Control of free-surface flow morphologies in anisotropic liquids

像水这样的液体和空气之间的界面被称为自由表面，它的形状是由界面的表面张力（它的作用很像儿童气球皮肤上的张力）和水分子与固体容器的相互作用（称为“润湿性”）之间的平衡决定的。这些影响之间的相互作用产生的日常现象是，当自由表面与部分装满水的杯子表面相遇时形成的典型弯曲半月板，以及水黾在池塘或河流表面停留的能力。在小（毫米）尺度上，这两种效应都很重要，因此了解表面张力和润湿性效应之间的微妙相互作用是理解和控制这些尺度上液体流动的关键。对于许多实际和工业环境，从窗户或挡风玻璃上的雨水到工业涂层过程，一个重要的现象是薄层流体如何以及何时分解成小溪流，或者大溪流分解成小溪流。这个看似日常的问题表现出迷人而复杂的行为，它以一种复杂的方式取决于一系列参数，包括流体体积、基材的斜率和润湿性，以及液体的固有特性（如密度和粘度）。我们在这个提案中的首要研究目标是探索、理解并因此积极地操纵流动液体可以采用的自由表面形状，其尺寸范围从几十微米（1/100毫米）到毫米尺度。虽然这种长度尺度的影响可能不存在于标准液体中，但我们将使用向列液晶，这是一种复杂的液体，其粘度取决于流动的速度和方向，而不是构成这种液体的细长分子的方向。控制分子的方向本身就会对流动的方式产生深远的影响。然而，这个提议远不止于此，旨在通过选择内部、表面和外部施加的力的相对强度来产生自由表面形状操纵的新方法。创造这种控制形状和流动形态的相对稳定性的能力，为各种拓扑结构不同的自由表面流动状态提供了一条新的途径。虽然从基础科学的角度来看，这项工作很吸引人，但它也将在许多应用领域产生相当大的影响。事实上，液晶无处不在——从电视和手机上的液晶显示器（LCD）到构成你身体每个细胞壁的微观分子层，这些材料都非常重要。在过去的50年里，液晶研究和显示设备的发展一直受到理解和利用弹性、外加电场和固体边界之间相互作用的需要的推动。理解和控制这些相互竞争的相互作用催生了一个价值约950亿美元的LCD产业。然而，超越当前技术的进步将使流动增强和微流体应用的创新成为可能，这需要对上述所有效应之间的动态相互作用以及流动向列型液晶中流动诱导排列、缺陷纹理和自由表面的影响有更好的基础科学理解。阐明这些相互作用是本提案的重点，希望我们的工作能够在新领域带来洞察力和发展，例如：用于所有光存储和软计算的缺陷介导的3D光子器件；可重构微货物运输等微流控应用；在大规模制造和小规模先进设备开发方面取得进展，其中设备填充工艺必须可靠。

The interface between a liquid, such as water, and air is called a free surface, and its shape is determined by a balance between the surface tension of the interface (which acts rather like the tension in the skin of a child's balloon) and how the molecules of the water interact with those of the solid container (called the "wettability"). Everyday phenomena arising from the interplay between these effects are the characteristic curved meniscus which forms as the free surface meets the surface of a partly-filled glass, and the ability of a water strider insect to sit on the surface of a pond or river. At small (millimetre) scales, both effects are important, and so understanding the subtle interplay between surface tension and wettability effects is key to understanding and controlling the flow of liquids at these scales. An important phenomenon for many practical and industrial settings, ranging from rain on a window or windscreen to industrial coating processes, is how and when a thin layer of fluid breaks up into small rivulets, or a larger rivulet breaks up into smaller rivulets. This seemingly everyday problem exhibits fascinating and complex behaviour which depends in a complicated manner on an array of parameters, including the fluid volume, the slope of the substrate and the wettability, as well as the inherent properties of the liquid (such as density and viscosity). Our over-arching research ambition in this proposal is to explore, understand, and hence actively manipulate, the free surface shapes that can be adopted by a flowing liquid, in the size range from tens of microns (1/100th of a millimetre) to millimetre scales. While effects at this length-scale may not be present in standard liquids, we will use nematic liquid crystals, which are complex liquids with viscosities dependent on the speed and direction of the flow compared to the orientation of the elongated molecules that make up this type of liquid. Exerting control of the orientation of the molecules will itself have a profound influence on the manner of flow. However, this proposal goes significantly further than this, aiming to generate new approaches to free surface shape manipulation via the selection of the relative strengths of internal, surface and externally imposed forces. Creating this ability to control the relative stabilities of shape and flow morphologies provides a novel route between a variety of topologically distinct free surface flow regimes.Although fascinating from a fundamental scientific point of view, this work will also have considerable impact in a number of application areas. Indeed, liquid crystals are ubiquitous - from the Liquid Crystal Display (LCD) in your TV and mobile phone to the microscopic layer of molecules that make up the wall of every cell in your body, these materials are hugely important. Over the last 50 years, liquid crystal research and display device development has been driven by a need to understand and exploit interactions between elasticity, applied electric fields, and solid boundaries. Understanding and controlling these competing interactions has spawned an LCD industry worth around $95 billion. However, progress beyond the current technology that would enable innovation in flow-enhancement and microfluidic applications requires an improved fundamental scientific understanding of the dynamic interactions between all of the above effects as well as the effects of flow-induced alignment, defect textures, and free surfaces in flowing nematic liquid crystals. Elucidating these interactions are the focus of this proposal and it is hoped that our work can then lead to insight and developments in new areas such as: defect-mediated 3D photonic devices for all optical storage and soft computing; microfluidic applications such as reconfigurable micro-cargo transport; and advances in large-scale manufacturing and small-scale advanced device development where device filling processes must be reliable.