The physics of nonequilibrium transitions in sheared complex fluids

软凝聚态物理涉及复杂流体，如聚合物、表面活性剂、胶体、泡沫和乳液。工业上，这些材料广泛应用于食品、个人护理产品、石油开采、消防、显示技术、涂料等领域。它们的共同点是存在纳米到微米尺度的介观内部亚结构：聚合物分子、乳液液滴、泡沫气泡等。这些相对容易通过外部施加的剪切流进行重组。反过来，这种重组对流场产生反馈，导致奇异的非平衡流动现象，并赋予与简单液体和结晶固体截然不同的宏观力学特性。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，该系统分裂成“粘度不等、内部组织不一”的共存剪切带。这种效应可以等效地视为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，塑料行业由于制造过程中的不稳定性导致的产品缺陷造成了巨大的浪费，而许多生产线的吞吐率有限，超过这个吞吐率，过程就会变得不稳定。因此，了解这些材料的奇特流动特性具有重要的实际意义。同时，它形成了一个活跃和具有挑战性的基础研究领域，借鉴和促进了非平衡统计物理学的概念。近年来实验技术的快速发展导致大量数据表明剪切带状流动往往表现出复杂的动力学。在这种情况下，系统对稳定施加剪切流的响应本质上是不稳定的。例如，这可以在带之间界面的不稳定运动中看到。目前，尚不清楚这种不稳定的反应是否源于其中一个波段的整体不稳定；流体在壁上滑动的动力学；或者在带间界面的波动不稳定性中。提出研究的一个重要目的是阐明主导机制。除了刚才描述的流动诱导相变外，更常见的（平衡）相变是热力学诱导的。一个例子是在低温下将不混溶的流体（如油和水）分离成两种成分不等的相。一个有趣的问题是，施加剪切流对这一过程的影响。对于聚合物混合物，剪切可以在高于正常（平衡）转变温度的温度下过早触发脱混。这种效应被称为“剪切诱导脱混”。相反，在低于平衡转变温度时，热力学分解实际上可以被施加的剪切阻止：系统不能完全分解，并且在由两种不同相的小域组成的乳化状态中停止。这就产生了一个明显的悖论：机械驱动可以引发剪切带化和剪切脱混；但是要阻止热力学分解。为了解决这个问题，我的目标是在复杂流体中建立一个统一的剪切带、剪切诱导脱混和剪切下热力学脱混的理论认识。其中，我建议探讨粘弹性脱混和乳化动力学在剪切作用下的突出问题。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.