EAGER: Molecular and hybrid simulations of nanobubble stability

1256838PI：壳表面纳米气泡仍然是界面科学中最重要的难题之一。这些小而扁平的气泡在疏水表面的水中形成，宽度为50-600纳米，高度为10-100纳米，含有最初溶解在液体中的气体。气泡寿命的经典预测表明，它们应该只持续几微秒，这在很大程度上是由于预测的显著内部压力（Young-Laplace）。然而，值得注意的是，观察到纳米气泡至少持续了几天，比预期长了大约9个数量级。人们一直在努力理解这一明显的差异，最近的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不寻常稳定性的确定理论。该项目的主要目标是开发基础模拟方法和模型，以解决目前实验无法解决的纳米泡稳定性的基本分子方面的问题。这项工作是热力学和连续介质力学的交叉，有两个综合目标。(1)首先是量化与疏水界面处液态水的物理和独特性质相关的热力学对稳定性的贡献。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。这些计算将与宏观的宏观参数（例如，基于表面张力和溶解度）进行比较，以评估它们是否在纳米尺度上失效。(2)第二个目标是了解一个动态的主动传输机制如何成为气泡稳定性的基础。最近提出的一种机制表明，可能存在一种溶解气体的再循环流动，这些气体离开气泡顶端，并在接触线附近返回气泡，AFM实验现在已经检测到纳米气泡上方的流体射流的特征，这有力地支持了这一观点。这项工作的最终目标是开发与实验一致的这种再循环气体输送机制的详细模拟图像，批判性地评估其作为稳定性解释的可行性。这一部分的研究将开发和适应混合（分子连续体）模拟方法来解决纳米气泡问题，这些方法可以跨越该机制所期望的大范围长度尺度。本研究旨在通过详细的模拟和最先进的分子和混合技术首次解决纳米气泡稳定性的重要问题。这将为研究疏水界面中水的基本相互作用提供新的视角。反过来，这将影响对界面科学中长期存在的问题的理解，如疏水边界的流动滑移、疏水表面之间的吸引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，微流控装置中的运输，医学治疗的输送，以及涉及生物或气液反应的化学过程，表面诱导结晶和催化。同时，本研究旨在为不同层次的学生提供优秀的教育机会，包括那些代表性不足的群体的参与。特别是，本科生将通过参与UCSB优秀的专业发展和研究培训项目，积极参与这项工作。美国加州纳米系统研究所和材料研究实验室（尤其是RISE、SIMS、GRIP和SABRE项目）。

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).