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。当液体冷却时，它经常经历结晶，分子排列成有序的模式。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。然后液体分子在冷却时移动得越来越慢，如果冷却速度足够高，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，结构仍然是无序的，像液体一样，但材料是坚硬的——由此产生的固体被称为玻璃。玻璃材料在各种人造材料和应用中都很重要，包括电池电解质和电极、太阳能电池、药品和大多数塑料。无论是玻璃形成的微观机制还是玻璃态的行为都没有得到很好的理解；这是值得注意的，因为人类生产玻璃已有数千年的历史，而玻璃是经过数百万年的地质作用自然形成的。因此，了解玻璃及其形成是基础科学和技术领域尚未解决的关键问题。值得注意的是，形成玻璃的分子（直径0.1-10纳米）和悬浮在液体中的粒子（直径0.1-10微米）的行为，也就是所谓的胶体，可以进行类比。对于胶体，玻璃的形成是由一定体积内的颗粒浓度控制的；当粒子浓度较低时，系统是液体，但当浓度增加时，系统变得拥挤，从而导致玻璃的形成。实际的例子包括油漆、乳剂、润滑剂和增稠剂。用胶体作为模型系统来研究玻璃形成的优势在于其大的粒径，这意味着可以用光作为探针来研究胶体的运动，同时胶体的尺寸、弹性和粒子间相互作用等特性也有很大的可控性。在这项工作中，我们将使用一种多功能的胶体模型系统，该系统由在溶剂中膨胀的凝胶颗粒组成，即所谓的微凝胶。除了作为模型系统的作用外，这种微凝胶悬浮液在生物传感和医学诊断、化学分离技术、石油回收、药物输送和可切换材料等应用中也很重要。我们将通过控制颗粒凝胶的交联来合成具有不同力学性能的微凝胶颗粒。每个微凝胶批次都将根据粒径、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，因为颗粒在浓度上聚集在体积上。微凝胶粒子的排列和运动都将在接近玻璃态时进行研究，使用光散射和流变学技术。光散射研究可以获得微凝胶结构、微凝胶粒子排列和微凝胶在大范围时间尺度（10ns - 1000s）内运动的信息。利用流变学，研究了材料对机械扰动的响应。该研究的具体目标是：(i)发现单个微凝胶性质与相应的悬浮排列和运动之间的关系，当玻璃态接近时；（ii）确定哪些类型的微凝胶运动与玻璃形成过程相关，以及这些运动如何相互关联；（iii）研究施加的剪切如何影响并最终“融化”微凝胶玻璃。这项工作解决了对一般玻璃材料理解的关键问题。通过对一个优秀模型系统的系统研究，我们的目标是为未来的玻璃化转变工作建立一个基准。

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.