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

水滴在-40摄氏度之前不会自发冻结，除非有粒子表面作为冻结过程的核。冰的成核和由颗粒和表面引起的水的最终冻结在许多过程中是重要的。 它在器官保存以及药物、食品、生物分子、化妆品和纳米材料的制造和纯化中发挥着关键作用。 冰成核在云的冰川作用中也起着关键作用，云中的水滴转化为冰晶。 冰川云是陆地上大部分降水的来源。 大气中的冰成核粒子会导致云的结构和行为发生重大变化。这种云的冻结极大地改变了云的寿命，并增加了其沉淀的能力。然而，我们缺乏严格的理解，这种异质冰成核过程。我们仍然不知道是什么特殊的属性使这些罕见的百万分之一的大气颗粒有效地成核冰晶。我们还缺乏小型便携式仪器，可以通过对悬浮气溶胶颗粒进行采样，实时确定大气冰核颗粒的浓度和冻结温度。这项研究通过开发一种新的微流体方法来连续捕获和测量冰成核颗粒，解决了许多这些短期问题。单个颗粒被激活成液滴，然后被捕获到流过由软聚合物制成的装置中的微通道的油中。通过将微滴发送通过施加到微流体装置的温度梯度，每个颗粒-液滴对的冷冻温度被光学地确定，因为每个液滴在冷冻时变得不透明。然后将使用我们独特的设备探索使用工程碳纳米管和金属氧化物纳米颗粒作为具有明确定义和可重现的冷冻温度的强大冰成核颗粒标准。非均质冰成核和相变的重要性将传达给公众，特别是代表性不足的K-12学生，通过课后计划在几所当地公立学校开展实践教育活动。学生们将在一个小云室中进行诱导冻结的实验。这些活动将被开发成独立的教育模块，将与更多的教育者共享。将开发一种新的方法，用于将微滴中的单个尺寸选择的雾化颗粒转移到微流体芯片中的连续油流中。这将在许多领域开辟新的实验途径。将使用缩放分析来优化将颗粒（首先使用云凝结核计数器激活成液滴）捕获到连续油流中，以调整气溶胶和油流率，以确保液滴撞击和聚结，从而避免反弹或液滴破碎。使用微珀耳帖元件沿微通道沿着施加线性温度梯度，并使用薄膜热电偶阵列进行测量。温度测量沿着与数值模拟将提供温度梯度场沿着的芯片，使我们能够相关的液滴冻结的观察到的位置，其临界冰成核温度。工程纳米颗粒的晶格和表面性质将被确定，并与我们使用微流体装置确定的冰成核能力相关联。这将用于测试我们的假设，即工程纳米粒子将具有紧密和稳健的冻结温度谱。这些工程化纳米颗粒的均匀特性应导致非常可再现的冷冻特性，从而产生冰成核测量非常需要的改进的冰成核颗粒标准。该项目汇集了气溶胶科学和技术、大气化学、传输现象和微流体学方面的专家，进行真正的跨学科研究。由于这项研究的重点是推进对关键相变的基本理解，它将对科学和工程领域的广泛领域产生直接影响，如材料合成和纯化，物理化学，地球物理科学，该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响进行评估，被认为值得支持审查标准。

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.