Introductory Chapter: Liquid Crystals and Applications

1. Introduction

Liquid crystalline state is the one having the physical properties that fall in between conventional crystalline (solid) and isotropic fluid (liquid). Liquid crystals have been of great interest owing to the amazing physical and chemical properties these exhibit, thereby proving to be potentially useful in versatile technological applications [1]. Though liquid crystals may flow like a liquid, the molecular orientations of them may be of the kind that a solid crystal possesses. Furthermore, liquid crystals may exist naturally or these can also be synthesized [2, 3]. Within the context, the lyotropic phase of liquid crystal can be abundantly found in living organisms, such as proteins and cell membranes. Synthesized forms of liquid crystals are widely used in display applications [4]. The use of liquid crystals in display technology stems from the nature of chirality and the high electro-optic coefficient of these mediums, thereby making them significantly advantageous. Apart from display-related applications, liquid crystals are greatly attractive for several kinds of sensing applications as well. Some other applications would be in the areas of lasers [5] and medical diagnostics [6]. However, the flat panel display is the most recognized device where liquid crystals are widely used. Apart from the display panels, there are a host of other avenues where the synthesized versions of liquid crystals are indispensable.

2. Liquid crystal types and properties

As reported vastly, the nematic, smectic, and cholesteric are the three basic phases of liquid crystals classified according to the molecular orientations [7]. Apart from this, liquid crystals are also technically classified into the thermotropic [3, 8] and lyotropic categories [9], distinguished by the mechanisms responsible for their self-organization. These two kinds of liquid crystals generally do have some similarities in their physical and chemical properties, in spite of the fact that the chemical structures of their molecules greatly differ. The thermotropic class of liquid crystals undergo thermally induced transitions to the liquid crystalline state [3]. As such, raising the temperature of a solid and/or lowering the temperature of a liquid can result in a thermotropic kind of liquid crystals. On the other hand, the lyotropic class of liquid crystals exhibits solvent-induced transitions [9], and therefore, they are amphiphilic in nature. That is, such liquid crystals are composed of both the lyophilic (solvent-attracting) and lyophobic (solvent-repelling) kinds of mediums.

Describing the nematic, smectic, and cholesteric phases of liquid crystals, as stated before, they are categorized by the kind of molecular orientation they possess. In the nematic state, the molecules acquire no positional order, but they are aligned along the director, which represents the direction of molecular orientation. Thus, the molecules in the nematic phase tend to be parallel, but their positions are just random. In the smectic phase, the liquid crystal molecules show a translational order, which is not found in the nematic phase. Therefore, the molecules in this state are aligned parallel, but their centers are stacked in parallel layers within which they acquire random positions. This essentially indicates the restriction imposed over the motion of liquid crystal molecules in the smectic phase, which is confined to the planes in which they are aligned. The molecules of liquid crystals in the cholesteric phase show helicoidal orientation, which makes this phase as a distorted kind of the nematic state, wherein the molecular orientation undergoes helical rotation about an axis. Clearly, the director in the cholesteric phase of liquid crystals exhibits a continuous helical pattern about the axis perpendicular to the two-dimensional nematic layers (of molecules). Within the context, chirality of cholesteric liquid crystals is of great potential in developing many electro-optic components, such as sensors and display panels.

Liquid crystals are anisotropic in nature, that is, these exhibit directional properties. This makes them acquire the birefringence characteristic [10]. Therefore, in liquid crystals, the light-polarized parallel to the director experiences a different refractive index than that polarized perpendicular to the director. This results in the propagation of light waves (in these mediums) in two different directions with different phase velocities, and the respective propagating waves are called the fast and the slow waves (or the ordinary and the extraordinary waves, respectively). Because of splitting of the incoming wave, the ordinary and extraordinary waves propagate with different phase velocities in the birefringent medium, and therefore, the waves become out of phase. Furthermore, the anisotropic characteristic of liquid crystals depends on the ambient temperature, and therefore, the property of birefringence (in liquid crystals) can be eliminated at the nematic-to-isotropic phase transition owing to the altering thermal ambience.

3. Applications of liquid crystals

Due to their possessing very high electro-optic coefficients, they are of great potential in many photonic applications [11]. However, till today liquid crystals find the major applications in flat panel electronic displays (or the LCDs) [12] as these offer several advantages over traditional displays comprising the cathode ray tubes (CRTs). Some of the notable advantages of LCD would be low power consumption and significantly less weight. At the same time, LCDs suffer from the drawback of limited viewing angle and relatively shorter lifetime.

Apart from the LCD panels, liquid crystals are used in many other photonic applications. For example, the chiral nematic phase of liquid crystal can selectively reflect light, which essentially depends on the wavelength of light and the pitch of the helix that the director assumes. To be more specific, if the wavelength of the incoming light is equal to the helix pitch, it will be reflected by the medium. This way, the cholesteric phase of liquid crystals can be used to develop optical filters and imaging systems [13, 14]. Since the cholesteric state is highly temperature dependent, variation in thermal ambience would modify the orientation of director between the successive layers (of the liquid crystal). This, in turn, modifies the length of helix pitch of the director, thereby altering the selectivity of the reflection spectrum. This operation can be utilized in devising thermal sensors [15].

Because of the large electro-optic coefficient of liquid crystals, these are of great potential in electromagnetic field sensing [7]. Upon applying an electric field, the director of the liquid crystal structure would be aligned along the applied field. In photonic applications, these are of special mention in the context of evanescent field sensing as the use of liquid crystals makes the field confinement in the outermost section of a liquid crystal clad-based fiber/guide very large, which can be affected in the presence of a measurand [16]. Within the context, the radially anisotropic liquid crystals have been greatly dealt with in the literature focusing on the analyses of the light wave propagation through optical waveguides composed of such mediums and having different kinds of geometrical features [17, 18, 19]. Liquid crystal-based guides with conducting sheath and tape helixes are new of their kinds, which provide control over the dispersion properties, thereby allowing the on-demand kinds of applications [20, 21]. It must be noted that the liquid crystal-based guides have promising potential in developing sensors in the electrical and chemical industries.

4. Summary

Liquid crystals are of great potential in varieties of technology-oriented applications. Apart from the most recognized usage in the development of display panels, these are also useful in many other applications that include optical imaging and recording, erasable optical disks, electronic slides, light modulators, lasers, etc. Research reports also indicate the use of liquid crystals in biomedical applications and metamaterial-based reconfigurable antennas. R&D scientists have been involved in investigating the properties of varieties of artificially synthesized liquid crystals with the positive thought of finding prudent and novel applications of these.

Finally, the book provides the basic understanding of liquid crystals and also the advanced discussions on phase transitions, which would make a fairly good platform for applications of liquid crystals in developing sensors. It is believed the volume would be useful to the undergraduate students in framing their research topics in the relevant direction.