No description available.

Tensile properties, namely the elastic modulus, tensile strength, percent of elongation at yield and at the break were determined for the pure components and blends. The results are connected to the respective phase diagrams and demonstrate that blending makes property manipulation possible. Blends for which the mechanical properties are better than those of pure EPs can be obtained.

In this thesis we study the physics of colloidal dispersions in active and passive liquid crystals by computer simulations. Liquid crystals are materials that exhibit long-range orientational order, with characteristics intermediate between the ones of simple, isotropic fluids and the ones of crystalline solids. Active fluids are suspensions of particles that continuously stir their ambient fluid. Like liquid crystals, active fluids undergo phase transitions to orientationally ordered phases. The framework that we apply here to describe them extends hydrodynamic equations for liquid crystals to the active case, in which their constituent particles exert local stresses on the simple fluid in which they are embedded. Studying systems of colloids embedded in these materials can be done with multiple aims. Here we use colloids as probe particles to investigate the rheological properties of active nematics. To do so we apply a constant force to a spherical particle embedded therein and define an effective viscosity, which we determine by measuring the velocity in steady state. We find an important dependence of the effective viscosity on the size of the particle, and a regime characterised by a steady state of negative drag. We also consider collective properties for systems of many colloids and analyse how they are affected by activity. We find that spontaneous flow can either hinder or favour colloidal aggregation, depending mainly on whether a fixed orientation of the liquid crystal is imposed close to the colloidal surface. This remains true independently of the initial condition chosen for the liquid crystal, which only affects the transition to spontaneous flow.

The growth of bacterial colonies is a very ubiquitous phenomenon occurring in nature and observed in the laboratories. However, there is a limited knowledge on how at the microscopic level these colonies develop and the individual cells spatially organize. In this thesis, we experimentally investigate the physics of growing microcolonies at the level of the individual Escherichia coli (E. coli ) cells, focussing on the order-disorder evolution and the understanding of it in the context of active matter. Bacterial cells are indeed constantly transducing energy from the environment to move and grow, therefore they behave as active microscopic units, defining an inherently far from equilibrium system. In Part I, we present a brief summary of passive liquid crystals that provide us with some basic concepts and tools for investigating the bacterial microcolony evolution. Then an overview of the biology of E. coli cell is given, both as part of multicellular structures (biofilm) and as individuals. Active matter is then discussed along with some examples of active nematics. This first part ends with the materials and methods used in the experiments and analysis. In Part II, we provide our experimental results on the study of growing bacterial microcolonies as active nematics. We describe the way a bacterial microcolony evolves from the first mother cell into a layer of hundreds of cells, and we study the global and local orientational order. We find that a transition from an anisotropic to an isotropic phase occurs as the colony increases and that instabilities and topological defects develop, in analogy to the case of an active nematic. We also compare the real system with simulated ones by investigating (i ) the case of equilibrated configurations with respect to experimental nonequilibrium ones, and (ii ) long-time behaviour of nonequilibrium analogues. In Part III, we discuss the buckling of bacterial microcolonies, that is, the transition from a 2D layer of cells to a 3D structure. We investigate the link between the buckling event and the presence of topological defects in the nematic map of the bacterial microcolony, finding that the buckling sites tend to be closer to topological defects with a negative charge. Later, we present some results of buckling in microcolonies composed of mutants lacking some appendages that play a role in the motion in and attachment to the surrounding environment, finding that buckling occurs at earlier times in the case of these mutants than the wild type. The aim of this work is to show that a growing bacterial microcolony is an interesting model of active matter to experiment on, and that the investigation tools developed for the study of liquid crystals can be useful for describing the evolution of these living systems in mechanistic terms, and for improving the current understanding of nonequilibrium phenomena.

Um novo método de medida (SAM -Spectral Analysis Method) mostra que, devído a um erro sistemático tradicionalmente desprezado, os valores medidos de polarização espontânea do cristal líquido ZLI-3654 (Merck) não seguem o que é previsto pela teoria de Landau das transições contínuas. Porém, a variação da susceptibilidade elétrica relativa com a temperatura proporcional à (T Tc) IND.-1, onde Te é a temperatura onde ocorre o máximo da susceptibilidade ...... está de acordo com o modelo de Landau. A resposta em freqüência da constante dielétrica mostra que, a fim de minimizar a corrente iônica, as medidas de polarização elétrica devem ser realizadas em frequências acima de 20 Hz. Por sua vez, os valores dos ângulos de fase dos harmônicos da polarização, obtidos pelo novo método, indicam que a intensidade do campo elétrico aplicado deve ser inferior a 1, 1 x 10 IND.6 Vrms/m (em 23Hz) para que não ocorram instabilidades elctrohidrodinâmicas, Finalmente, o ajuste das amplitudes dos harmônicos da polarização elétrica obtidos pelo SAM aos parâmetros da equação de Landau-Kalatnikov mostra um bom acordo dos valores medidos com o modelo, em contraste com os valores de polarização espontânea, A solução da equação de equilíbrio, levando em consideração os parâmetros ajustados, mostra como seriam os valores da polarização espontânea sem os erros sistemáticos. / A new proposed measurement method (SAM - Spectral Analysis Method) shows that, due a traditionally neglected systematic error, the values of the measured spontaneous polarization cf the ferroeleotric liquid crystal ZLI-3654 (Merck) doesn\'t follow what is predicted by the Landau theory of continuous transition. In contrast, the decay of the relative electric susceptibility with the temperature (T) - proportional to (T Tc)-l, where Tc is at the maximum of the susceptibility - is in good agreement with the Landau model. The frequency response of the dielectric constant shows that the measurements of the electric polarization of the sample must be done at frequencies hígher than 20Hz to avoid the actíons of the ionic currents. The phase values of the electric polarization harmonics obtained by the SAM indicates that the electric field must be lower thau 1,1 x l06Vrms/m (at 23Hz) to avoid electrohydrodynamic instabilities, Finally, the fitting of the amplitudes of the electric polarization harmonics obtained by the SAM to the parameters of the Landau-Kalatnikov equation shows a good agreement with the model, in contrast with the spontaneous polarization measurements. The solution of the Landau model, considering the fitted parameters, predicts how should be the spontaneous polarization values without the systematic errors.

Self-assembly of nanomaterials over macroscopic dimensions and development of novel nano-electromechanical systems (NEMS) hold great promise for numerous nanotech applications. However, it has always been a great challenge to find a general route for controlled self-assembly of nanomaterials and generating electromechanical response at the nanoscale level. This work indicates that self-organized anisotropic nematic liquid crystals (LC) can be exploited for nanotemplating purposes to pattern carbon nanotubes (CNTs) and Quantum dots (QDs) over a macroscopic dimension. The pattern formed by the CNTs or QDs can be controlled by applying external electric and magnetic fields, developing novel nano-electromechanical and nano-magnetomechanical systems. Self-organizing nematic liquid crystals (LC) impart their orientational order onto dispersed carbon nanotubes (CNTs) and obtain CNT-self-assembly on a macroscopic dimension. The nanotubes-long axis, being coupled to the nematic director, enables orientational manipulation via the LC nematic reorientation. Electric field induced director rotation of a nematic LC+CNT system is of potential interest due to its possible application as a nano-electromechanical system. Electric field and temperature dependence of dielectric properties of an LC+CNT composite system have been investigated to understand the principles governing CNT-assembly mediated by the LC. In the LC+CNT nematic phase, the dielectric relaxation on removing the applied field follows a single exponential decay, exhibiting a faster decay response than the pure LC above a threshold field. Due to a strong LC-CNT anchoring energy and structural symmetry matching, CNT long axis follows the director field, possessing enhanced dielectric anisotropy of the LC media. This strong anchoring energy stabilizes local pseudo-nematic domains, resulting in nonzero dielectric anisotropy in the isotropic LC phase. These anisotropic domains respond to external electric fields and show intrinsic frequency response. The presence of these domains makes the isotropic phase electric field-responsive, giving rise to a large dielectric hysteresis effect. These polarized domains maintain local directors, and do not relax back to the original state on switching the field off, showing non-volatile electromechanical memory effect. Assembling quantum dots (QDs) into nanoscale configurations over macroscopic dimensions is an important goal to realizing their electro-optical potential. In this work, we present a detailed study of a pentylcyanobiphenyl liquid crystal (LC) and a CdS QD colloidal dispersion by probing the dielectric property  and relaxation as a function of an applied ac-electric field Eac. In principle, dispersing QDs in a nematic LC medium can direct the dots to align in nearly one-dimensional chain-like structures along the nematic director and these assemblies of QDs can be directed by external electric fields. In a uniform planar aligned cell, the Fréedericksz switching of the LC+QDs appears as a two-step process with the same initial switching field as the bulk but with the final value larger than that for an aligned bulk LC. The relaxation of  immediately following the removal of Eac follows a single-exponential decay to its original value that is slower than the bulk but becomes progressively faster with increasing Eac, eventually saturating. These results suggest that the arrangement of the QDs is mediated by the LC.

No description available.

Song Liang = 向列液晶中激光束感应的取向重联非线性光学效应和取向性光折变光栅形成 / 宋亮. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2006. / Includes bibliographical references. / Text in English; abstracts in English and Chinese. / Song Liang = Xiang lie ye jing zhong ji guang shu gan ying de qu xiang zhong lian fei xian xing guang xue xiao ying he qu xiang xing guang zhe bian guang zha xing cheng / Song Liang. / Abstract --- p.i / 论文摘要 --- p.iii / Acknowledgement --- p.iv / Contents --- p.v / Chapter Chapter 1 --- Introduction --- p.1 / References --- p.5 / Chapter Chapter 2 --- Brief review of liquid crystals and nonlinear optics of liquid crystals --- p.9 / Chapter 2.1 --- Brief review of liquid crystals --- p.9 / Chapter 2.1.1 --- General description --- p.9 / Chapter 2.1.2 --- Types of liquid crystals --- p.11 / Chapter 2.1.3 --- Order parameter --- p.14 / Chapter 2.1.4 --- Liquid crystal alignment --- p.14 / Chapter 2.1.5 --- Continuum theory of liquid crystals --- p.16 / Chapter 2.2 --- Nonlinear optics of liquid crystals --- p.20 / Chapter 2.2.1 --- Basic mechanism of nonlinear optics --- p.20 / Chapter 2.2.2 --- Self-phase modulation --- p.23 / Chapter 2.2.3 --- Orientational photorefractive effects --- p.29 / Chapter 2.3 --- Sample preparation --- p.34 / Chapter 2.4 --- Conclusion --- p.37 / References --- p.39 / Chapter Chapter 3 --- Domain formation in homeotropic nematic liquid crystal cell --- p.42 / Chapter 3.1 --- Introduction --- p.42 / Chapter 3.2 --- Domain formation under applied dc and ac electric fields --- p.44 / Chapter 3.3 --- Light scattering of domains under applied dc and ac electric fields --- p.49 / Chapter 3.4 --- Conclusion --- p.53 / References --- p.54 / Chapter Chapter 4 --- Laser induced self-phase modulation and effects of applied electric field --- p.56 / Chapter 4.1 --- Introduction --- p.56 / Chapter 4.2 --- Self-phase modulation without applied electric field --- p.58 / Chapter 4.2.1 --- Experimental setup --- p.58 / Chapter 4.2.2 --- Experimental results and discussions --- p.59 / Chapter 4.3 --- Effects of applied electric field --- p.67 / Chapter 4.3.1 --- Effects of applied dc electric field --- p.67 / Chapter 4.3.2 --- Effects of applied ac electric field --- p.72 / Chapter 4.4 --- Conclusion --- p.75 / References --- p.77 / Chapter Chapter 5 --- Orientational photorefractive grating in nematic liquid crystals --- p.80 / Chapter 5.1 --- Introduction --- p.80 / Chapter 5.2 --- Orientational photorefractive grating under dc electric field --- p.82 / Chapter 5.2.1 --- Experimental setup --- p.82 / Chapter 5.2.2 --- Experimental results and discussion --- p.84 / Chapter 5.3 --- Orientational photorefractive grating under ac electric field --- p.90 / Chapter 5.3.1 --- Raman-Nath grating --- p.90 / Chapter 5.3.2 --- Quasi-Bragg grating --- p.96 / Chapter 5.4 --- Conclusion --- p.98 / References --- p.100 / Chapter Chapter 6 --- Summary and future outlook --- p.103 / List of Publications and Conference Presentations --- p.107

本論文致力於棒狀液晶體動力學系統的幾個方面的研究，其中包括:空間週期問題及初邊值問題弱解的整體存在性，初值問題及初邊值問題強解的局部存在唯一性及相應的爆破準則。 / 首先，借助於一類新的逼近系統，我們證明了非等溫液晶體系統的二維空間週期問題存在整體弱解。與經典的Ginzburg Landau逼近系統不同的是，我們所採用的系統不是奇異型逼近。由於無法從基本能量等式中獲得方向場的二階導數的估計，我們採用局部能量不等式，於局部時間內，重新獲得了這些估計。與等溫系統不同，我們所獲得的弱解保持整體能量不變。由此可以看出，系統所損失的內能和動能全部轉化為熱能。此外，我們所獲得的弱解至多含有限個奇異時間點。而且，在每個奇異時間點上，由於能量集中現象的發生，系統的溫度必然會在某些區域內突然升高。 / 其次，通過Ginzburg- Landau逼近，我們證明了三維Ericksen-Leslie系統的初值問題存在唯一的局部強解，並建立了相應的爆破準則。我們共建立了如下四種爆破準則: (i) Serrin型準則; (ii) Beale-Kato-Majda (縮寫為BKM) 型準則; (iii) 混合型準則，即對速度場和方向場之一提Serrin型條件，而對另一場提BMK型條件;(iv) 一個新型準則，即用Ginzburg-Landau逼近系統的強解的Serrin型範數來刻畫Ericksen- Leslie 系統的強解的最大存在區間。其中，借助於一新的對數型Sobolev不等式，我們對速度場所建立的BKM型條件是經典條件的一個BMO型弱化。此外，我們還證明了，于強解的存在區間內， Ginzburg-Landau系統強收斂至Ericksen- Leslie 系統。 / 然後，我們將我們的關於強解的局部存在唯一性的結果推廣至有界域的情形。我們採用的依舊是Ginzburg-Landau逼近。為克服由於缺乏方向場的法向三階導教估計而帶來的困難，我們採用了一新的Sobolev型嵌入不等式。該嵌入不等式將Sobolev函數嵌入至一適當的混合範教LP 空間。此外，我們亦證明了局部強解的相應爆破準則 。 / 最後，通過利用前述提及的技街，我們證明了二維有界域上Ericksen-Leslie 系統的初邊值問題存在唯一的局部強解並存在一整體弱解。特別地，通過利用局部能量不等式，我們證明了強解的存在區間以及局部時間內的估計只依賴於初值的基本能量及其L²積分連續性。借助於此，通過對強解取極限，我們獲得了Ericksen-Leslie 系統的弱解。 / This thesis is devoted to some studies on the hydrodynamical model of nematic liquid crystals, including: the global existence of weak solutions to the spacial periodic and the initial-boundary value problems, the local well-posedness and blow-up criteria of strong solutions to the Cauchy and the initial-boundary value problems. / First, we prove the global existence of weak solutions to the non-isothermal nematic liquid crystal system in T², based on a new approximate system. Different from the classic Ginzburg-Landau approximation, this new system is not a singular type one. Local energy inequalities are employed to recover the estimates on the second order spacial derivatives of the director fields locally in time, which cannot be derived from the basic energy balance. Different from the isothermal case, the weak solutions we obtained conserve the total energy, and thus the kinetic and potential energies transfer to nothing but the heat energy. Also our weak solutions have at most finite many singular times at which the energy concentration occurs, and as a result, the temperature must increase suddenly at each singular time on some part of T². / Next, we prove the local well-posedness and blow-up criteria of strong solutions to the Ericksen-Leslie system in R³, based on the Ginzburg-Landau approximation. Four kinds of blow-up criteria are established, including: (i) the Serrin type; (ii) the Beale-Kato-Majda (BKM for short) type; (iii) a mixed type, i.e., a Serrin type condition for one field and a BKM type condition on the other one; (iv) a new one, which characterizes the maximal existence time of a strong solution to the Ericksen-Leslie system in terms of the Serrin type norms of the strong solutions to the Ginzburg-Landau approximate system. Besides, thanks to a new logarithmic Sobolev type inequality, our BKM type condition for the velocity is a BMO type improvement version. We also show that the strong solutions of the Ginzburg-Landau approximate system converge to a strong solution of the Ericksen-Leslie system up to the maximal existence time of this solution. / Then, we generalize our results on the local well-posedness of strong solutions to the Ericksen-Leslie system for the whole space to bounded domains of R³, still by the Ginzburg-Landau approximation method. A new Sobolev embedding inequality into mixed-norm L{U+1D3E} space is exploited to overcome the difficulty caused by the lack of the uniform estimates on the third order normal derivative of the director field to the Ginzburg-Landau approximate system. We also establish a blow-up criterion of the local strong solutions to the Ericksen-Leslie system. / Finally, using the technics exploited in the previous results, we obtain the local existence of strong solutions and the global existence of weak solutions to the Ericksen-Leslie system in bounded domains of R². In particular, by employing the local energy inequality, we prove that the lower bound of the existence time and the local in time estimates of a strong solution depend only on the basic energy and the L² integral continuity of the initial data. Thanks to these properties, by taking the limit of a sequence of strong solutions, we obtain a weak solution to the Ericksen-Leslie system. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Li, Jinkai. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2013. / Includes bibliographical references (leaves 179-185). / Abstracts also in Chinese. / Introduction --- p.1 / Chapter 1 --- Global existence of weak solutions to the nematic liquid crystals in T² --- p.14 / Chapter 1.1 --- Introduction and main results --- p.14 / Chapter 1.2 --- Preliminaries --- p.21 / Chapter 1.3 --- Faedo-Galerkin scheme --- p.24 / Chapter 1.4 --- The limit n → ∞ --- p.27 / Chapter 1.5 --- The limit M → ∞ --- p.33 / Chapter 1.6 --- The limit N → ∞ and the local existence --- p.47 / Chapter 1.7 --- The global existence --- p.51 / Chapter 2 --- Local well-posedness and blow-up criteria of strong solutions to the Ericksen-Leslie system in R³ --- p.60 / Chapter 2.1 --- Introduction and main results --- p.60 / Chapter 2.2 --- Local existence --- p.67 / Chapter 2.3 --- Blow-up criteria --- p.83 / Chapter 2.4 --- Convergence of Ginzburg-Landau to Ericksen-Leslie --- p.99 / Chapter 3 --- Local well-posedness and blow-up criteria of strong solutions to the Ericksen-Leslie system in bounded domains of R³ --- p.112 / Chapter 3.1 --- Introduction and main results --- p.112 / Chapter 3.2 --- Local well-posedness --- p.119 / Chapter 3.3 --- Blow-up criteria --- p.141 / Chapter 4 --- Global existence of weak solutions to the Ericksen-Leslie system in bounded domains of R² --- p.156 / Chapter 4.1 --- Introduction and main results --- p.156 / Chapter 4.2 --- Strong solutions and blow-up criteria --- p.161 / Chapter 4.3 --- Global weak solutions --- p.172 / Chapter 5 --- Discuss on future works --- p.177 / Bibliography --- p.178

No description available.