The non-linear physics of driven colloids and bacteria

统计力学是物理学的一个分支，研究大量实体的集体属性如何取决于它们如何相互作用。这门学科中最难的问题是那些处理被某种外力驱使而失去平衡的系统的问题。我提出了一个实验研究计划，以探索两种驱动系统：致密胶体悬浮液的变形和流动，以及细菌的收集。浓胶体悬浮液中相互作用的实体是悬浮在液体中的微观颗粒。这种悬浮液的流动特性既是基础科学的魅力，也是重要的应用。其中一个日常的例子是，牙膏从牙膏管中挤出来时表现得像液体，但放在牙刷上时却表现得像固体。我将使用一系列新的实验工具来研究非常明确的“模型”悬架的变形和流动。特别是，现在有可能使用先进的光学显微镜来跟踪流动中悬浮液中单个颗粒的轨迹。获得的数据将为我们提供一幅前所未有的详细图片，说明致密悬浮液的奇怪流动特性是如何与它们的组成粒子相关的。今天出现了一个统计力学的新领域，致力于研究“代理”——相互作用的复杂实体，导致新的集体行为。这些实体可以是网络上的移动电话，或者是交易大厅里的股票经纪人，或者是一群细菌。单个细菌的大小和惰性胶体粒子差不多（千分之一毫米）。主要的区别在于细菌是活跃的——它们可以推动自己穿过周围的液体。它们还通过分泌和“解码”一系列化学“信息”来相互传递“信号”。因此，大量细菌可以表现出新的集体行为。因此，例如，它们可以在表面上“群集”（就像鸟类在空中一样）。我的研究将涉及一系列细菌的集体行为，比如它们如何聚集在一起形成“生物膜”——复杂的二维细菌“城市”，以及生物膜如何处理“骗子”，即利用邻居的个体突变体。我还将利用胶体的类比，并比较细菌悬浮液与惰性粒子悬浮液的区别。相互作用因子理论的新结果也将以细菌作为“模型”进行实验测试。

Statistical mechanics is the branch of physics dealing with how the collective properties of a large number of entities depend ib how they interact with each other. The hardest problems in the subject are those that deal with systems driven off equilibrium by some external force. I propose a programme of experimental research to probe two kinds of driven systems: the deformation and flow of dense colloidal suspensions, and collections of bacteria.The interacting entities in a concentrated colloidal suspension are microscopic particles suspended in a liquid. The flow properties of such suspensions are both a fascination for fundamental science, and important for applications. One of the many everyday examples is the way tooth paste behaves like a liquid when being squeezed out of its tube, but acts like a solid while sitting on the tooth brush. I will use a range of new experimental tools to study the deformation and flow of very well defined, 'model' suspensions. In particular, it is now possible to use advanced optical microscopy to follow the trajectories of individual particles in a suspension in flow. The data obtained will give us an unprecedentedly detailed picture of how the strange flow properties of dense suspensions are related to their constituent particles.There is today emerging a new area of statistical mechanics devoted to the study of 'agents' - complex entities interacting with each other, resulting in novel collective behaviour. The entities can be mobile phones on a network, or stock brokers on a trading floor, or a collection of bacteria. Individual bacteria are about the same size as inert colloidal particles (a thousandth of a millimeter). The main difference is that bacteria are active - they can propel themselves through the surrounding liquid. They also 'signal' to each other by secreting and 'decoding' a range of chemical 'messages'. A large number of bacteria can therefore show novel collective behaviour. Thus, e.g., they can 'swarm' on a surface (much like birds do in air). My research will address a range of bacterial collective behaviour, such as how they clump together to form 'biofilms' - complex two-dimensional bacterial 'cities', and how biofilms deal with 'cheaters', individual mutants who take advantage of their neighbours. I will also draw on the analogy with colloids and compare how suspensions of bacteria differ from suspensions of inert particles. Emerging results from the theory of interacting agents will also be tested experimentally using bacteria as 'models'.