Liquid crystals interfacial properties

In the previous sections, we have seen how computer simulations have contributed to our understanding of the microscopic structure of liquid crystals. By applying periodic boundary conditions preferably at constant pressure, a bulk fluid can be simulated free from any surface interactions. However, the surface properties of liquid crystals are significant in technological applications such as electro-optic displays. Liquid crystals also show a number of interesting features at surfaces which are not seen in the bulk phase and are of fundamental interest. In this final section, we describe recent simulations designed to study the interfacial properties of liquid crystals at various types of interface. First, however, it is appropriate to introduce some necessary terminology. 

Modem scaling theory is a quite powerful theoretical tool (applicable to liquid crystals, magnets, etc) that has been well established for several decades and has proven to be particularly useful for multiphase microemulsion systems (46). It describes not just interfacial tensions, but virtually any thermodynamic or physical property of a microemulsion system that is reasonably dose to a critical point. For example, the compositions of a microemulsion and its conjugate phase are described by equations of the following form ... 

Over the last two decades the exploration of microscopic processes at interfaces has advanced at a rapid pace. With the active use of computer simulations and density functional theory the theory of liquid/vapor, liquid/liquid and vacuum/crystal interfaces has progressed from a simple phenomenological treatment to sophisticated ah initio calculations of their electronic, structural and dynamic properties [1], However, for the case of liquid/crystal interfaces progress has been achieved only in understanding the simplest density profiles, while the mechanism of formation of solid/liquid interfaces, emergence of interfacial excess stress and the anisotropy of interfacial free energy are not yet completely established [2],... 

Emulsion stability is determined by the strength of the interfacial film and the way the adsorbed molecules in it are packed. If the adsorbed molecules in the film are closely packed, and it has some strength and viscoelasticity, it is difficult for the emulsified liquid droplets to break the film. In other words, coalescence is difficult. The emulsion is therefore stable. The molecular structure and the properties of the emulsifiers in the film affect the film s properties. The molecules in the film are more closely packed if the emulsifier has straight chains rather than branched chains. The film strength is increased if mixed emulsifiers are used rather than a single one. The reasons are that (1) the molecules in the film are closely packed, (2) mixed liquid crystals are formed between droplets, and (3) molecular complexes are formed in the interface by emnlsifier compositions. For example, an oil-soluble surfactant mixed with a water-solnble snrfactant works very well to stabilize emulsions (Kang, 2001). 

The presence of a third phase can promote or impair foam stability, and in some cases, even prevent foaming. As mentioned previously, stable foams can be formed from mixtures of an isotropic liquid with a liquid-crystal phase The foam lamellae become covered with layers of liquid-crystal the foam stability is increased through surface viscosity. Foam stability can also be affected by the presence of other dissolved species, an additional liquid phase such as oil in an aqueous foam, or fine solids. In these cases, whether the effect is one of stabilizing or destabilizing depends on several factors. First, it depends on whether or not the third-phase species have a strong affinity for the liquid phase, and therefore whether they tend to accumulate at the gas—liquid interface. Second, once accumulated, any effect they may have on the interfacial properties is important. 

As discussed abfeve, ellipsometry is directly sensitive only to the interfacial variations of the nematic order parameter, which is connected to the optical refraction indices. The interfacial smectic order, which has no direct influence on the optical properties, can only be observed due to its coupling to the nematic (orientational) order. The same experimental setup as described in Sect. 4.1.3 has been used to study the interface between smectic liquid crystal dodecylcyanobiphenyl (12CB) in the isotropic phase and the silanated glass. Although only orientational order is observed, the temperature dependence of pb is in this case quite different from the case with the nematic liquid crystal, as evident from Fig. 4.4. 

Now we are interested in phenomena at an interface between a liquid crystal and another phase (gas, liquid or sohd) [1,2]. Why is it important First, the structure of a liquid crystal in a thin interfacial layer is different from that in the bulk and manifests many novel features. Second, the interface plays a decisive role in applications, because liquid crystals are always used in a cmifined geometry. There are two approaches to the surface problems, microscopic and macroscopic. In the first approach, we are interested in a structure and properties of interfacial liquid crystal layers at the molecular level in the second one, we ignore the microscopic details and use only symmetry properties and the concept of the director. 

Finally we mention a work by Guyot-Sionnest et in which SHG from liquid crystal - glass and liquid crystal - air interfaces were studied. This work demonstrated the value of such studies in the very important field of interfacial properties of liquid crystals. 

In the present volume we discuss a relatively new and rapidly developing branch of the field, namely nonlinear optical effects in liquid crystals. Optical studies have always played a significant role in liquid crystal science. Research of optical nonlinearities in liquid crystals began at the end of the sixties. Since then it became a powerful tool in the investigation of symmetry properties, interfacial phenomena or dynamic behaviour. Furthermore, several new aspects of nonlinear processes were demonstrated and studied extensively in liquid crystals. The subject covered in this book is therefore of importance both for liquid crystal research and for nonlinear optics itself. 

It is possible to create materials with either multi-layered structures, continuously varying mixes of materials, or nanostructures, such that RI varies continuously across an interfacial region rather than at a definite optical interface. These materials, analogies of which are found in nature, offer enhanced optical properties for a number of applications, such as reduced glare from liquid crystal display (LCD) computer monitors and televisions and improved signal-to-noise ratio in photodetectors. 

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