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Threshold phenomena in soft matter.

机译：软物质中的阈值现象。

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摘要

Although two different fields are covered, this thesis is mainly focused on some threshold behaviors in both liquid crystal field and fluid dynamic systems.; A method of rubbed polyimide is used to obtain pretilt. Sufficiently strong rubbing of a polyimide (SE-1211) results in a large polar pretilt of liquid crystal director with respect to the homeotropic orientation. There exists a threshold rubbing strength required to induce nonzero pretilt. For the homologous liquid crystal series alkyl-cyanobyphenyl, we found that the threshold rubbing strength is a monotonic function of the number of methylene units. A dual easy axis model is then used to explain the results.; Freedericksz transition measurements have been used to determine the quadratical and quartic coefficients associated with the molecules' tilt with respect to the layer normal in surface-induced smectic layers in the nematic phase above the smectic-A-nematic phase transition temperature. Both the quadratic and quartic coefficients are consistent with the scaling relationship as predicted in theory, and their ratio is approximately constant.; A Rayleigh-Taylor instability experiment is performed by using a magnetic field gradient to draw down a low density but highly paramagnetic fluid below a more dense fluid in a Hele-Shaw cell. When turning off the magnetic field, the RT instability occurs in situ and the growth of the most unstable wavevector is measured as a function of time. The wavelength of the RT instability along with the growth rate was measured as a function of capillary number (which is related to the density difference and interfacial tension between two fluids). A theory for the instability that permits different viscosities for two immiscible fluids was developed, and good agreement was found with the experimental results. The technique of magnetic levitation promises to broaden significantly the accessible parameter space of gravitational interfacial instability experiments.; A method is introduced to control initial conditions for a Rayleigh-Taylor instability, wherein a dense fluid sits metastably atop a less dense fluid, a configuration that can be stabilized using a magnetic field gradient when one fluid is highly paramagnetic. On switching off the magnetic field, the instability occurs as the dense fluid falls under gravity. By affixing appropriately shaped magnetically-permeable wires to the outside of the cell, arbitrarily-chosen and well-controlled initial perturbations are imposed on the interface. Using this technique, the perturbation amplitudes, growth rates, nonlinear growth coefficients, and terminal velocities are obtained to examine both the linear and nonlinear growth regimes.
机译：尽管涵盖了两个不同的领域，但本文主要关注液晶领域和流体动力学系统中的一些阈值行为。使用摩擦的聚酰亚胺的方法获得预倾角。聚酰亚胺（SE-1211）足够强力的摩擦会导致液晶指向矢相对于垂直取向的极性大倾斜。存在诱导非零预倾角所需的阈值摩擦强度。对于同源液晶系列烷基-氰基亚苯基，我们发现阈值摩擦强度是亚甲基单元数的单调函数。然后使用双易轴模型来解释结果。 Freedericksz跃迁测量已用于确定与分子相对于近晶-A-向列相转变温度以上的向列相中的表面诱导近晶层中的层法向层的分子倾斜相关的二次和四次系数。二次和四次系数都与理论上预测的比例关系一致，并且它们的比率近似恒定。通过使用磁场梯度将Hele-Shaw电池中密度较低但高度顺磁的流体吸引到密度更高的流体之下，进行了Rayleigh-Taylor不稳定性实验。当关闭磁场时，RT不稳定性会原位发生，并且最不稳定的波矢的增长将作为时间的函数进行测量。 RT失稳的波长以及增长率与毛细管数的函数有关（与两种流体之间的密度差和界面张力有关）。建立了允许两种不混溶流体具有不同粘度的不稳定性理论，并与实验结果很好地吻合。磁悬浮技术有望大大拓宽重力界面失稳实验的可及参数空间。引入了一种控制瑞利-泰勒不稳定性初始条件的方法，其中稠密流体平稳地位于密度较小的流体之上，当一种流体是高度顺磁性时，可以使用磁场梯度来稳定这种配置。在关闭磁场时，当稠密的流体在重力作用下掉落时会发生不稳定性。通过将适当形状的导磁丝固定到单元的外部，可以在界面上任意选择和控制良好的初始扰动。使用这种技术，获得了摄动幅度，增长率，非线性增长系数和最终速度，以检验线性和非线性增长方式。

著录项

  • 作者

    Huang, Zhibin.;

  • 作者单位

    Case Western Reserve University.;

  • 授予单位 Case Western Reserve University.;
  • 学科 Physics Condensed Matter.; Physics Fluid and Plasma.
  • 学位 Ph.D.
  • 年度 2008
  • 页码 187 p.
  • 总页数 187
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 等离子体物理学;
  • 关键词

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