Integrated studies of nanodroplet freezing

化学系化学结构、动力学和机制（CSDM-A）项目将通过该奖项资助俄亥俄州立大学的Barbara Wyslouzil教授开展一项综合实验和分子建模项目，探索冷冻的物理特性。这包括确定液体可以过冷的极限，转变动力学和新出现的固相的结构。研究的重点是短链类分子，即碳少于12个的正构烷烃和正醇，它们被限制在纳米液滴中。这些链式分子是脂质、表面活性剂和聚合物等多种材料的组成部分。冻结是一种基本的物理过程，在工业和自然界中都起着核心作用，例如在大气气溶胶中。冷冻转变将在远离平衡状态的条件下用极好表征的样品进行定量研究，并使用一套功能强大的实验工具探测非常接近分子模拟理想化要求的条件。Wyslouzil教授还将与高中生一起进行研究，并与蒸汽工厂合作开展公共宣传。蒸汽工厂是她所在机构的一项倡议，旨在促进科学、技术、工程、艺术和数学的跨学科合作。Wyslouzil和她的研究小组将追求以下具体的实验目标：(1)测量纳米液滴冻结时液体到固体的成核速率，(2)探索分子结构对表面和体积冻结的相对重要性的作用，以及(3)进一步开发x射线散射技术来直接跟踪这种转变。分子模拟工作的目标是：(1)了解链状分子表面冻结的驱动力，(2)计算液-固成核速率，直接与测量值进行比较。超声速喷嘴流动是产生高度非平衡状态的液滴，用静压测量来表征流动，用空间分辨小广角x射线散射（SAXS/WAXS）来表征气溶胶，用红外光谱来量化可冷凝部分在汽、液、固三态之间的分布。开发单纳米液滴/纳米晶体x射线衍射测量方法，利用自由电子激光在粒子的基础上测量冻结团簇的结构的努力将得到扩展。分子动力学（MD）模拟是为了直接测试表面冻结的竞争理论，并利用正醇和正烷烃在统一原子或全原子水平上的可用相互作用势，在分子水平上深入了解均相液固成核。

With this award,the Chemical Structure, Dynamics and Mechanisms (CSDM-A) Program of the Division of Chemistry is funding Professor Barbara Wyslouzil at Ohio State University to pursue an integrated experimental and molecular modeling program exploring the physics of freezing. This includes determining the limits to which liquids can be supercooled, the transformation kinetics, and the structure of the emerging solid phase. The focus is on short chain-like molecules, i.e. n-alkanes and n-alcohols with less than 12 carbons, confined in nanodroplets. These chain molecules are the building blocks of materials as diverse as lipids, surfactants, and polymers. Freezing is a fundamental physical process that plays a central role in both industry and nature, such as in atmospheric aerosols. The freezing transitions will be quantitatively investigated with extremely well characterized samples under conditions that are far from equilibrium, and a powerful set of experimental tools probes conditions very close to the still idealized requirements of molecular simulations. Prof. Wyslouzil will also engage with high school students in her research and conduct public outreach in collaboration with the Steam Factory, an initiative at her institution to promote interdisciplinary collaboration across Science, Technology, Engineering, Arts, and Math.Prof. Wyslouzil and her research group will pursue the following specific experimental objectives: (1) measure liquid to solid nucleation rates as nanodroplets freeze, (2) explore the role of molecular structure on the relative importance of surface versus bulk freezing, and (3) further develop X-ray scattering techniques to follow this transformation directly. The objectives of the molecular modeling work are to (1) understand the driving force for surface freezing in chain-like molecules, and (2) calculate liquid-solid nucleation rates to compare directly to the measured values. Supersonic nozzle flows are to generate liquid droplets in highly non-equilibrium states with static pressure measurements to characterize the flow, spatially resolved small and wide angle X-ray scattering (SAXS/WAXS) to characterize the aerosol, and infrared spectroscopy to quantify the distribution of the condensable fraction among the vapor, liquid and solid states. Efforts to develop single nanodroplet/nanocrystal X-ray diffraction measurements to measure the structure of freezing clusters on a particle-by-particle basis using free electron lasers are to be extended. Molecular dynamics (MD) simulations are to directly test competing theories of surface freezing, and develop molecular level insight into homogeneous liquid-solid nucleation using available interaction potentials for both n-alcohols and n-alkanes at the united atom or all-atom level.