressures. The contact mechanics model incorporates the measurement of the tip–bubble interaction forces by atomic force microscopy to determine the internal pressures and the hardness of NBs (e.g., Young’s modulus). Both the colloidal force balance model and our contact mechanics model yielded consistent predictions of the internal pressures of various NBs (120–240 psi). The developed methods and model framework will be useful to unravel properties of NBs and support engineering applications of NBs (e.g., aeration or ozonation). Finally, the bulk NBs under sealed storage could be stable for around a week and progressively reduce in concentrations over the next 30–60 days.

中文翻译：

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探测水中纳米气泡的内部压力和长期稳定性

与大体积气泡相比，液体中的纳米气泡（NB）表现出许多有趣的特性，例如低浮力，高传质效率和反应性。但是，仍然难以确定为什么将大量的NB或如何在水中稳定，尤其是由于缺乏适当的方法或仪器而难以测量NB的内部压力状态。本研究采用通过疏水化陶瓷膜注入高压气体来产生不同的气态NB（例如N ₂，O ₂，H ₂和CO ₂）在水中，这不同于具有潜在内部低压和非冷凝气体的空化气泡。结果表明，增加注入气体压力（60–80 psi）和溶液温度（6–40°C）均可将气泡尺寸从约400 nm减小至200 nm，这由Young-Laplace方程开发的两个独立模型验证和联系技工。特别地，胶体力模型可以解释表面张力和表面电荷排斥对气泡尺寸和内部压力的影响。接触力学模型结合了原子力显微镜对尖端-气泡相互作用力的测量，以确定内部压力和NB的硬度（例如，杨氏模量）。胶体力平衡模型和我们的接触力学模型都对各种NB（120-240 psi）的内压产生了一致的预测。所开发的方法和模型框架将有助于阐明NB的特性并支持NB的工程应用（例如充气或臭氧化）。最后，密封保存的大块NB可能会稳定一周左右，并在接下来的30-60天内逐渐降低其浓度。