Liquid crystals flow characteristics

Liquid crystals are interesting supramolecular systems which can show second harmonic generation when they are aligned appropriately. Ferroelectric LCs [250] as well as bent-core molecules have been used to this purpose, and show reasonable second harmonic generation [251]. These materials combine non-linear optical effects with simple processing procedures on account of their liquid crystalline flow characteristics and the possibility of organising them with electric and magnetic fields. 

Electro-optic The liquid crystal plastics exhibit some of the properties of crystalline solids and still flow easily as liquids (Chapter 6). One group of these materials is based on low polymers with strong field interacting side chains. Using these materials, there has developed a field of electro-optic devices whose characteristics can be changed sharply by the application of an electric field. 

Liquid crystals have a degree of order characteristic of solid crystals, but they can flow like viscous liquids. They are mesophases, intermediate between solids and liquids their properties can be modified by electric fields and changes in temperature. 

On a molecular level the director is not rigorously defined, but the molecular director is typically considered to be the average long axis of the molecules, oriented along the macroscopic director with some order parameter less than one. This type of anisotropic order is often called long-range orientational order and, combined with the nonresonant optical properties of the molecules, provides the combination of crystal-like optical properties with liquidlike flow behavior characteristic of liquid crystals. 

Different types of liquid crystals exhibit different rheological properties [16,17]. With an increase in organization of the microstructure of the liquid crystal its consistency increases and the flow behavior becomes more viscous. The coefficient of dynamic viscosity r, although a criterion for the viscosity of ideal viscous flow behavior (Newtonian systems), is high for cubic and hexagonal liquid crystals but fairly low for lamellar ones. However, the flow characteristics are not Newtonian but plastic or pseudoplastic, respectively. 

The rheological and flow properties of ordered block copolymers are extraordinarily complex these materials are well-deserving of the apellation complex fluids. Like the liquid-crystalline polymers described in Chapter 11, block copolymers combine the complexities of small-molecule liquid crystals with those of polymeric liquids. Hence, at low frequencies or shear rates, the rheology and flow-alignment characteristics of block copolymers are in some respects similar to those of small-molecule liquid crystals, while at high shear rates or frequencies, polymeric modes of behavior are more important. 

The liquid crystal state (LCS) shows order in one or two dimensions it lacks the three-dimensional long-range order of the crystalline state. LCS has characteristics intermediate between those of the crystalline and the disordered amorphous states. These phases are called liquid crystals because many of them can flow like ordinary liquids but they display-birefringence and other properties characteristic of crystalline soHds. In liquid crystal phases the molecules can move but the orientational order is conserved in at least ne direction. The LCS can be displayed by small molecules and by polymersj but in both cases a characteristic chemical structure is needed. The existence of the liquid crystal state is related to the molecular asymmetry and the presence of strong anisotropic intermolecular interactions (19-21). Thus, molecules with a rigid rod structure can form highly ordered... 

In order to show how these lamps are used, it is necessary to describe liquid crystal displays (LCD). We will not describe this technology thoroughly since it does not directly relate to the use of phosphors in the display, except in the fluorescent lamps used for "backlighting" the LCD. A "liquid crystal"- (LC) is an organic compound whose molecules align themselves with an applied electric field. An LC substance flows as a liquid but maintains some of the ordered structure characteristic of a crystal. 

Figure 2 shows the basic physical idea of the microstructure of the continuum rheologicS model we proposed earlier (2). The layers can be idealized as separated by porous slabs, which are connected by elastic springs. Liquid crystals may flow parallel to the planes in the usual Newtonian manner, as if the slabs were not there. In the direction normal to the layers, liquid crystals encounter resistance through the porous medium, proportional to the normal pressure gradient, which is known as permeation. The permeation is characterized by a body force which in turn causes elastic compression and splay of the layers. Applied strain from the compression causes dislocations to move into the sample from the side in order to relax the net force on the layers. When the compression stops and the applied stress is relaxed the permeation characteristic has no influence on stress strain field. 

The exact scale-up of crystallizers is not possible because it would be necessary to preserve similar flow characteristics of both liquid and solid phases together with identical temperatures and supersaturations in all equivalent regions. The scale-up of simple agitated vessels containing a liquid phase alone has long been recognized as a difficult problem. The two dimensionless numbers most frequently encountered in the analysis of stirrers and agitators are the Reynolds number, Re, and the Froude number, Fr ... 

The fibrillation of LCPs in thermoplastic melts is influenced by several parameters, including the thermal characteristics of the component polymers and their compatibility, and processing parameters such as viscosity ratio, melt temperature, flow mode, and shear rate. Among these parameters, thermal characteristics of LCPs are basic factors for the formation of LCP fibrils. They should have the matched processing window with the matrix resin, when the former is in the liquid crystal state. It has been found that the spinnability of LCPs can be taken as a prerequisite for the accomplishment of submicrometer reinforcing with LCP fibrils [18]. 

This expression yields a surface relaxation time (Ys/ s the bulk, the second derivative of orientation with respect to z replaces the simple direction orientation difference In the surface term. Ignoring flow, the bulk relaxation has a characteristic time where h Is the liquid crystal film... 

The formation of banded textures in thin-film samples of solutions of hquid crystalline polymers (LCPs), subjected to shear, has been reported in the literature since 1979 [15]. Because of the symmetrical properties of the liquid crystal solutions, large domains of weU-oriented polymer chains are formed during shear flow, while defects are squeezed into small regions. The shear accounts for an additional energy stored in the solution. When the shear is stopped, the system will first relax with a characteristic time fb to a transient state. In this state the distortion energy is minimized, and the orientational order is kept, resulting in a banded stmcture. This behavior is observed only if two conditions are fulfilled [16] ... 

The above equations can be related to measurable viscosity coefficients. The coefficients allow one to develop the concepts of stability of the flow in the applied field and have allowed examination of the dynamics of switching of the liquid crystal systems. The above theory illustrates that although the liquid crystals have many facets of liquids they exhibit coupled behaviour that is characteristic of solids. 

The mechanism involved in the formation of the bands is not properly understood, and there is also some contention as to whether they form during shear or on stress relaxation afterwards. However, their appearance under suitable conditions of melt or solution flow is an almost universal characteristic of main-chain rigid liquid crystal polymers. 

Generally speaking, excitation of a medium by short laser pulses can be used to study dynamic properties of the medium over a very wide time range. Here, we have shown that nanosecond-pulse excitation can yield information about the dynamics of molecular reorientation on the -10-sec time scale, and thermal effect on the 10—l(X)-msec time scale. The power of this technique lies in the fact that a single 6-function-like laser pulse may induce a number of fundamental excitation modes of vastly different time constants. Consider, for example, molecular reorientation coupled with flow induct by a picosecond laser pulse in a liquid crystal. It can be shown that, aside from the thermal effect, the transient behavior will manifest itself with three characteristic time constants ... 

Polymer-dispersed liquid crystals (PDLCs) is a relatively new class of promising material for many applications such as, switchable windows, display devices, infrared shutters, angular-discriminating filters, thermoelectrooptic switches, memories, gas-flow sensors, optical sensors, and optical gratings etc. These materials are examples of combined application of polymers and liquid crystals and command the attention of the display industry as well as the researchers. These consist of LC droplets which are dispersed in a polymer matrix. These tiny droplet characteristics are responsible... 

Rheology is one of the most common methods used to analyze the flow characteristics of various materials, including liquid crystal polymers (Mezzenga et al. 2005). There are several types of flow behaviors, which are broadly divided into two main categories (Shaw 2012 Osswald and Rudolph 2014) ... 

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