State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China.
Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland.
J Colloid Interface Sci. 2017 Feb 1;487:123-129. doi: 10.1016/j.jcis.2016.10.027. Epub 2016 Oct 14.
Surface nanobubbles are stable gaseous phases in liquids that form on solid substrates. While their existence has been confirmed, there are many open questions related to their formation and dissolution processes along with their structures and properties, which are difficult to investigate experimentally. To address these issues, we carried out molecular dynamics simulations based on atomistic force fields for systems comprised of water, air (N and O), and a Highly Oriented Pyrolytic Graphite (HOPG) substrate. Our results provide insights into the formation/dissolution mechanisms of nanobubbles and estimates for their density, contact angle, and surface tension. We found that the formation of nanobubbles is driven by an initial nucleation process of air molecules and the subsequent coalescence of the formed air clusters. The clusters form favorably on the substrate, which provides an enhanced stability to the clusters. In contrast, nanobubbles formed in the bulk either move randomly to the substrate and spread or move to the water-air surface and pop immediately. Moreover, nanobubbles consist of a condensed gaseous phase with a surface tension smaller than that of an equivalent system under atmospheric conditions, and contact angles larger than those in the equivalent nanodroplet case. We anticipate that this study will provide useful insights into the physics of nanobubbles and will stimulate further research in the field by using all-atom simulations.
表面纳米气泡是在固体基底上形成的稳定的液相气相。尽管它们的存在已经得到证实，但仍有许多与它们的形成和溶解过程以及它们的结构和性质有关的问题尚未解决，这些问题难以通过实验进行研究。为了解决这些问题，我们基于原子力场对由水、空气（N 和 O）和高取向热解石墨（HOPG）基底组成的系统进行了分子动力学模拟。我们的结果提供了对纳米气泡形成/溶解机制的深入了解，并对其密度、接触角和表面张力进行了估计。我们发现，纳米气泡的形成是由空气分子的初始成核过程以及随后形成的空气簇的聚结驱动的。这些簇在基底上形成有利，从而为簇提供了更高的稳定性。相比之下，在本体中形成的纳米气泡要么随机移动到基底并扩散，要么移动到水-气表面并立即弹出。此外，纳米气泡由一个具有小于在大气条件下的等效系统的表面张力的凝聚气相组成，并且接触角大于等效纳米液滴的情况。我们预计，这项研究将为纳米气泡的物理特性提供有用的见解，并通过使用全原子模拟来激发该领域的进一步研究。
