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的影响，到它们在向肺部输送药物及其对人类健康的影响方面的使用，再到它们在燃烧燃料输送和喷雾干燥中制造功能化微颗粒方面的使用。气溶胶是高度动态的，大小可变，组成不均匀。物理和化学过程可以跨越时间尺度，从粒子表面的分子过程的纳秒，延伸到大气中长达数小时的氧化老化。粒子的大小从成核团的纳米尺寸延伸到雨滴的毫米尺寸。颗粒的大小通过挥发性和半挥发性气相成分的冷凝或蒸发或通过颗粒与颗粒的相互作用和碰撞而演变，从而导致合并。气溶胶中凝结和蒸发过程中的传质和传热耦合问题已经得到了广泛的研究。然而，尽管聚结过程在稠密的气溶胶羽流中很重要，但它的特征却不那么好。在燃料输送和喷雾干燥中)、大气中云滴对带电气溶胶颗粒的清除以及使用干粉输送药物时固体颗粒的凝聚。事实上，粒子-粒子相互作用的机制和实际的结合过程具有相当大的基础性科学兴趣。实验将提供一个机会来测试介电带电粒子相互作用的模型，以及由著名科学家如Rayleigh勋爵、Lamb和Chadrashekar发展的支撑无粘或粘性液滴形状的理论。我们已经开发了光学镊子，作为研究单个颗粒或气溶胶阵列的通用平台。利用光来捕捉和操纵粒子，我们可以改变粒子的分离，使它们接近甚至合并。从拉曼光谱中，我们可以以纳米级的精度确定球形颗粒的大小，并可以高精度地测定它们的折射率(&lt；0.03%)。粒子可以使用按需液滴生成器一次一个地加载到光学陷阱中，并可以获得选定的电荷量。捕获颗粒周围的气相组成、温度和压力是可以控制的。我们将利用这一平台详细研究粒子之间的相互作用和合并过程。更具体地说，通过改变气溶胶的组成和气相相对湿度，我们将能够测量粒子内水的分子扩散速度和粒子的粘度，通过跟踪粒子形状的松弛到合并后的球体。事实上，在初步工作中，我们已经表明，我们可以在超过12个数量级的史无前例的范围内测量粘度。这些测量将为分子和宏观尺度之间的关系提供宝贵的见解，通过成核和结晶与缓慢的扩散动力学竞争来探索材料的相行为。我们还将研究类似电荷粒子之间的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，接触冻结(被认为对大气中的冰成核很重要)等重要过程将首次以受控方式进行研究。总而言之，我们将利用多种光学技术，在基础水平和应用背景下调查气溶胶研究的一个新前沿，这在很大程度上是迄今为止尚未探索的，这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.