Tailoring emulsions of complex fluids for precise wetting and coalescence rates

乳剂是两种不混溶液体的混合物，一种液体以滴状悬浮在另一种液体中。尽管乳剂在日常生活和工业中无处不在，但由于缺乏将乳剂的组成、加工和结构与其性质联系起来的基本关系，乳剂的配制仍然主要采用组合方法。本项目旨在弥补文献中两个重要的乳液相关现象的空白：润湿，即乳液滴在表面上的扩散，以及聚结，即两滴结合成一个更大的滴。在许多应用中，调整聚结和润湿速率以达到所需的产品性能或特定功能是至关重要的。但是，虽然有一些研究解释了润湿和聚结的热力学方面，但对这些现象发生速度的研究却很少。基于乳化的产品形成了一个价值数十亿美元的产业，因此，对这两个关键现象的基本理解的任何改进都可以为该行业带来实质性的好处。我们聚结和润湿研究的重点将放在一类称为复杂流体的流体上。这些流体是双重人格流体，可以表现得像固体或液体，这取决于施加在它们身上的压力。尽管大多数工业乳剂是由复杂的流体制成的，但人们对这些材料的聚结和润湿作用了解得最少。在这项工作中，我们将重点关注三种复杂流体：聚合物共混物中的聚合物熔体，乳液配方中广泛使用的表面活性剂溶液，用于特定的扩散性能和抗聚结稳定性，以及用于稳定乳液抗重力的屈服应力流体。为了揭示这些复杂流体的聚结或润湿的控制机制和时间尺度，我们将采用悬臂式微移液装置（CMD）与一种称为反射干涉对比显微镜（RICM）的光学技术相结合。CMD将使我们能够测量液滴与固体-流体或流体-流体界面之间的力，精确到纳米牛顿的几分之一。同时，我们将使用RICM测量薄膜接触轮廓到纳米级，并尽可能使用冷冻扫描电子显微镜补充数据。从这项工作中获得的知识和工具将使我们能够量化由复杂流体组成的乳液的聚结和表面润湿速率，促进预先选择润湿和聚结时间的乳液设计，并允许从目前的经验设计范式过渡。在加拿大有几个以乳化为基础的工业，每一个都至少是一个价值数百万美元的产业。当HQP在我们的实验室接受最先进方法的培训后，他们将为加拿大工业提供巨大的竞争优势。

An emulsion is a mixture of two immiscible liquids in the form of drops of one liquid suspended in the other. In spite of being ubiquitous in everyday life and in the industry, emulsions are still being formulated largely by combinatorial methods, due to a lack of the fundamental relationships that connect their composition, processing and structure to their properties. This project aims to remedy this gap in the literature for two important emulsion-related phenomena: wetting, which is the spreading of an emulsion drop on a surface, and coalescence, which is the union of two drops into one larger drop. In many applications, it is critical to tune the coalescence and wetting rates to achieve the desired product properties or specific functions. But while there are several studies explaining thermodynamic aspects of wetting and coalescence, investigations of the rates at which these phenomena occur are sparse. Emulsion-based products form a billion dollar industry, hence any improvement in the fundamental understanding with respect to these two key phenomena can lead to substantial benefits to the industry.The emphasis of our coalescence and wetting studies will be on a class of fluids called complex fluids. These fluids are Jekyll and Hyde fluids, and can behave like solids or liquids depending on how stresses are imposed on them. Coalescence and wetting are least understood for these materials, in spite of the fact that most industrial emulsions are made out of complex fluids. In this work, we will focus on three complex fluids: polymer melts in the context of polymer blends, surfactant solutions which are used extensively in emulsion formulations for specific spreading properties and for stability against coalescence, and yield stress fluids which are used to stabilize emulsions against gravity.To unravel the governing mechanisms and time scales of coalescence or wetting for these complex fluids, we will employ the Cantilevered Micropipette Device (CMD) in conjunction with an optical technique called Reflection Interference Contrast Microscopy (RICM). The CMD will allow us to measure the force between a drop and a solid-fluid or fluid-fluid interface down to a fraction of a nanoNewton. Simultaneously, we will measure the film contact profile down to a nanometer using RICM, and supplement the data with cryo-Scanning Electron Microscopy wherever possible. The knowledge and tools from this work will allow us to quantify the rates of coalescence and surface wetting in emulsions composed of complex fluids, facilitating emulsion design with preselected wetting and coalescence times and allow a transition away from the current empirical design paradigm. There are several emulsion-based industries in Canada, and each of these is, at least, a multimillion dollar industry. When HQP trained in our lab in state-of-the-art methods join the workforce, they will provide Canadian industry a huge competitive edge.