The physics of nonequilibrium transitions in sheared complex fluids

软凝聚态物理学涉及复杂的流体，如聚合物，表面活性剂，胶体，泡沫和乳液。在工业上，这些材料在食品，个人护理产品，石油提取，消防，显示技术，涂料等方面有着广泛的应用。它们的共同点是存在纳米到微米尺度的介观内部子结构：聚合物分子，乳液液滴，泡沫气泡等。这些都是相对容易重组的外部施加的剪切流。反过来，这种重组反馈流场，导致异国情调的非平衡流动现象，并赋予宏观力学性能，是惊人的不同，从简单的液体和结晶固体。在许多情况下，均质流在临界剪切速率以上是不稳定的。然后，系统分离成不同粘度和内部组织的共存剪切带。这种效应可以等效地看作是一种非平衡相变或流动不稳定性。在工业上，诸如此类的不稳定性通常会阻碍加工。事实上，塑料行业通过制造过程中的不稳定性造成的产品缺陷看到了巨大的浪费，而许多生产线的生产率有限，超过该生产率，工艺就会变得不稳定。因此，了解这些材料的奇异流动特性具有重要的实际意义。与此同时，它形成了一个活跃且具有挑战性的基础研究领域，借鉴并贡献了非平衡统计物理学的概念。实验技术的快速发展带来了大量数据，表明剪切带流往往表现出复杂的动力学。在这种情况下，系统对稳定剪切流的响应本质上是不稳定的。例如，这可以在带之间的界面的不规则运动中看到。目前，还不知道这种不稳定的响应是否源于其中一个带中的体积不稳定性;流体在壁处滑动的动力学方式;或者带之间界面的波动不稳定性。本研究的一个重要目的是阐明主导机制，除了上述的流动诱导相变，更常见的（平衡）相变是热诱导的。一个例子是不混溶的流体（如油和水）在低温下分层成组成不相等的两相。一个迷人的问题，然后关注的效果，一个应用剪切流对这一过程。对于聚合物混合物，剪切可在高于正常（平衡）转变温度的温度下过早地触发分层。这种效应被称为剪切诱导的分层。相反，低于平衡转变温度，热力学分层实际上可以通过施加的剪切来阻止：该系统不能完全分层，并且在包含两个不同相的小区域的乳化状态下阻止。这带来了一个明显的悖论：机械驱动可以触发剪切带和剪切诱导的分层，但逮捕热力学分层。为了解决这个问题，我的目标是发展一个统一的理论理解剪切带，剪切诱导的分层，和热力学分层剪切下复杂的流体。在它里面，我建议探索悬而未决的问题，粘弹性分层和乳化动力学剪切。在商业上，这些问题直接关系到这些物质的保质期和可加工性。

Soft condensed matter physics concerns complex fluids such as polymers, surfactants, colloids, foams, and emulsions. Industrially, these materials find widespread applications in foodstuffs, personal care products, oil extraction, fire-fighting, display technologies, coatings, etc. Common to them all is the presence of mesoscopic internal substructures on the scale of nanometers to microns: polymer molecules, emulsion droplets, foam bubbles, etc. These are relatively easily reorganised by an externally applied shear flow. In turn, this reorganisation feeds back on the flow field, leading to exotic nonequilibrium flow phenomena, and conferring macroscopic mechanical properties that are strikingly different from those of simple liquids and crystalline solids. In many cases, homogeneous flow is unstable above a critical shear rate. The system then separates into coexisting shear bands'' of unequal viscosities and internal organisation. This effect can equivalently be viewed as a nonequilibrium phase transition, or a flow instability. Industrially, instabilities such as these routinely hinder processing. Indeed, the plastics industry sees huge wastage through product defects caused by instabilities during manufacturing, while many production lines have a limited throughput rate above which the process becomes unstable. Understanding the exotic flow properties of these materials is thus of great practical importance. At the same time, it forms an active and challenging field of fundamental research, drawing on and contributing to concepts of nonequilibrium statistical physics.Rapid recent advances in experimental techniques have resulted in a large body of data showing that shear banded flows often display complex dynamics. In such cases, the response of the system to a steady applied shear flow is intrinsically unsteady. This can be seen in an erratic motion of the interface between the bands, for example. At present, it is not known whether this erratic response stems from a bulk instability in one of the bands; in the dynamics of the way the fluid slips at the wall; or in an undulatory instability of the interface between the bands. An important aim of the proposed research is to elucidate the dominant mechanism.Besides the flow-induced transitions just described, more familiar (equilibrium) phase transitions are thermodynamically induced. One example is the demixing of immiscible fluids (such as oil and water) at low temperatures into two phases of unequal compositions. A fascinating question then concerns the effect of an applied shear flow on this process. For polymeric mixtures, shear can prematurely trigger demixing at temperatures above the normal (equilibrium) transition temperature. This effect is called shear-induced demixing''. In contrast, below the equilibrium transition temperature, thermodynamic demixing can actually be arrested by an applied shear: the system fails to demix fully, and arrests in a emulsified state comprising small domains of the two different phases. This brings an apparent paradox: that mechanical driving can trigger shear banding and shear-induced demixing; but arrest thermodynamic demixing. To resolve this, I aim to develop a unified theoretical understanding of shear banding, shear-induced demixing, and thermodynamic demixing under shear in complex fluids. Within it, I propose to explore outstanding questions concerning the kinetics of viscoelastic demixing and emulsification under shear. Commercially, issues such as these directly concern the shelf life and processability of these substances.