Dynamic arrest and non-equilibrium behaviour in suspensions of deformable colloids

液体的分子很容易移动，分子排列的快照显示出完全的无序。当液体被冷却时，它通常会经历结晶，分子排列成有序的图案。最常见的例子是水变成冰。然而，这种结晶可以通过快速冷却来避免。冷却后，液体分子的移动速度越来越慢，如果冷却速度足够快，在所有远程运动停止之前，没有足够的时间重新排列成有序的模式；因此，结构保持无序和液体状，但材料是硬的-生成的固体被称为玻璃。玻璃材料在广泛的人造材料和应用中非常重要，包括电池电解液和电极、太阳能电池、制药和大多数塑料。无论是玻璃形成的微观机制还是玻璃状态的行为都不是很清楚；这是值得注意的，因为人类生产玻璃已有数千年的历史，而玻璃是由数百万年的地质过程自然形成的。因此，对玻璃及其形成的认识是基础科学和技术中尚未解决的关键问题。值得注意的是，分子玻璃的分子(尺寸：0.1-10纳米)和悬浮在液体中的颗粒(尺寸：0.1-10微米)的行为可以类比，即所谓的胶体。对于胶体，玻璃的形成受控于一定体积内的颗粒浓度；对于低颗粒浓度，系统是液体，但随着浓度的增加，系统变得拥挤，这导致玻璃的形成。实际的例子包括涂料、乳液、润滑剂和增稠剂。用胶体作为研究玻璃形成的模型系统的优点是颗粒尺寸大，这意味着可以用光作为探针来研究胶体的运动，以及胶体尺寸、弹性和颗粒间相互作用等性质的很大控制。在这项工作中，我们将使用一个通用的胶体模型系统，由在溶剂中膨胀的凝胶颗粒组成，即所谓的微凝胶。除了作为模型体系的作用外，这种微凝胶悬浮液在生物传感和医疗诊断、化学分离技术、采油、药物输送和可切换材料等应用中也是重要的。我们将通过控制颗粒凝胶的交联度来合成具有不同机械性能的微凝胶颗粒。每个微凝胶批次都将根据颗粒大小、凝胶结构和机械性能进行表征。然后，我们将研究这些微凝胶悬浮液是如何形成玻璃的，因为这些颗粒在浓度达到一定体积时会挤在一起。在接近玻璃态时，将利用光散射和流变学技术来研究微凝胶颗粒的排列和运动。光散射研究产生了关于单个微凝胶结构、微凝胶颗粒排列和微凝胶在大范围时间尺度(10 ns-1000 S)上的运动的信息。利用流变学，研究了材料对机械扰动的响应。这项研究的具体目的是(I)在接近玻璃状态时，找出单个微凝胶的性质与相应的悬浮排列和运动之间的关系(Ii)确定哪些类型的微凝胶运动与玻璃形成过程相关，以及这些运动是如何相互关联的(Iii)研究外加剪切如何影响并最终熔化微凝胶玻璃。通过对一个优秀的模型系统的系统研究，我们的目标是为未来的玻璃过渡工作形成一个基准。

The molecules of a liquid move easily and a snap-shot of the molecular arrangements reveal complete disorder. As a liquid is cooled, it often undergoes crystallization, where the molecules arrange into an ordered pattern. The most common example is water turning into ice. However, such crystallization can be avoided by cooling at a fast rate. The liquid molecules then move slower and slower upon cooling and if the cooling rate is high enough there is not enough time to rearrange into an ordered pattern before all long-range motions come to a halt; thus, the structure remains disordered and liquid-like but the material is hard - the resulting solid is called a glass. Glassy materials are important in a wide range of man-made materials and applications including battery electrolytes and electrodes, solar cells, pharmaceuticals and most plastics. Neither the microscopic mechanisms involved in glass-formation nor the behaviour of the glassy state are well understood; this is remarkable since humans have been producing glass for thousands of years and glasses have been naturally formed by geological processes for millions of years. Thus, reaching an understanding of glasses and their formation is a key unsolved problem in both fundamental science and technology. Remarkably, an analogy can be drawn between and the molecules (size: 0.1-10 nm) of molecular glass-formers and the behaviour of particles (size: 0.1-10 microns) suspended in a liquid, so called colloids. For colloids, glass-formation is controlled by the concentration of particles within a certain volume; for low particle concentrations the system is a liquid but as the concentration is increased the system gets crowded, which leads to the formation of a glass. Practical examples include paints, emulsions, lubricants and thickeners. The advantage of using colloids as a model system to study glass-formation is the large particle size, which means that the colloid motions can be studied using light as a probe, together with the great control of properties such as colloid size, elasticity and inter-particle interactions. In this work we will use a versatile colloidal model system consisting of gel particles swollen in a solvent, so called microgels. In addition to their role as model systems, such microgel suspensions are important in applications including biosensing and medical diagnostics, chemical separation technologies, oil recovery, pharmaceutical delivery, and switchable materials.We will synthesize microgel particles with varying mechanical properties, by controlling the cross-linking of the particle gels. Each microgel batch will be characterized with regards to particle size, gel structure and mechanical properties. We will then study how these microgel suspensions form glasses as the particles crowd the volume upon concentration. Both the arrangement and the motions of the microgel particles will be studied as the glassy state is approached, using light scattering and rheology techniques. Light scattering studies yield information about the individual microgel structure, the microgel particle arrangements and the microgel motions over a wide range of time-scales (10 ns-1000 s). With rheology, the response of the material to a mechanical disturbance is investigated. Specific aims of the study are to (i) find the relationship between single microgel properties and the corresponding suspension arrangements and motions as the glassy state is approached (ii) determine which types of microgel motions are relevant to the glass formation process and how these motions are inter-related (iii) investigate how an applied shear affects and eventually 'melts' a microgel glass.This work addresses questions that are key to an understanding of glassy materials in general. By systematic studies of an excellent model system, we aim to form a benchmark for future glass-transition work.