Ice nucleation properties of well characterized single particle-droplet pairs assessed using a microfluidic platform

水滴在零下40摄氏度之前不会自发冻结，除非有一个颗粒表面作为冻结过程的核心。冰核和由此导致的由颗粒和表面引起的水冻结在许多过程中都是重要的。它在器官保存以及药品、食品、生物分子、化妆品和纳米材料的制造和提纯中发挥着关键作用。冰核在云层冰化中也起着关键作用，即云层中的水滴转化为冰晶。冰川云是陆地上大部分降水的来源。导致云层冰化的大气冰核粒子使云层的结构和行为发生重大变化。云层的冻结极大地改变了云层的寿命，增加了它的沉淀能力。然而，我们对这种非均质成冰过程缺乏严格的了解。我们仍然不知道是什么特殊的性质使这些百万分之一的稀有大气粒子有效地成核了冰晶。我们也缺乏小型便携式仪器，可以通过从悬浮气溶胶粒子中采样来实时确定大气冰核粒子的浓度和冻结温度。这项研究通过开发一种新的微流控方法来连续捕获和测量冰核颗粒，从而解决了其中的许多缺点。单个颗粒被激活成液滴，然后在由软聚合物制造的设备中被捕获到流经微通道的油中。通过使微滴通过施加到微流控装置的温度梯度，当每个液滴在冻结时变得不透明时，光学地确定每个颗粒-液滴对的冻结温度。然后，我们将使用我们独特的设备探索使用经过工程设计的碳纳米管和金属氧化物纳米颗粒作为坚固的冰核粒子标准，以及定义良好且可重复的冻结温度。异质冰成核和相变的重要性将向公众和代表不足的K-12学生传达，特别是通过课外计划在当地几所公立学校开展动手教育活动。学生们将进行实验，在一个小的云室中诱导结冰。这些活动将被开发成独立的教育模块，将与更多的教育工作者共享。将开发一种新的方法，将微滴中个别大小选定的气雾化颗粒转化为微流控芯片中的连续油流。这将使许多领域的新的实验途径成为可能。首先使用云凝结核计数器将颗粒激活为液滴，然后将其捕获到连续的油流中，这将使用比例分析来调整气溶胶和油流率，以确保液滴的撞击和结合，避免反弹或液滴破碎。将使用微珀尔蒂埃元件沿微通道施加线性温度梯度，并使用薄膜热电偶阵列进行测量。温度测量和数值模拟将提供沿芯片的温度梯度场，使我们能够将观测到的液滴冻结位置与其临界冰核温度相关联。工程纳米颗粒的晶格和表面性质将被确定，并与我们使用微流控装置确定的冰核能力相关联。这将被用来检验我们的假设，即工程纳米颗粒将具有紧密而强劲的冻结温度谱。这些经过工程改造的纳米颗粒的统一特性应该会产生非常可重复性的冻结特性，创造出改进的冰核形成粒子标准，这是冰核形成测量所急需的。该项目汇集了气溶胶科学和技术、大气化学、传输现象和微流体方面的专家，进行真正的跨学科研究。由于这项研究的重点是促进对关键相变的基本理解，它将对科学和工程中的广泛领域产生直接影响，如材料合成和提纯、物理化学、地球物理科学和人为气候变化。这一奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

Water droplets do not freeze spontaneously until -40 degrees C, unless there is a particle surface to act as a nucleus for the freezing process. Ice nucleation and the resulting freezing of water induced by particles and surfaces is important in many processes. It plays a critical role in organ preservation and in the manufacturing and purification of pharmaceuticals, food, biomolecules, cosmetics, and nanomaterials. Ice nucleation also plays a key role in cloud glaciation, the transformation of water droplets in a cloud to ice crystals. Glaciated clouds are responsible for most precipitation over land. Atmospheric ice nucleating particles that cause clouds to glaciate create significant changes in the structure and behavior of the clouds. This cloud freezing dramatically changes the lifetime of the cloud and increases its ability to precipitate. However, we lack a rigorous understanding of this heterogeneous ice nucleation process. We still do not understand what special properties make these rare one-in-a-million atmospheric particles effective at nucleating ice crystals. We also lack small transportable instruments that can determine the concentration and freezing temperature of atmospheric ice nucleating particles in real-time by sampling from suspended aerosol particles. This research addresses many of these short-comings through the development of a new microfluidic approach to continuously capture and measure ice nucleating particles. Individual particles are activated into liquid droplets, which are then captured into an oil flowing through a microchannel in a device fabricated from a soft polymer. By sending the microdroplets through a temperature gradient applied to the microfluidic device, the freezing temperature of each particle-droplet pair is determined optically as each droplet turns opaque upon freezing. The use of engineered carbon nanotubes and metal oxide nanoparticles as robust ice nucleating particle standards with well-defined and reproducible freezing temperatures will then be explored using our unique device. The importance of heterogeneous ice nucleation and phase transitions will be communicated to the public and to underrepresented K-12 students in particular by conducting hands-on educational activities at several local public schools through after-school programs. The students will conduct experiments where they induce freezing in a small cloud chamber. These activities will be developed into self-contained educational modules that will be shared with a larger number of educators.A new approach will be developed for transferring individual size-selected aerosolized particles in microdroplets into a continuous oil flow in a microfluidic chip. This will enable new experimental avenues in numerous fields. The capture of particles - first activated into droplets using a cloud condensation nuclei counter - into the continuous oil flow will be optimized using scaling analysis to tune the aerosol and oil flow rates to ensure droplet impaction and coalescence that avoids bouncing or droplet shattering. A linear temperature gradient will be applied along the microchannel using micro-Peltier elements, and measured using an array of thin-film thermocouples. Temperature measurements along with numerical simulations will provide the temperature gradient field along the chip, allowing us to correlate the observed position of droplet freezing to its critical ice nucleation temperature. The lattice and surface properties of engineered nanoparticles will be determined and correlated to the ice nucleation ability we determine using the microfluidic device. This will be used to test our hypothesis that engineering nanoparticles will have tight and robust freezing temperature spectra. The uniform characteristics of these engineered nanoparticles should result in very reproducible freezing properties, creating improved ice nucleating particle standards that are greatly needed for ice nucleation measurements. This project brings together experts in aerosol science and technology, atmospheric chemistry, transport phenomena, and microfluidics, to perform truly interdisciplinary research. As this research focuses on advancing fundamental understanding of a key phase transition, it will have immediate impact on a wide range of fields in science and engineering, such as materials synthesis and purification, physical chemistry, the geophysical sciences, and anthropogenic climate change.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.