Itoh Shintaro, Imura Yuuichi, Fukuzawa Kenji, Zhang Hedong
Department of Micro-Nano Systems Engineering, Nagoya University , Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan.
Department of Complex Systems Science, Nagoya University , Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan.
Langmuir. 2015 Oct 20;31(41):11360-9. doi: 10.1021/acs.langmuir.5b02713. Epub 2015 Oct 7.
In the context of the use of liquid crystals (LCs) as lubricants and lubricant additives, this study investigates the anisotropic shear viscosity of LCs confined in nanometer-sized gap widths subject to both shearing and photoalignment. The photoalignment is achieved using anisotropically dimerized polyvinyl cinnamate (PVCi) films coated on substrates. We simultaneously measure the viscosity and order parameter of a liquid crystal (4-cyano-4'-pentylbiphenyl) confined and sheared in the gap range of 500 nm down to a few nm. We achieve this simultaneous measurement using an original method that combines a highly sensitive viscosity measurement and a sensitive birefringence measurement. When the LC is sheared in the same direction as the photoalignment (parallel shearing), the order parameter, which is around 0.3 in the bulk state, increases up to around 0.4 at a gap width of less than 50 nm and the viscosity is smaller than half the bulk viscosity. We consider that this increase in the order parameter is due to the highly ordered photoaligned LC layer near the PVCi film, and the viscosity decrease is due to shear thinning of this layer enhanced by both confinement and molecular ordering. In addition, we observe a gradual decrease in viscosity starting at a gap of less than around 300 nm in the parallel shearing. Based on the apparent slip model, we show that the LC layer near the PVCi film can also cause this gradual viscosity decrease. In contrast, when the LC is sheared in the direction perpendicular to the photoalignment direction (perpendicular shearing), the viscosity increases as the gap decreases. We speculate that this is due to the rotational motion of the LC molecules caused by the competing effect of shear alignment and photoalignment. We believe our findings can significantly contribute to a better understanding of the confined LCs utilized for lubrication.
在将液晶用作润滑剂和润滑添加剂的背景下，本研究考察了受限在纳米级间隙宽度内的液晶在剪切和光取向作用下的各向异性剪切粘度。光取向是通过在基板上涂覆各向异性二聚化的聚乙烯基肉桂酸酯（PVCi）薄膜来实现的。我们同时测量了受限在500纳米至几纳米间隙范围内并受到剪切作用的液晶（4-氰基-4'-戊基联苯）的粘度和有序参数。我们采用一种将高灵敏度粘度测量和灵敏双折射测量相结合的原始方法来实现这种同时测量。当液晶在与光取向相同的方向上剪切（平行剪切）时，在本体状态下约为0.3的有序参数在间隙宽度小于50纳米时增加到约0.4，并且粘度小于本体粘度的一半。我们认为这种有序参数的增加是由于PVCi薄膜附近高度有序的光取向液晶层，而粘度降低是由于这种层在受限和分子有序化的共同作用下增强的剪切变稀效应。此外，我们观察到在平行剪切中，从间隙小于约300纳米开始粘度逐渐降低。基于表观滑移模型，我们表明PVCi薄膜附近的液晶层也会导致这种粘度逐渐降低。相反，当液晶在垂直于光取向方向的方向上剪切（垂直剪切）时，粘度随着间隙减小而增加。我们推测这是由于剪切取向和光取向的竞争效应导致液晶分子的旋转运动。我们相信我们的发现能够显著有助于更好地理解用于润滑的受限液晶。
