The behaviour of nematic liquid crystals in chemically patterned millimetre-sized rectangular surfaces

We describe a method to produce nematic liquid crystal ‘semi-droplets’ with potential use in sensing by chemically patterning rectangles on solid surfaces, surrounded by a hydrophobic monolayer to confine the liquid crystal. The substrate has directional planar anchoring, while the top surface is a liquid crystal/air interface. The effects of varying the semi-droplet of E7 height between ~20 and 420 μm; aspect ratios between 0.5 and 1; widths between 6 and 13 mm and lengths between 6 and 20 mm were investigated, significantly extending the range of semi-droplets considered in the literature; their increased size is expected to be useful in real-life devices. A geometric model for the semi-droplet was derived and compared with an existing theoretical model, offering a simple method to predict the semi-droplet interface angle, which can subsequently be used to predict the maximum height and height variation along the semi-droplet. Polarised light imaging was used to deduce the semi-droplet height profile using the liquid crystal’s birefringent properties, with excellent agreement with the model for heights ≲ 50 μm, even when the semi-droplet width was larger than E7’s capillary length. It is suggested that the confinement surface is reusable by washing with the relatively cheap and safe solvent isopropanol.

我们提出一种制备向列相液晶（nematic liquid crystal）半液滴的方法，该半液滴具备传感应用潜力：通过在固体表面化学图案化矩形区域，并以疏水单分子层环绕该区域以约束液晶的分布。基底具备定向平面锚定特性，而半液滴的上表面为液晶-空气界面。本研究系统考察了E7型液晶半液滴的多项参数变化对其的影响：高度范围约20~420 μm、长径比0.5~1、宽度6~13 mm、长度6~20 mm。该研究大幅拓展了已有文献中报道的半液滴参数范围，其更大的尺寸有望在实际器件中发挥应用价值。我们推导了半液滴的几何模型，并与现有理论模型进行对比，由此提出一种可简便预测半液滴界面角的方法，该方法后续可用于预测半液滴的最大高度及沿其分布的高度变化。利用液晶的双折射特性，通过偏振光成像技术推导半液滴的高度分布；当半液滴高度≤50 μm时，实验结果与模型吻合极佳，即便此时半液滴宽度大于E7的毛细管长度。研究表明，通过使用相对廉价且安全的异丙醇溶剂进行清洗，该约束表面可重复使用。