Lock and key colloids: Controlling self assembly via depletion forces

这是一个著名的数学结果：堆积球形粒子的最有效方法是将它们分层堆叠，就像把橙子放在盒子里一样。另一个不太为人所知的结果是，微观塑料球（胶体）在浸泡在简单的化学混合物中时，会自发地形成相同的有序结构。这是一个自我组装的例子：没有能量被耗尽，单个粒子也不需要人工操纵，而是一个有序的产品被组装起来。此外，这些系统足够简单，粒子之间的力可以从理论上计算出来，并且可以在计算机中模拟。自组装在包括生物学在内的许多其他环境中也是相关的。在那里，复杂的生物分子聚集在一起，形成有序的结构，这些结构可能对生命至关重要（例如，赋予细胞形状的微观细丝），也可能导致疾病（例如，病毒）。受这些生物系统的启发，有人提出在纳米技术中使用自组装技术，建造新型太阳能电池或计算机。然而，尽管这些想法令人兴奋，但设计这种自组装产品的问题是控制问题之一。生物系统可以组装成复杂的功能结构，但粒子之间的相互作用是复杂的，不容易为我们自己的特定目的设计和建造类似的系统。另一方面，简单的球形胶体可以被精确地控制，但只能用于制造堆积层的球体。最近，研究人员取得了重要的进展：通过制造不同形状的胶体，新的有序结构可以自组装：不是堆叠层，而是粒子簇和粒子串。这些新粒子被称为“锁”和“钥匙”，因为它们在组装时相互配合的方式。本提案将使用计算机模拟来探索这些锁和钥匙粒子的自组装。就像在球形的情况下一样，我们已经理解了基本的物理原理，所以我们可以用计算机模拟来预测和解释实验结果。特别是，我们将研究可能自组装的结构范围，既包括现有的锁和钥匙粒子，也包括未来可能制造的其他类似粒子。通过这种方式，我们的目标是指导未来在这一领域的实验，并研究锁和钥匙系统可能的技术应用。更一般地说，通过从一个相对简单的不同形状的胶体粒子系统开始，我们的目标是开发可以更广泛地用于设计和控制自组装的指导原则。如果一个系统可能形成几种不同的结构，我们如何从众多选择中选择一种？柔性结构和刚性结构哪一个更容易组装？如何控制这种灵活性？我们能否设计出即使它们的形状不完美或彼此略有不同，仍能组装的结构？这样的问题出现在许多不同的自组装系统中：通过在锁和钥匙粒子的相对简单的背景下研究它们，我们寻求有朝一日可能用于模拟或破坏生物组装，或构建纳米级机器产品的见解。

It is a famous result from mathematics that the most efficient way to pack spherical particles is to stack them in layers, like oranges in a box. Another result, less well-known, is that microscopic plastic spheres (colloids) will form the same ordered structure spontaneously, on immersion in a simple mixture of chemicals. This is an example of self-assembly: no energy is used up and individual particles are not manipulated by hand, but an ordered product is assembled. Furthermore, these systems are simple enough that the forces between the particles can be calculated theoretically and the systems simulated in computers.Self-assembly is also relevant in many other contexts, including biology. There, complicated biological molecules come together and form ordered structures that might might be essential for life (for example, microscopic filaments that give cells their shape) or might cause disease (for example, viruses). Inspired by these biological systems, proposals have been made to use self-assembly in nanotechnology, building novel solar cells or computers.However, while these ideas are exciting, the problem in designing such self-assembled products is one of control. The biological systems can assemble into complex functional structures but the interactions between the particles are complicated and it is not easy to design and build similar systems for our own specific purposes. On the other hand, simple spherical colloids can be controlled accurately but can only be used to make stacked layers of spheres. Recently, an important step was made: by making colloids with different shapes, new ordered structures could self-assemble: not stacked layers but clusters and strings of particles. The new particles are called "locks" and "keys" because of the way they fit together as they assemble.This proposal will use computer simulation to explore the self-assembly of these lock and key particles. As in the spherical case, the fundamental physics are understood, so we can use computer simulations to predict and explain the results of experiments. In particular, we will investigate the range of structures that might self-assemble, both with existing lock and key particles and with other similar particles that might be made in the future. In this way we aim to guide future experiments in this area, and look into possible technological applications of lock and key systems.More generally, by starting from a relatively simple system of colloidal particles with different shapes, we aim to develop guiding principles that can be used more generally in designing and controlling self-assembly. If a system might form several different structures, how can we select one out of the many choices? Is it easier to assemble flexible structures or rigid ones, and how might this flexibility be controlled? Can we design structures that can still assemble even if their shapes are imperfect or slightly different from each other? Such questions occur in many different self-assembling systems: by investigating them in the relatively simple context of lock and key particles, we look for insight that might one day be used to mimic or disrupt biological assembly, or to build nano-scale products of machines.