Control of free-surface flow morphologies in anisotropic liquids

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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.