New Frontiers in Aerosol Particle Measurements

气溶胶颗粒在广泛的学科中发挥着关键作用，从它们对大气化学和物理学的影响，到它们在将药物输送到肺部中的用途及其对人类健康的影响，再到它们在输送燃烧燃料和在喷雾干燥中制造功能化微粒子中的用途。气溶胶是高度动态的，大小可变，成分不均匀。物理和化学过程可以跨越时间尺度，从粒子表面分子过程的纳秒，一直延伸到大气中的氧化老化，持续数小时。粒子的大小从纳米大小的成核簇延伸到毫米大小的雨滴。颗粒的尺寸通过挥发性和半挥发性气相组分的冷凝或蒸发或通过颗粒-颗粒相互作用和碰撞导致聚结而演变。气溶胶凝结和蒸发过程中的质热耦合已经得到了广泛的研究。然而，尽管聚结过程在密集的气溶胶羽流中很重要，但它的特征却少得多（例如。在燃料输送和喷雾干燥中），在大气中云滴对带电气溶胶颗粒的清除中，以及在使用干粉输送药物中固体颗粒的凝聚中。事实上，粒子间相互作用的机制和实际的聚结过程具有相当大的基本科学意义。实验将提供一个机会，以测试介电带电粒子的相互作用模型和理论支撑的形状的非粘性或粘性液滴开发的著名科学家，如主瑞利，兰姆和Chadrashekar。我们已经开发了一个多功能的平台，用于研究单个粒子或阵列的气溶胶光镊。利用光来捕获和操纵粒子，我们可以改变粒子的分离，使它们接近甚至合并。通过拉曼光谱，我们可以以纳米精度确定球形颗粒的尺寸，并可以以高精度（<0. 03%）确定它们的折射率。使用按需液滴发生器，粒子可以一次一个地加载到光阱中，并且可以被赋予选定的电荷量。可以控制捕获颗粒周围的气相组成、温度和压力。我们将利用该平台详细研究颗粒之间的相互作用和凝聚过程，具体而言，通过改变气溶胶的组成和气相相对湿度，我们将能够测量颗粒内水分子的扩散速率，并通过跟踪凝聚后颗粒形状松弛成球形来测量颗粒的粘度。事实上，在初步工作中，我们已经表明，我们可以测量粘度超过12个数量级的前所未有的范围。这些测量将提供宝贵的见解分子和宏观尺度之间的关系，探索材料的相行为，通过竞争的成核和结晶与缓慢的扩散动力学。我们也将研究相似电荷粒子之间的吸引力和排斥力的相互作用，并将测量结果与最近发展的理论进行比较。最后，接触冷冻等重要工序（被认为是重要的冰成核在大气中）将首次在一个受控的方式进行研究。总之，使用多功能的光学技术，我们将调查在一个基础水平和应用背景下，在气溶胶研究的一个新的前沿，在很大程度上是未开发的日期，为我们提供了一个研究基础物理化学和应用气溶胶科学问题的机会。

Aerosol particles play a critical role in a broad range of disciplines, extending from their impact on atmospheric chemistry and physics, to their use in the delivery of drugs to the lungs and their impact on human health, through to their use in the delivery of fuels for combustion and for fabricating functionalised micropaticles in spray-drying. Aerosols are highly dynamic, variable in size and heterogeneous in composition. Physical and chemical processes can span timescales from the nanosecond for molecular processes at the surface of a particle, extending through to oxidative aging in the atmosphere over a time period of many hours. Particle sizes extend from the nanometre size of nucleation clusters to the millimetre size of rain drops. The size of a particle evolves through the condensation or evaporation of volatile and semi-volatile gas phase components or through particle-particle interaction and collision leading to coalescence. The coupling of mass and heat transfer during condensation and evaporation in aerosol has been widely studied. However, the process of coalescence has been much less well characterised despite its importance in dense aerosol plumes (eg. in fuel delivery and spray drying), in the scavenging of charged aerosol particles by cloud droplets in the atmosphere, and in the agglomeration of solid particles in the delivery of drugs using dry powders. Indeed, the mechanisms of particle-particle interactions and the actual process of coalescence are of considerable fundamental scientific interest. Experiments will provide an opportunity to test models of the interactions of dielectric charged particles and the theory underpinning the shape of inviscid or viscous liquid droplets developed by eminent scientists such as Lord Rayleigh, Lamb and Chadrashekar. We have developed optical tweezers as a versatile platform for studying either individual particles or arrays of aerosol. Using light to capture and manipulate particles, we can change the separation of particles and bring them to close approach and even coalescence. From Raman spectroscopy, we can determine the size of spherical particles with nanometre accuracy and can determine their refractive index with high accuracy (<0.03 %). Particles can be loaded one at a time into an optical trap using droplet-on-demand generators and can be given a chosen amount of charge. The gas phase composition, temperature and pressure around the trapped particles can be controlled. We will use this platform to investigate in detail the interactions between particles and the coalescence process.More specifically, by varying the composition of the aerosol and the gas phase relative humidity, we will be able to both measure the rate of the molecular diffusion of water within a particle and the viscosity of a particle by following the relaxation in particle shape to a sphere after coalescence. Indeed, in preliminary work we have shown that we can measure the viscosity over an unprecedented range of more than 12 orders of magnitude. These measurements will provide invaluable insights into the relationship between the molecular and macroscopic scales, exploring the phase behaviour of materials through the competition of nucleation and crystallisation with the slow kinetics of diffusion. We will also investigate the interplay of attractive and repulsive forces between particles of like charge, comparing measurements with recently developed theories. Finally, important processes such as contact freezing (thought to be important for ice nucleation in the atmosphere) will be studied in a controlled way for the first time.In summary, using versatile optical techniques, we will investigate at a fundamental level and in applied contexts a new frontier in aerosol research that has largely been unexplored to date, providing us with an opportunity to study problems in fundamental physical chemistry and in applied aerosol science.