Liquid crystals ordered, fluid mechanics

Even though liquid crystals are fluids, the fact that orientational order exists ensures that all directions in the fluid are not equivalent. This has a profound effect on all the properties of the phase, producing a complex response to external factors such as electric fields and mechanical distortions. Yet it is this combination of factors, namely the flow properties of fluids and the anisotropic behaviour normally absent in fluids, that makes the behaviour of liquid crystals both intrinsically interesting and ripe for technical applications. 

Liquid crystals are fluids in which there occurs a certain order in the arrangement of the molecules. As a result, there is anisotropy in the mechanical, electrical, magnetic, and optical properties. Although liquid crystals, or mesophases, combine the properties of a solid and an isotropic liquid, they exhibit very specific electrooptical phenomena. As a rule these have no corresponding analogues in solids or in isotropic liquids. 

As its name suggests, a liquid crystal is a fluid (liquid) with some long-range order (crystal) and therefore has properties of both states mobility as a liquid, self-assembly, anisotropism (refractive index, electric permittivity, magnetic susceptibility, mechanical properties, depend on the direction in which they are measured) as a solid crystal. Therefore, the liquid crystalline phase is an intermediate phase between solid and liquid. In other words, macroscopically the liquid crystalline phase behaves as a liquid, but, microscopically, it resembles the solid phase. Sometimes it may be helpful to see it as an ordered liquid or a disordered solid. The liquid crystal behavior depends on the intermolecular forces, that is, if the latter are too strong or too weak the mesophase is lost. Driving forces for the formation of a mesophase are dipole-dipole, van der Waals interactions, 71—71 stacking and so on. 

We note that earlier research focused on the similarities of defect interaction and their motion in block copolymers and thermotropic nematics or smectics [181, 182], Thermotropic liquid crystals, however, are one-component homogeneous systems and are characterized by a non-conserved orientational order parameter. In contrast, in block copolymers the local concentration difference between two components is essentially conserved. In this respect, the microphase-separated structures in block copolymers are anticipated to have close similarities to lyotropic systems, which are composed of a polar medium (water) and a non-polar medium (surfactant structure). The phases of the lyotropic systems (such as lamella, cylinder, or micellar phases) are determined by the surfactant concentration. Similarly to lyotropic phases, the morphology in block copolymers is ascertained by the volume fraction of the components and their interaction. Therefore, in lyotropic systems and in block copolymers, the dynamics and annihilation of structural defects require a change in the local concentration difference between components as well as a change in the orientational order. Consequently, if single defect transformations could be monitored in real time and space, block copolymers could be considered as suitable model systems for studying transport mechanisms and phase transitions in 2D fluid materials such as membranes [183], lyotropic liquid crystals [184], and microemulsions [185],... 

Most liquid crystals are mechanically fluid, but there is some residual orientation (anisotropy) order of the long, rodlike molecules (Fig. 2.15). When these liquid crystals are heated to sufficiently high temperatures, they will pass through the clearing point, where they loose their anisotropy and become an isotropic melt. 

Solidification First order phase transformation, in which a liquid (melt) crystallizes Sol-gel transition Transition of a colloidal solution into a solid network extending through the whole sample space. The network is made up of the colloidal particles attached to each other mechanically and might be bonded by various potentials. At the point of contact so-called necks establish. The sol-gel transition might be considered as exactly that point when a percolating net of particles (a cluster) spans through the wet fluid. See also gelation... 

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