Evaporative Drying of Droplets and the Formation of Micro-structured and Functional Particles and Films

“看着油漆干”是一个比喻无聊和无意义的活动。在现实中，液体的干燥是一个复杂的过程，眼睛不受干扰的外观可以隐藏液体内部发生的丰富动态。这些内部过程的作用是改变干燥后留下的沉积物中物料的分布。我们都很熟悉咖啡环效应，即裂开的咖啡干燥后在溢出物的边缘形成一个固体环——如果你想要均匀地涂在表面上，这个效应就没什么用了。这个项目是关于液滴的干燥，无论是在空气中还是在表面上；一个孤立的液滴，两个液滴合并或许多液滴在一个喷雾中。我们试图了解液滴是如何干燥的，以及如何控制液滴中的颗粒或分子在液滴蒸发后的最终位置。什么时候会得到固体粒子或空心粒子？圆的还是尖的？是均匀粒子还是带壳粒子？或者在一个表面上：一个咖啡圈或一个煎饼？是均匀的，分层的还是靶心的？颗粒是结晶的还是非晶的？不同的成分是混合的还是分离的？有无数的可能性来控制最终颗粒或薄膜的微观结构和性能。干燥之所以复杂，主要有三个原因。首先，许多输运过程（蒸发、热流、扩散、对流）同时发生，并且是强耦合的。例如，在表面上蒸发的一小滴酒精和水中，滴内的液体每秒会以甜甜圈的形式流动数十次。其次，干燥液滴中的条件往往远离平衡。例如，空气或光滑干净表面上的小水滴可以冷却到-35摄氏度而不会结冰。所以为了理解干燥，我们需要了解远离平衡状态的流体的性质。从接近平衡的简单溶液的性质来预测干燥的最终结果通常是不可能的。第三，滴液不会孤立干燥。它们可能合并或反弹，合并或在一个表面上相互追逐。一个液滴的蒸发影响到它的邻近液滴。移动的液滴会改变其他液滴周围的空气流动，从而耦合液滴的运动。除了对液滴干燥的奇异结果的智力迷恋之外，为什么还有人关心这个？液滴干燥在实际应用中是一个相当重要的过程：喷漆、图形印刷、喷墨制造、作物喷洒、种子或片剂涂层、喷雾冷却、喷雾干燥（广泛用于食品、制药和个人护理产品）、药物吸入器和消毒，仅举几个例子。所有这些应用背后的物理和化学原理都是相同的，但它们以不同的方式表现出来，期望的结果也因应用而异。这个项目解决的第一个挑战是测量：你如何计算出一个直径不到十分之一毫米、可能在一秒钟内变干的液滴中发生了什么？我们已经开发了复杂的测量工具，但需要进一步扩展这些工具。另一个挑战是建模：为了理解干燥过程，我们需要一个理论框架和计算机模型来解释和预测实验观察结果。我们将开始研究发生在空气中的单个液滴和表面上的基本过程，然后探索当液滴相互作用或合并时会发生什么。这种基本的理解将被用于在大多数实际应用（如喷涂、喷雾干燥、吸入器或喷墨制造）中遇到的阵列、云或液滴喷雾的改进模型。我们将利用一个行业俱乐部与来自不同行业的公司进行接触。该俱乐部将提供一个论坛，分享问题、想法和解决办法，并传播项目中产生的知识。

'Watching paint dry' is a metaphor for a boring and pointless activity. In reality, the drying of liquids is a complex process and the imperturbable appearance to the eye can hide a wealth of dynamics occurring inside the liquid. The effect of these internal processes is to change the distribution of materials in the deposit left after drying. We are all familiar with the coffee-ring effect, where split coffee dries to form a ring of solids at the edge of the spill - of little use if you are trying to coat a surface uniformly. This project is all about the drying of droplets, either in air or on a surface; one isolated droplet, two droplets merging or many droplets in a spray. We seek to understand how drops dry and how to control where the particles or molecules in the drop end up after the drop evaporates. When do you get a solid particle or a hollow particle? A round one or a spiky one? A uniform particle or one with shells? Or on a surface: a coffee-ring or a pancake? A uniform deposit, a layered one or a bull's eye? Are particles crystalline or amorphous, are different components mixed or separated? There are a myriad of possibilities for controlling the microstructure and properties of the final particle or film. Drying is complicated for three main reasons. First, many transport processes (evaporation, heat flow, diffusion, convection) occur simultaneously and are strongly coupled. For example, in a small droplet of alcohol and water evaporating on a surface, the liquid inside the drop will flow around in a doughnut pattern tens of times each second. Second, the conditions in a drying droplet are often far from equilibrium. For example, a small water droplet in air or on a smooth clean surface can be cooled to -35 degrees C without freezing. So to understand drying one needs to understand the properties of fluids far from equilibrium. It is generally not possible to predict the final outcome of drying from the properties of simple solutions near equilibrium. Third, drops do not dry in isolation. They may merge or bounce, coalesce or chase each other across a surface. The evaporation of one droplet affects its neighbours. Moving droplets change the flow of air around other droplets, coupling the motion of droplets.Why does anyone care, beyond the intellectual fascination with the bizarre outcomes of droplet drying? Drying of droplets turns out to be a rather important process in practical applications: spray painting, graphics printing, inkjet manufacturing, crop spraying, coating of seeds or tablets, spray cooling, spray drying (widely used in food, pharmaceutical and personal care products), drug inhalers and disinfection, to give a few examples. The physics and chemistry underlying all these applications is the same, but if manifests itself in different ways and the desired outcome varies between applications.The first challenge addressed by this project is one of measurement: how do you work out what is going on in a droplet that is less than a tenth of a millimetre across and may dry in less than a second? We have already developed sophisticated measurement tools but will need to extend these further. Another challenge is one of modelling: to understand the drying process we need a theoretical framework and computer models to explain - and predict - experimental observations. We will begin looking at the fundamental processes occurring in single drops in air and on a surface and then explore what happens when drops interact or coalesce. This fundamental understanding will be fed into improved models of arrays, clouds or sprays of droplets that are encountered in most practical applications (such as spray coating, spray drying, inhalers or inkjet manufacturing).We will use an Industry Club to engage with companies from a range of different sectors. This Club will provide a forum for sharing problems, ideas and solutions and for disseminating the knowledge generated in the project.