Zhang Hongguang, Guo Zhenjiang, Zhang Xianren
State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China.
Soft Matter. 2020 Jun 21;16(23):5470-5477. doi: 10.1039/d0sm00116c. Epub 2020 Jun 2.
Numerous experiments have shown that bulk nanobubble suspensions are often characterized by a high magnitude of zeta potential. However, the underlying physical mechanism of how the bulk nanobubbles can stably exist has remained unclear so far. In this paper, based on theoretical analysis, we report a stability mechanism for charged bulk nanobubbles. The strong affinity of negative charges for the nanobubble interface causes charge enrichment, and the resulting electric field energy gives rise to a local minimum for the free energy cost of bubble formation, leading to thermodynamic metastability of the charged nanobubbles. The excess surface charges mechanically generate a size-dependent force, which balances the Laplace pressure and acts as a restoring force when a nanobubble is thermodynamically perturbed away from its equilibrium state. With this negative feedback mechanism, we discuss the nanobubble stability as a function of surface charge and gas supersaturation. We also compare our theoretical prediction with recent experimental observations, and a good agreement is found. This mechanism provides new fundamental insights into the origin of the unexplained stability of bulk nanobubbles.
大量实验表明，大量纳米气泡悬浮液的一个常见特征是具有较高的ζ电位。然而，到目前为止，大量纳米气泡能够稳定存在的潜在物理机制仍不清楚。在本文中，基于理论分析，我们报告了带电大量纳米气泡的一种稳定性机制。负电荷对纳米气泡界面的强亲和力导致电荷富集，由此产生的电场能量使得气泡形成的自由能成本出现局部最小值，从而导致带电纳米气泡的热力学亚稳性。过量的表面电荷机械地产生一种与尺寸相关的力，该力平衡拉普拉斯压力，并在纳米气泡受到热力学扰动而偏离其平衡状态时起到恢复力的作用。通过这种负反馈机制，我们讨论了纳米气泡稳定性与表面电荷和气体过饱和度的关系。我们还将理论预测与最近的实验观察结果进行了比较，发现两者吻合良好。该机制为大量纳米气泡难以解释的稳定性起源提供了新的基本见解。
