Teshima Hideaki, Nishiyama Takashi, Takahashi Koji
Department of Aeronautics and Astronautics, Kyushu University, Nishi-Ku, Motooka 744, Fukuoka 819-0395, Japan.
J Chem Phys. 2017 Jan 7;146(1):014708. doi: 10.1063/1.4973385.
Classical thermodynamics theory predicts that nanosized bubbles should disappear in a few hundred microseconds. The surprisingly long lifetime and stability of nanobubbles are therefore interesting research subjects. It has been proposed that the stability of nanobubbles arises through pinning of the three-phase contact line, which results from intrinsic nanoscale geometrical and chemical heterogeneities of the substrate. However, a definitive explanation of nanobubble stability is still lacking. In this work, we examined the stability mechanism by introducing a "pinning force." We investigated nanobubbles at a highly ordered pyrolytic graphite/pure water interface by peak force quantitative nano-mechanical mapping and estimated the pinning force and determined its maximum value. We then observed the shape of shrinking nanobubbles. Because the diameter of the shrinking nanobubbles was pinned, the height decreased and the contact angle increased. This phenomenon implies that the stability results from the pinning force, which flattens the bubble through the pinned three-phase contact line and prevents the Laplace pressure from increasing. The pinning force can also explain the metastability of coalesced nanobubbles, which have two semispherical parts that are joined to form a dumbbell-like shape. The pinning force of the semispherical parts was stronger than that of the joint region. This result demonstrates that the contact line of the semispherical parts is pinned strongly to keep the dumbbell-like shape. Furthermore, we proposed a nanobubble generation mechanism for the solvent-exchange method and explained why the pinning force of large nanobubbles was not initially at its maximum value, as it was for small nanobubbles.
经典热力学理论预测，纳米气泡应在几百微秒内消失。因此，纳米气泡令人惊讶的长寿命和稳定性是有趣的研究课题。有人提出，纳米气泡的稳定性源于三相接触线的钉扎，这是由基底固有的纳米级几何和化学不均匀性导致的。然而，对于纳米气泡稳定性仍缺乏明确的解释。在这项工作中，我们通过引入“钉扎力”来研究稳定性机制。我们通过峰值力定量纳米力学映射研究了高度有序热解石墨/纯水界面处的纳米气泡，估计了钉扎力并确定了其最大值。然后我们观察了收缩纳米气泡的形状。由于收缩纳米气泡的直径被固定，其高度降低而接触角增大。这种现象意味着稳定性源于钉扎力，该力通过固定的三相接触线使气泡变平，并防止拉普拉斯压力增加。钉扎力还可以解释合并纳米气泡的亚稳性，合并纳米气泡有两个半球形部分连接在一起形成哑铃状。半球形部分的钉扎力比连接区域的更强。这一结果表明，半球形部分的接触线被强烈钉扎以保持哑铃状。此外，我们提出了溶剂交换法的纳米气泡生成机制，并解释了为什么大纳米气泡的钉扎力最初不像小纳米气泡那样处于最大值。
