EAGER: Molecular and hybrid simulations of nanobubble stability

1256838 PI：壳表面纳米气泡仍然是界面科学中最重要的难题之一。 这些小的、非常平坦的气泡在疏水表面上的水中形成，宽度为50-600 nm，高度为10-100 nm，并且包含最初溶解在液体中的气体。 对气泡寿命的经典预测表明，它们应该只持续几微秒，这主要是由于预测的内部（杨-拉普拉斯）压力很大。 然而，值得注意的是，观察到纳米气泡至少持续了几天，比预期长了9个数量级。 人们一直在努力理解这种明显的差异，最近实验成功地产生了纳米气泡几何形状和尺寸分布的详细特征，并描绘了气泡对变化条件（溶解气体浓度，温度，盐，pH值等）的响应。 尽管取得了这些令人印象深刻的成就，但仍然缺乏解释纳米气泡异常稳定性的明确理论。 该项目的主要目标是开发基础模拟方法和模型，可以解决目前实验无法解决的纳米气泡稳定性的基本分子方面。 这项工作是热力学和连续介质力学的交叉点，并有两个综合目标。 (1)第一个是量化的相关性的热力学贡献的稳定性，涉及到的物理和独特的性质的液体水在疏水界面。 先进的分子模拟方法将用于计算纳米气泡形成的自由能，特别关注溶解气体的影响。 这些计算将与大量宏观参数（例如，基于本体表面张力和溶解度）以评估它们是否在纳米级分解。 (2)第二个目的是了解一个动态的，主动传输机制可能是泡沫稳定性的基础。 最近提出的一种机制表明，可能存在溶解气体离开气泡顶点并在接触线附近返回的再循环流，AFM实验现在已经检测到纳米气泡上方的流体射流的特征，这强烈支持了这一观点。 这项工作的最终目标是开发一个详细的模拟图片，这种再循环气体传输机制与实验一致，严格评估其可行性作为稳定性的解释。 这部分研究将开发和适应混合（分子连续）模拟方法的纳米气泡的问题，可以跨越大范围的长度尺度预期这种机制。这项研究的目的是解决纳米气泡稳定性的重要问题，首次使用详细的模拟和最先进的分子和混合技术。 它将提供新的观点在水的疏水界面的基本相互作用。 反过来，这将影响对界面科学中长期存在的问题的理解，例如疏水边界处的流动滑移、疏水表面之间的吸引力相互作用和胶体凝聚。 最终，这些知识将影响一系列令人兴奋的新技术和新兴技术，这些技术依赖于纳米气泡从根本上改变固体表面特性的能力，包括防污和表面清洁技术，微流体设备中的运输，医疗治疗的提供，以及涉及生物或气液反应，表面诱导结晶和催化的化学过程。 这项研究同时旨在为多层次的学生提供出色的教育机会，包括来自代表性不足群体的学生的参与。 特别是，本科生将积极参与这项工作，在UCSB优秀的专业发展和研究培训计划的参与？美国加州纳米系统研究所和材料研究实验室（特别是RISE，西姆斯，GRIP和SABRE计划）。

1256838PI: ShellSurface nanobubbles remain among the most significant puzzles in interfacial science. These small, very flat bubbles form in water on hydrophobic surfaces, with widths 50-600 nm and heights 10-100 nm, and contain gases that were originally dissolved in the liquid. Classic predictions of bubble lifetimes suggest that they should last only for microseconds, owing largely to the significant predicted internal (Young-Laplace) pressure. Remarkably, however, nanobubbles are observed to persist for at least days, some nine orders of magnitude longer than expected. There have been intense efforts to understand this glaring discrepancy, and experiments have recently succeeded in producing detailed characterizations of nanobubble geometries and size distributions, and in delineating the response of the bubbles to changing conditions (dissolved gas concentration, temperature, salt, pH, etc.) Despite these impressive achievements, a definitive theory that explains the unusual stability of nanobubbles remains lacking. The main goal of this project is to develop foundational simulation methodologies and models that can address fundamental, molecular aspects of nanobubble stability where experiments currently cannot. This work lies at the intersection of thermodynamics and continuum mechanics, and has two integrated aims. (1) The first is to quantify the relevance of thermodynamic contributions to stability that relate to the physics and unique properties of liquid water at hydrophobic interfaces. Advanced molecular simulation methods will be used to compute free energies of nanobubble formation, with a particular focus on the impact of dissolved gases. These calculations will be compared with bulk macroscopic arguments (e.g., based on bulk surface tensions and solubilities) to assess if they break down at the nanoscale. (2) The second aim is to understand how a dynamic, active-transport mechanism may underlie bubble stability. A recently-proposed mechanism suggests that there may be a recirculating flow of dissolved gases leaving the bubble apex and returning to it in the vicinity of contact line, and AFM experiments have now detected signatures of a jet of fluid above a nanobubble that strongly supports this picture. The ultimate goal of this work is to develop a detailed simulation picture of this recirculating gas transport mechanism consistent with experiments, critically assessing its viability as an explanation for stability. This part of the study will develop and adapt hybrid (molecular-continuum) simulation methods to the nanobubble problem that can span the large range of length scales expected for this mechanism. This study aims to addresses the important issue of nanobubble stability for the first time using detailed simulations and state-of-the-art molecular and hybrid techniques. It will provide new perspectives on fundamental interactions in water at hydrophobic interfaces. In turn, this will impact the understanding of longstanding issues in interfacial science such as flow slip at hydrophobic boundaries, attractive interactions between hydrophobic surfaces, and colloidal coagulation. Ultimately such knowledge will impact a range of exciting new and emerging technologies that rely on the ability of nanobubbles to radically modify the properties of solid surfaces, including anti-fouling and surface cleaning techniques, transport in microfluidic devices, delivery of medical therapeutics, and chemical processes involving biological or gas-liquid reactions, surface-induced crystallization, and catalysis. This study simultaneously aims to provide outstanding educational opportunities for students at multiple levels, including involvement of those from underrepresented groups. In particular, undergraduates will be actively incorporated in this work by involvement in outstanding professional development and research training programs at UCSB?s California Nanosystems Institute and Materials Research Lab (notably the RISE, SIMS, GRIP, and SABRE programs).