Skip to content Skip to navigation Skip to collection information

OpenStax-CNX

You are here: Home » Content » Physical Methods in Chemistry and Nano Science » The Analysis of Liquid Crystal Phases using Polarized Optical Microscopy

Navigation

Table of Contents

Lenses

What is a lens?

Definition of a lens

Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

Who can create a lens?

Any individual member, a community, or a respected organization.

What are tags? tag icon

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

This content is ...

Affiliated with (What does "Affiliated with" mean?)

This content is either by members of the organizations listed or about topics related to the organizations listed. Click each link to see a list of all content affiliated with the organization.
  • Rice Digital Scholarship

    This collection is included in aLens by: Digital Scholarship at Rice University

    Click the "Rice Digital Scholarship" link to see all content affiliated with them.

Recently Viewed

This feature requires Javascript to be enabled.
 

The Analysis of Liquid Crystal Phases using Polarized Optical Microscopy

Module by: Changsheng Xiang, Andrew R. Barron. E-mail the authors

Summary: Liquid crystals (LCs) have tremendous applications in industry fields such as LCs display, LCs thermometer and polymers. In this paper, different types of LCs with its unique property will be introduced. The theory of birefringence and working mechanism of polarized optical microscope (POM) are demonstrated. We also illustrate the methods for analyzing LCs by using polarized optical microscope.

Introduction

Liquid crystal phases

Liquid crystals are a state of matter that has the properties between solid crystal and common liquid. There are basically three different types of liquid crystal phases:

  • Thermotropic liquid crystal phases are dependent on temperature.
  • Lyotropic liquid crystal phases are dependent on temperature and the concentration of LCs in the solvent.
  • Metallotropic LCs are composed of organic and inorganic molecules, and the phase transition not only depend on temperature and concentration, but also depend on the ratio between organic and inorganic molecules.

Thermotropic LCs are the most widely used one, which can be divided into five categories:

  • Nematic phase in which rod-shaped molecules have no positional order, but they self-align to have long-range directional order with their long axes roughly parallel (Figure 1a).
  • Smactic phase where the molecules are positionally ordered along one direction in well-defined layers oriented either along the layer normal (smectic A) or tilted away from the layer normal (smectic C), see Figure 1b).
  • Chiral phase which exhibits a twisting of the molecules perpendicular to the director, with the molecular axis parallel to the director (Figure 1c).
  • Blue phase having a regular three-dimensional cubic structure of defects with lattice periods of several hundred nanometers, and thus they exhibit selective Bragg reflections in the wavelength range of light (Figure 2).
  • Discotic phase in which disk-shaped LC molecules can orient themselves in a layer-like fashion (Figure 3).
Figure 1: Schematic representations of (a) a nematic LC phase, (b) smactic LC phases oriented along (left) and away (right) from the normal of the layer, and (c) a chiral LC phase.
Figure 1 (FigLC1.jpg)
Figure 2: A schematic representation of the ordered structure of a blue LC phase.
Figure 2 (FigLC3.jpg)
Figure 3: Schematic representations of (a) a discotic nematic LC phase and (b) a discotic columnar LC phase.
Figure 3 (FigLC2.jpg)

Thermotropic LCs are very sensitive to temperature. If the temperature is too high, thermal motion will destroy the ordering of LCs, and push it into a liquid phase. If the temperature is too low, thermal motion is hard to perform, so the material will become crystal phase.

The existence of liquid crystal phase can be detected by using polarized optical microscopy, since liquid crystal phase exhibits its unique texture under microscopy. The contrasting areas in the texture correspond to domains where LCs are oriented towards different directions.

Polarized optical microscopy

Polarized optical microscopy is typically used to detect the existence of liquid crystal phases in a solution.The principle of this is corresponding to the polarization of light. A polarizer is a filter that only permits the light oriented in a specific direction with its polarizing direction to pass through. There are two polarizers in a polarizing optical microscope (POM) (Figure 4) and they are designed to be oriented at right angle to each other, which is termed as cross polar. The fundamental of cross polar is illustrated in Figure 5, the polarizing direction of the first polarizer is oriented vertically to the incident beam, so only the waves with vertical direction can pass through it. The passed wave is subsequently blocked by the second polarizer, since this polarizer is oriented horizontally to the incident wave.

Figure 4: The basic configuration of polarized optical microscope. Copyright: Nikon Corporation.
Figure 4 (Picture 1.jpg)
Figure 5: A schematic representation of the polarization of light waves. Copyright: Nikon Corporation.
Figure 5 (Picture 2.png)

Theory of birefringence

Birefringent or doubly-refracting sample has a unique property that it can produce two individual wave components while one wave passes through it, those two components are termed as ordinary and extraordinary waves. Figure 7 is an illustration of a typical construction of Nicol polarizing prism, as we can see, the non-plarized white light are splitted into two ray as it passes through the prism. The one travels out of the prism is called ordinary ray, and the other one is called extraordinary ray. So if we have a birefringent specimen located between the polarizer and analyzer, the initial light will be separated into two waves when it passes though the specimen. After exiting the specimen, the light components become out of phase, but are recombined with constructive and destructive interference when they pass through the analyzer. Now the combined wave will have elliptically or circularly polarized light wave, see Figure 7, image contrast arises from the interaction of plane-polarized light with a birefringent specimen so some amount of wave will pass through the analyzer and give a bright domain on the specimen.

Figure 6: A schematic representation of a Nicol polarzing prism. Copyright: Nikon Corporation.
Figure 6 (FigLC4.jpg)
Figure 7: A schematic representation of elliptically and circularly polarized light waves. Copyright: Nikon Corporation.
Figure 7 (FigLC5.jpg)

Liquid crystal display

The most common application of LCs are in liquid crystals displays (LCD). Figure 8 is a simple demonstration of how LCD works in digit calculators. There are two crossed polarizers in this system, and liquid crystal (cholesteric spiral pattern) sandwich with positive and negative charging is located between these two polarizers. When the liquid crystal is charged, waves can pass through without changing orientations. When the liquid crystal is out of charge, waves will be rotated 90° as it passes through LCs so it can pass through the second polarizer. There are seven separately charged electrodes in the LCD, so the LCD can exhibit different numbers from 0 to 9 by adjusting the electrodes. For example, when the upper right and lower left electrodes are charged, we can get 2 on the display.

Figure 8: Demonstration of a seven-segment liquid crystal display. Copyright: Nikon Corporation.
Figure 8 (Picture 4.png)

Microscope images of liquid crystal phase

The first order retardation plate

The first order retardation plate is frequently utilized to determine the optical sign of a birefringent specimen in polarized light microscopy. The optical sign includes positive and negative. If the ordinary wavefront is faster than the extraordinary wavefront (see Figure 7), the specimen displays positive birefringence. Conversly, a negative birefringence will be detected if the ordinary wavefront is slower than the extraordinary wavefront. In addition, the retardation plate is also useful for enhancing contrast in weakly birefringent specimens. Figure 9 shows the effect of first order retardation plate on the contrast of birefringence. The birefringence is so weak that the morphology on the edge of the tissue is hard to image, Figure 9a. When a first order retardation plate is added, the structure of the cell become all apparent compared with the one without retardation plate, Figure 9b.

Figure 9: Microscope images of thin section of human tongue, (a) without first order retardation plate and (b) with first order retardation plate. Copyright: Olympus.
Figure 9 (Picture 5.png)

Images of liquid crystal phases

Figure 10 shows the images of liquid crystal phases from different specimens. First order retardation plates are utilized in all of these images. Apparent contrasts are detected here in the image which corresponds to the existence of liquid crystal phase within the specimen.

Figure 10: Microscope images in polarized light with a first-order retardation plate inserted between the specimen and analyzer: (a) polyethylene glycol, (b) polycarbonate, and (c) liquid crystalline DNA. Copyright from Nikon.
Figure 10 (Picture 6.png)

The effect of rotation of the polarizer

The effect of the angle between horizontal direction and polarizer transmission axis on the appearance of liquid crystal phase may be analyzed. In Figure 11 is shown images of an ascorbic acid (Figure 12) sample under cross polar mode. When the polarizer rotates from 0° to 90°, big variations on the shape of bright domains and domain colors appear due to the change of wave vibrating directions. By rotating the polarizer, we can have a comprehensive understanding of the overall texture.

Figure 11: Cross polarized Microscope images of ascorbic acid specimen with polarizer rotation of (a) 0°, (b) 45°, and (c) 90°. Copyright: Nikon Corporation.
Figure 11 (Picture 7.png)
Figure 12: The structure of ascorbic acid.
Figure 12 (FigLC6.jpg)

Bibliography

  • R. Weaver, Am. Lab., 2003, 35, 55.
  • F. Massoumian, R. Juskaitis, M. A. Neil, and T. Wilson, J. Microsc., 2003, 209, 13.
  • R. Oldenbourg, Nature, 1996, 381, 811.

Collection Navigation

Content actions

Download:

Collection as:

PDF | More downloads ...

Module as:

PDF | More downloads ...

Add:

Collection to:

My Favorites (?)

'My Favorites' is a special kind of lens which you can use to bookmark modules and collections. 'My Favorites' can only be seen by you, and collections saved in 'My Favorites' can remember the last module you were on. You need an account to use 'My Favorites'.

| A lens I own (?)

Definition of a lens

Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

Who can create a lens?

Any individual member, a community, or a respected organization.

What are tags? tag icon

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

| External bookmarks

Module to:

My Favorites (?)

'My Favorites' is a special kind of lens which you can use to bookmark modules and collections. 'My Favorites' can only be seen by you, and collections saved in 'My Favorites' can remember the last module you were on. You need an account to use 'My Favorites'.

| A lens I own (?)

Definition of a lens

Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

Who can create a lens?

Any individual member, a community, or a respected organization.

What are tags? tag icon

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

| External bookmarks