Using time-dependent interactions to control self-assembly of soft matter

在自组装过程中，新材料甚至功能设备都可以从简单的组件自发地组装起来。例如，微小粒子和纳米线的有序结构可以以不同寻常的方式与光和电相互作用：这些结构的自组装为廉价高效的有机太阳能电池或其他光子器件提供了一条新途径。如果功能材料的自发组装似乎好得令人难以置信，那么我们应该记住，许多生物结构都是由自组装形成的，而像冰这样高度有序的晶体在冷冻过程中也会自行组装。在这些例子的基础上，人工自组装系统现在在实验室中已经成为可能。然而，在设计和开发自组装的人造系统方面存在许多挑战。特别是，人们经常发现，粒子之间的相互作用必须非常精确地调整，以实现高质量的组装。粗略地说，自组装的实验和计算机模拟最常见的结果要么是粒子根本无法组装，要么是它们聚集成无序的团块（这被称为动能捕获）。有效的自组装处于这两种状态之间的边缘：粒子之间的相互作用必须足够强，才能使组装成为可能，但相互作用也必须足够弱，这样无序的聚合就不会主宰系统。在这里，我们建议考虑如果粒子之间的相互作用可以随时间变化，如何避免这个问题。这在实验胶体系统中变得越来越可能，这是自组装的理想选择，因为它们涉及微观粒子，其相互作用被很好地理解，因此可以控制和设计。然而，我们还不知道在这些系统中应该利用与时间相关的相互作用，以达到任何特定的结果。我们将使用理论和计算机模拟来解决这个问题。我们将研究如何利用胶体粒子在不同种类晶体的自组装中利用时间依赖的相互作用。我们还将考虑胶体凝胶，它有时会干扰结晶，但也是工业相关的材料。通过考虑几个实验相关的系统和一系列控制相互作用的协议，我们的目标是达到这些自组装过程的一般“设计规则”，展示如何使用时间相关的相互作用来控制和优化不同类型有序状态的生产。

In self-assembly processes, novel materials or even functional devices can spontaneously build themselves up from simple components. For example, ordered structures of tiny particles and nano-wires can interact with light and electricity in unusual ways: self-assembly of these structures offers a new route to cheap and efficient organic solar cells, or other photonic devices. If the spontaneous assembly of functional materials seems too good to be true, one should bear in mind that many biological structures are formed by self-assembly, and that highly-ordered crystals like ice also assemble themselves during freezing. Building on these examples, artificial self-assembling systems are now becoming possible in the laboratory.However, there are many challenges in designing and developing man-made systems that self-assemble. In particular, one often finds that interactions between particles must be tuned very accurately in order to achieve high-quality assembly. Loosely speaking, the most common results of experiments and computer simulations of self-assembly are either that the particles fail to assemble at all, or that they aggregate into a disordered clump (this is known as kinetic trapping). Effective self-assembly sits on a knife-edge between these regimes: particles must interact strongly enough to make assembly possible, but interactions must also be weak enough that disordered aggregates do not dominate the system.Here, we propose to consider how this problem can be avoided if interactions between particles can be varied with time. This is increasingly becoming possible in experimental colloidal systems, which are ideal for self-assembly because they involve microscopic particles whose interactions are well-understood and can therefore be controlled and designed. However, we do not yet know time-dependent interactions should be exploited in these systems, in order to arrive at any specific result.We will address this question using theory and computer simulation. We will investigate how time-dependent interactions can be exploited in self-assembly of different kinds of crystal, using colloidal particles. We will also consider colloidal gels, which sometimes interfere with crystallisation, but are also industrially-relevant materials in their own right. By considering several experimentally-relevant systems and a range of protocols for controlling interactions, we aim to arrive at general "design rules" for these self-assembly processes, showing how time-dependent interactions may be used to control and optimise the production of different kinds of ordered state.