Preface

Liquid crystals have been in the research limelight for the last couple of decades because of the exotic electromagnetic properties of these exhibits, which can be altered under certain external stimuli. In general, these exist in three different phases, namely the nematic, smectic and cholesteric phases, which are distinguished based on the orientation of molecules, termed as the director. Interestingly the property of the director can be altered due to external influence (such as electric/ magnetic fields), which confirms varieties of potential applications of these in our everyday life as well as advanced systems of technological use.

The scope of this book is limited to providing a glimpse of the basic knowledge and properties of liquid crystals followed by the discussions of possible phase transition characteristics of certain kinds of liquid crystalline polymers through simulations. Also, the use of liquid crystals in field sensing and some of the optical applications are also discussed, apart from the basic understanding of the interfacing of liquid crystal displays. To be more specific, Malik et al. in Chapter 2 discuss the fundamentals of liquid crystal mediums emphasizing right from the very basics, such as the molecular patterns in the different phases of this. Also, they put a cursory glance at the advanced classification of liquid crystals. Following these, the authors touch upon some of the novel applications of liquid crystals and the current challenges in the advancement of research in the relevant direction. Overall, this chapter simply provides a glimpse of liquid crystals, which serves as the introduction to these mediums before proceeding to further reading of the advanced research themes in the relevant direction.

The phase of liquid crystals remains the primary factor to determine the physical and/or chemical properties, and this can be altered depending on the external stimuli. In Chapter 3, the authors touch upon the phase transition aspect of liquid crystalline polymer systems under the influence of an external magnetic field using the phase diagrams that categorize their chemical structures. The authors report the process of self-assembly of liquid crystal nanomolecules in solutions exhibits a packet mechanism, which depends on the concentration of the polymer in the solution. Overall, the chapter explains well the effects of magnetic field on the molecular structure of liquid crystals resulting in a phase transition.

Cholesteric liquid crystals exhibit the helical orientation of the director and constitute an extremely important class owing to the chiral behavior that finds wide applications in display and other sensing technologies. In Chapter 4, Méndez et al. explore the synthesis and characterization of cholesteric liquid-crystal polyesteramides exploiting the molecular simulations. Within the context, they perform the conformational and structural analyses of polyesteramides. The results of the phase analyses in the cases of four different possible helical conformers demonstrate fairly good validation of the theoretical understanding.

Chapter 5 by Zixin Zhao focuses on the use of liquid crystal-based spatial light modulators in the generation of an optical wavefront. The nonlinear phase response and static aberration are the factors that affect phase modulation in such systems, which the author studies emphasizing the required calibration, followed by presenting some state-of-the-art methods to estimate the phase modulation implementing a certain form of system configuration. Also, the chapter presents the possibilities for future development of such light modulators in achieving phase modulation.

The most significant use of liquid crystals has been in the construction of different kinds of liquid crystal displays, which find applications in diverse areas of everyday life. Such a display device essentially relies on the principle that the optical properties of liquid crystals (used in the form of certain patterns of the active or passive matrix) greatly depend on the applied voltage and/or current. Chapter 6 of the book focuses on the fundamentals of the interfacing of liquid crystal displays based on the functioning of the liquid crystal matrix, which a display is composed of, and the relevant programming needs. The readers can acquire acquaintances of the underlying facts in the interfacing of such displays.

Finally, the book throws a glimpse of liquid crystals from the very basic understanding of the medium to the discussions of certain phase transition-related issues of these polymers depending on their molecular structure and characterization. Some of the applications of liquid crystals are also touched upon, which essentially remains important in sensing applications under certain external stimuli. The editor believes the undergraduate students would take benefit from it in conceptualizing their own thoughts.

> **Pankaj Kumar Choudhury** Institute of Microengineering and Nanoelectronics, Universiti Kebangsaan Malaysia, Bangi, Malaysia

## **Abdel-Baset M.A. Ibrahim**

Faculty of Applied Sciences, Universiti Teknologi MARA (UiTM), Shah Alam, Malaysia

## **Chapter 1**

## Introductory Chapter: Liquid Crystals and Applications

*Pankaj Kumar Choudhury and Abdel-Baset M.A. Ibrahim*

## **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.

*Introductory Chapter: Liquid Crystals and Applications DOI: http://dx.doi.org/10.5772/intechopen.105105*

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 [17–19]. 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 crystalbased 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.

## **Author details**

Pankaj Kumar Choudhury1 \* and Abdel-Baset M.A. Ibrahim2

1 Institute of Microengineering and Nanoelectronics, Universiti Kebangsaan Malaysia, UKM Bangi, Selangor, Malaysia

2 Faculty of Applied Sciences, Universiti Teknologi MARA (UiTM), Shah Alam, Selangor, Malaysia

\*Address all correspondence to: pankaj@ukm.edu.my

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

## **References**

[1] Chandrasekhar S. Liquid Crystals. Cambridge: Cambridge University Press; 1992

[2] Collings PJ. Liquid Crystals: Nature's Delicate Phase of Matter. Princeton: Princeton University Press; 1990

[3] de Gennes PG, Prost J. The Physics of Liquid Crystals. Oxford: Clarendon Press; 1993

[4] O'Mara WC. Liquid Crystal Flat Panel Displays. New York: Van Nostrand Reinhold; 1993

[5] Coles H, Morris S. Liquid-crystal lasers. Nature Photonics. 2010;**4**:676-685

[6] Lin Y-H. Liquid crystals for biomedical applications. In: Lee C-C, editor. The Current Trends of Optics and Photonics, Topics in Applied Physics. Vol. 129. Dordrecht: Springer; 2015

[7] Choudhury PK, editor. New Developments in Liquid Crystals and Applications. New York: Nova; 2013

[8] Shao Y, Zerda TW. Phase transitions of liquid crystal PAA in confined geometries. Journal of Physical Chemistry B. 1998;**102**:3387-3394

[9] Liang Q, Liu P, Liu C, Jian X, Hong D, Li Y. Synthesis and properties of lyotropic liquid crystalline copolyamides containing phthalazinone moiety and ether linkages. Polymer. 2005;**46**:6258-6265

[10] Madsen LA, Dingemans TJ, Nakata M, Samulski ET. Thermotropic biaxial nematic liquid crystals. Physical Review Letters. 2004;**92**: 145505-1-145505-4

[11] Moreno I. Liquid crystals for photonics. Optical Engineering. 2011;**50**:081201-1

[12] Castellano JA. Liquid Gold: The Story of Liquid Crystal Displays and the Creation of an Industry. Singapore: World Scientific; 2005

[13] Gebhart SC, Stokes DL, Vo-Dinh T, Mahadevan-Jansen A. Instrumentation considerations in spectral imaging for tissue demarcation: Comparing three methods of spectral resolution. Proceedings of SPIE. 2005;**5694**:41-52

[14] Levenson RM, Lynch DT, Kobayashi H, Backer JM, Backer MV. Multiplexing with multispectral imaging: From mice to microscopy. ILAR Journal. 2008;**49**:78-88

[15] Plimpton RG. Pool Thermometer. U.S. Patent 4738549. 1988

[16] Choudhury PK. Liquid crystal optical fibers for sensing applications. Proceedings of SPIE. 2013;**8818**: 88180E-1-88180E-10

[17] Choudhury PK, Yoshino T. TE and TM modes power transmission through liquid crystal optical fibers. Optik. 2011;**115**:49-56

[18] Choudhury PK, Soon WK. On the transmission by liquid crystal tapered optical fibers. Optik. 2011;**122**:1061-1068

[19] Ghasemi M, Choudhury PK. Propagation through complex structured liquid crystal optical fibers. Journal of Nanophotonics. 2014;**8**: 083997-1-083997-13

[20] Ghasemi M, Choudhury PK. On the conducting sheath double-helix loaded liquid crystal optical fibers. Journal of Electromagnetic Waves and Applications. 2015;**29**:1580-1592

[21] Ghasemi M, Choudhury PK. Conducting tape helix loaded radially anisotropic liquid crystal clad optical fiber. Journal of Nanophotonics. 2015;**9**:093592-1-0093592-15

## **Chapter 2**
