What is the Difference Between Nematic Smectic and Cholesteric Liquid Crystals

The key difference between nematic smectic and cholesteric liquid crystals is their structure. Nematic liquid crystals have no ordered structure of molecules, and smectic liquid crystals have a layered molecular structure, whereas cholesteric liquid crystals have the molecules in a twisted and chiral arrangement.

The term liquid crystals can be defined as a phase of matter having properties in between the properties of conventional liquids and solid crystals. For example, a liquid crystal can flow like a liquid, but the molecules of the liquid crystal tend to be oriented in a crystal-like nature. There are three major phases in which the liquid crystals may occur: thermotropic liquid crystals, lyotropic liquid crystals, and metallotropic liquid crystals. There are several types of thermotropic liquid crystals as well. These types include nematic crystals, smectic crystals, chiral phase or twisted nematics, discotic phase, and conic phases.

CONTENTS

1. Overview and Key Difference
2. What are Nematic Liquid Crystals 
3. What are Smectic Liquid Crystals
4. What are Cholesteric Liquid Crystals
5. Nematic Smectic vs Cholesteric Liquid Crystals  in Tabular Form
6. Summary – Nematic Smectic vs Cholesteric Liquid Crystals

What are Nematic Liquid Crystals?

Nematic liquid crystals are one of the most common types of liquid crystals. The term “nematic” comes from a Greek origin where it has the meaning “thread.” This is because this term originated from the topological defects that appear as threads. Normally, these topological defects in nematics are named disclinations. Moreover, nematics tend to exhibit a type of defect known as “hedgehog” topological defects.

Typically, the nematic phase has calamitic or rod-shaped organic molecules that have no positional order. But these molecules tend to self-align in order to have long-range directional order having long axes that are roughly parallel to each other. Therefore, the molecules in this phase are free to flow, and the center of mass of these molecules tends to randomly distribute, similar to a liquid. However, these molecules tend to maintain the long-range directional order similar to a solid phase.

Mostly, nematic phases are uniaxial. This means these molecules have one axis (it is named as “directrix”) that is long. This is the preferred axis, and the other two axes are equivalent to each other, and we can approximate them as cylinders or rods. Furthermore, there can be biaxial nematics having not only the orientation is similar in their long axis but also they tend to orient along a secondary axis.

The molecules of the nematic phase show fluidity that is similar to the fluidity in an ordinary isotropic liquid. However, we can easily align the molecules in this phase through the application of an external magnetic or electric field. When these molecules are aligned into an order, the nematic phase shows optical properties of uniaxial crystals, and therefore, these phases are very useful in liquid crystal displays.

What are Smectic Liquid Crystals?

Smectic liquid crystals are a type of liquid crystals that have well-defined layers of molecules that are able to slide over one another. This behavior is similar to the sliding effects given by soap. Moreover, smectic liquid crystals occur at temperatures lower than that of the nematic liquid crystals. The term smectic originates from the Latin word “smectius” which means “cleaning”; in other words, it means the presence of soap-like properties. Therefore, the smectic phase is positioned in order along one direction.

We can observe two different smectic phases as smectic A phase and smectic C phase. The smectic A phase has the molecules oriented along with the normal layer, whereas smectic C has its molecules tilted away from it. Moreover, this type of phase appears as a liquid within the layers. However, there can be many different types of smectic phases that are different from each other according to the type and degree of positional and orientational order.

What are Cholesteric Liquid Crystals?

Cholesteric liquid crystals are a type of liquid crystals made only of chiral molecules. This type of phase shows chirality. Often, we call this phase a chiral phase or a twisted nematic phase due to its chirality. The term cholesteric comes from its first observation where this phase of matter first appeared in cholesterol derivatives. We can observe this type of phase exhibiting a twisting of the molecules, which are perpendicular to the director where the molecular axis is parallel to the director.

There is a finite twist angle between the adjacent molecules in this phase because of the asymmetric packing of the molecules. This type of packing causes a longer-range chiral order. Usually, we define chiral pitch or “p” regarding the properties of cholesteric liquid crystals where it refers to the distance over which the liquid crystal molecules undergo a full-circle twist (360-degree twist).

What is the Difference Between Nematic Smectic and Cholesteric Liquid Crystals?

Nematic, smectic, and cholesteric phases are three different phases of matter that come under thermotropic liquid crystals. The key difference between nematic smectic and cholesteric liquid crystals is that nematic liquid crystals have no ordered structure of molecules, and smectic liquid crystals have a layered molecular structure, whereas cholesteric liquid crystals have molecules in a twisted and chiral arrangement.

Below is a summary of the difference between nematic smectic and cholesteric liquid crystals in tabular form.

Summary – Nematic Smectic vs Cholesteric Liquid Crystals

Nematic, smectic, and cholesteric phases are three different phases of matter that come under thermotropic liquid crystals. The key difference between nematic, smectic, and cholesteric liquid crystals is that nematic liquid crystals have no ordered structure of molecules, and smectic liquid crystals have layered molecular structure, whereas cholesteric liquid crystals have the molecules in a twisted and chiral arrangement.

Reference:

1. “Liquid Crystal.” Wikipedia, Wikimedia Foundation, 27 May 2021.

Image Courtesy:

1. “LiquidCrystal-MesogenOrder-SmecticPhases” By Kebes – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Cholesterinisch“By de:Benutzer:Heimoponnath – retirved form german Wikipedia originaly uploaded on 00:55, 19. Jan 2005 (CC BY-SA 3.0) via Commons Wikimedia