Dynamical properties of pure and mixed active matter suspensions

从细胞骨架动力学(分子水平)和细菌蜂拥(细胞水平)到成群的鸟类和人类群体(宏观水平)，活跃运动的相互作用剂的集合，也被称为“活性物质”，可以显示出意想不到的紧急性质。在活性物质的一般理论框架内理解这些性质是一项挑战，目前正在推动这一快速增长的物理学领域。活性物质的一些最重要的例子是微尺度的，包括活性胶体和微生物悬浮液，既作为简单的活性流体(单一物种)，也作为活性和被动元素的混合悬浮液。在这最后一类体系中，最近的实验和理论工作已经开始提供一个窗口，以了解新的现象，包括活性诱导的耗尽相互作用和相分离；以及从活性悬浮液中提取网络的可能性。控制这类系统集体行为的能力可能会导致从药物微释放到生物修复的一系列应用。我们最近开发了一个简单的实验系统来研究微生物悬浮液的行为，这些悬浮液通过微泳者对外部刺激(光)的反应而自我压缩，使我们能够通过实验调节局部微生物的浓度。该项目将在这个系统的基础上(并开发)解决以下两个主要问题：1.活性物质的相：我们的初步结果表明，有证据表明，相类似于由过渡区分隔的堵塞、液体和气体状态。学生将对这一现象和理解其行为的理论模型有一个定量的理解；2.混合系统中的运输特性：在我们小组当前研究的基础上，解开胶体-游泳者相互作用的物理学基础，我们感兴趣的是了解主动成分的动力学的外部控制(例如，诱导的微游泳者的各向异性/不均质性)如何可以用来改变被动货物的运输。学生将首先观察物理禁闭(我们已经有了有希望的初步结果)和外部光刺激的影响。这项工作将建立在实验、数值和理论模型相结合的基础上。为了实现对活性物质的外部控制，我们需要关键地提高我们对主动悬架动力学特性的理解。理论和数值工作最近探索了是否有可能发展一种热力学方法来研究活性物质，特别是努力集中在定义有意义的压强的可能性上，因此有可能定义类似于状态方程的东西。不幸的是，由于几乎完全缺乏实验结果，这一领域的进展受到阻碍。该项目将解决这一知识/技术差距。该项目与目前对软物质和生物物理学关于理解和控制活性物质的兴趣非常一致。该项目将建立在与Idan Tuval博士(西班牙IMEDEA)微生物建模和数值模拟专家以及Giorgio Volpe博士(UCL)正在进行的合作的基础上，Giorgio Volpe博士(UCL)是光学和活性胶体方面的专家。

From cytoskeletal dynamics (molecular level) and bacterial swarming (cellular level) to flocks of birds and human crowds (macroscopic level), collections of actively moving interacting agents, also called "active matter", can display unexpected emergent properties. Understanding these properties within the framework of a general theory of active matter is a challenge which is currently driving this rapidly growing area of physics. Some of the most important examples of active matter are at the microscale, and include active colloids and suspensions of microorganisms, both as a simple active fluid (single species) and as mixed suspensions of active and passive elements. In this last class of systems, recent experimental and theoretical work has started to provide a window into new phenomena including activity-induced depletion interactions and phase separation; and the possibility to extract net work from active suspensions. The ability to govern collective behaviour of such systems could lead to applications ranging from drug microdelivery to bioremediation.We have recently developed a simple experimental system to study the behaviour of microorganismal suspensions which are self-compressing through the microswimmers' response to an external stimulus (light), allowing us to tune experimentally the local microorganismal concentration. The project will build on (and develop) this system to address the following two main points:1.Phases of active matter: Our preliminary results have shown evidence of phases similar to a jammed, liquid and gas states, separated by transition regions. The student will develop a quantitative understanding of this phenomenon and a theoretical model to understand its behaviour;2.Transport properties in mixed systems: Building on current research in our group unraveling the physics of colloid-swimmer interactions we are interested in understanding how external control of the dynamics of the active component (e.g. induced microswimmer anisotropy/inhomogeneity) can be used to alter the transport of passive cargo. The student will begin by looking at the effect of physical confinement (for which we already have promising preliminary results) and external light stimuli. The work will be based on a combination of experiments, numerical and theoretical modelling.To achieve external control of active matter, we need to critically advance our understanding of the dynamical properties of active suspensions. Theoretical and numerical work has recently explored whether it is possible to develop a thermodynamic approach to active matter, with efforts focussing in particular on the possibility to define a meaningful pressure and therefore something akin an equation of state. Unfortunately, progress in this area is hindered by the almost complete lack of experimental results. The project will address this knowledge/technological gap.The project is very well aligned with current interest in Soft Matter and Biological Physics regarding the understanding and control of active matter.This project will build on ongoing collaborations with Dr. Idan Tuval (IMEDEA, Spain) expert in modelling and numerical simulations of microorganisms; and Dr. Giorgio Volpe (UCL), an expert in optics and active colloids.