Understanding and optimising self-assembly processes

秩序是如何从无序中产生的？这个深刻的问题已经困扰了科学家几十年。最近，新兴的纳米技术领域通过自组装的想法激发了人们对订购过程的新兴趣。与简单的组装过程（由适当设计的部件组装成产品）相比，自组装是一种更不寻常的效果。如果零部件设计得足够仔细，那么它们就能自发地组装成有序的产品。这样的过程似乎令人惊讶，因为直觉和物理定律都告诉我们，随着时间的推移，系统应该变得更加无序：也就是说，我们期望熵增加。然而，自发有序的产物在自然界中是很常见的：对称的雪花从天而降，多面宝石在地下形成。这些效应表明，在适当的条件下，可以发生自组装。（产品熵的减少由环境熵的增加来补偿。）雪花和宝石大得足以用肉眼看到，而现代科学的几个方面则与小得多的物体的自我组装有关，这些物体的尺寸从百万分之一毫米到千分之一毫米不等。在生物学中，这种结构的自组装是必不可少的，它允许细胞组织和控制生命所需的各种过程。另一方面，病毒的自组装是病毒繁殖所必需的，但它允许疾病传播，从被感染生物体的角度来看是极不可取的。在其他领域，如物理和化学，科学家们经常关注组装大规模有序结构，以达到特定的电子或机械性能。自组装太阳能电池甚至自组装计算机已经被提出。本研究的目的是开发普遍适用的方法来控制自组装。为了实现这一点，我将重点关注有序产品和不需要的无序结构之间的竞争。显然，在自然界中，无序的雨滴和岩石比有序的雪花和宝石要常见得多。这些常见的无序结构与我们的直觉预期一致，即熵随时间增加。为了避免陷入这些无序状态，物理学家可以从生物学中获得灵感，生物学中的组装过程已经通过进化得到了优化，以获得高质量的有序产品，如病毒、细胞器和细胞骨架。统计物理学为讨论有序与无序之间的竞争提供了自然的语言。特别地，这项工作的中心主题是有效装配和装配过程中固有的微妙逆时对称性之间的联系。也就是说，本质上不可逆的自组装过程可以通过利用牛顿运动定律中固有的可逆性来优化。如果一个系统开始形成一个无序的结构，可逆性允许它回溯其步骤并纠正其错误，朝着组装产品前进。为了理解这种效应的重要性及其含义，我将把数学和分析方法与模型系统的计算机模拟结合起来。首先集中于生物自组装过程的一般特征，我将讨论物理科学家如何适应或模仿这些生物系统，以设计新颖的有序产品。

How does order emerge from disorder? This profound question has concerned scientists for many decades.Recently, the emerging field of nanotechnology has stimulated renewed interest in ordering processes,through the idea of self-assembly. Compared with simple assembly processes, in which a product is builtup from appropriately designed components, self-assembly is a more unusual effect. If components can bedesigned carefully enough, then they can assemble themselves spontaneously into ordered products.Such proceses seem surprising, since both intuition and physical laws tell us that systems should becomemore disordered over time: that is, we expect entropy to increase. However, spontaneously ordered productsare familiar in nature: symmetric snowflakes fall from the sky, and faceted gemstones form under the earth.These effects demonstrate that self-assembly can occur, under the right conditions. (The reduced entropy ofthe product is compensated by an increase in the entropy of its environment.)While snowflakes and gemstones are large enough to be seen with the naked eye, several aspects of modernscience are concerned with the self-assembly of much smaller objects, with sizes ranging from a millionth toa thousandth of a millimeter. In biology, self-assembly of such structures can be essential, allowing cells toorganise and control various processes that are required for life. On the other hand, self-assembly of virusesis essential for viral reproduction, but it allows diseases to spread, and is extremely undesirable from the pointof view of the infected organism. In other fields such as physics and chemistry, scientists are often concernedwith assembling large-scale ordered structures, to achieve particular electronic or mechanical properties. Self-assembled solar cells and even self-assembled computers have been proposed.The aim of this research is to develop generally applicable methods for controlling self-assembly. To achievethis, I will focus on the competition between ordered products and unwanted disordered structures. Clearly,disordered raindrops and rocks are much more common in nature than ordered snowflakes and gemstones.These common disordered structures are consistent with our intuitive expectation that entropy increaseswith time. To avoid getting trapped in these disordered states, physicists can draw inspiration from biology, whereassembly processes have been optimised by evolution, in order to achieve high-quality ordered products such as viruses, organelles and cellular skeletons.Statistical physics provides the natural language in which to discuss the competition between order anddisorder. In particular, a central theme of this work is a link between effective assembly and subtle time-reversalsymmetries inherent in the assembly process. That is, intrinsically irreversible self-assembly processes canbe optimised by exploiting the inherent reversibility in Newton's laws of motion. If a system starts to form a disorderedstructure, reversibility allows it to retrace its steps and correct its mistakes, making progress towards the assembledproduct. In order to understand the importance of this effect and its implications, I will combine mathematical and analytic methods with computer simulations of model systems. Initially concentrating on generic features of biological self-assembly processes, I will discuss how physical scientists might adapt or mimic these biological systems, in order to engineer novel ordered products.