Dynamical properties of pure and mixed active matter suspensions

从细胞骨架动力学（分子水平）和细菌群集（细胞水平）到鸟群和人群（宏观水平），活跃移动的相互作用剂（也称为“活性物质”）的集合可以显示出意想不到的涌现特性。在活性物质的一般理论框架内理解这些性质是一个挑战，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在微观尺度上，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。在这最后一类系统中，最近的实验和理论工作已经开始提供一个窗口，进入新的现象，包括活动引起的耗尽相互作用和相分离;和可能性，以提取净工作从主动悬浮液。管理集体行为的能力，这样的系统可能会导致应用范围从药物微输送到bioremediation.We最近开发了一个简单的实验系统来研究的微生物悬浮液的行为是自我压缩通过microswimmer的响应外部刺激（光），使我们能够调整实验的本地微生物浓度。该项目将建立（和开发）这个系统，以解决以下两个主要问题：1.活跃物质的阶段：我们的初步结果已经显示出类似于堵塞，液体和气体状态的阶段的证据，由过渡区域分开。学生将发展对这种现象的定量理解和理解其行为的理论模型; 2.混合系统中的传输特性：基于我们小组目前的研究，即解开胶体-游动分子相互作用的物理学，我们有兴趣了解活性组分动力学的外部控制是如何实现的。（例如，诱导的微泳者各向异性/不均匀性）可用于改变被动货物的运输。学生将开始看物理限制的影响（我们已经有了有希望的初步结果）和外部光刺激。这项工作将基于实验、数值和理论模型的结合。为了实现对活性物质的外部控制，我们需要严格推进对主动悬浮液动力学特性的理解。最近的理论和数值工作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一个有意义的压力的可能性上，因此类似于状态方程。不幸的是，这一领域的进展受到几乎完全缺乏实验结果的阻碍。该项目将解决这一知识/技术差距。该项目非常符合当前软物质和生物物理学对活性物质的理解和控制的兴趣。该项目将建立在与微生物建模和数值模拟专家Idan Tuval博士（IMEDEA，西班牙）和光学和活性胶体专家Giorgio Volpe博士（UCL）的持续合作基础上。

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.