Difference Between Isotropic and Anisotropic

Main Difference – Isotropic vs Anisotropic

Isotropic and anisotropic are two important terms widely used to explain the material properties in material science and crystal morphology in basic crystallography. In certain materials like crystals, the orientation of atoms is very important as it affects their physical and mechanical properties. Based on the orientation of atoms, materials are broadly divided into two classes namely: isotropic materials and anisotropic materials. The main difference between isotropic and anisotropic is that the properties of isotropic materials are the same in all directions, whereas in anisotropic materials, the properties are direction dependent.

This article looks at,

1. What is Isotropic
      – Definition, Properties, Examples
2. What is Anisotropic
      – Definition, Properties, Examples
3. What is the difference between Isotropic and Anisotropic

What is Isotropic

If the properties (mechanical, physical, thermal and electrical properties) of a material do not change with different crystallographic orientations, or in other words, the properties are direction independent, that material is called isotropic. Isotropic crystals have one refractive index in all directions. Crystals with cubic symmetry and amorphous materials such as glasses are regarded as isotropic materials. Examples of cubic crystals include rock salt and sodium chloride. However, not all properties of cubic crystals are isotropic. Usually, cubic crystals are isotropic with respect to their electrical conductivity and pyroelectric effect. However, cubic crystals are not direction independent with respect to their elastic properties such as stiffness, shear and bulk moduli. Isotropic crystals are often used for windows and lenses. Plant cell walls are considered to be isotropic as they are more or less the same everywhere.

Figure 01: Glass is an example of an isotropic material .

What is Anisotropic

The term anisotropic is used to refer to materials that have atomic arrangements which are direction dependent; in other words, the physical properties vary along the different directions within the material. Usually, anisotropic materials are very common in nature than isotropic materials due to the large variation of atomic orientations. Almost all the crystals except cubic crystals are considered as anisotropic. Anisotropic crystals have many refractive indices. Because of that, anisotropic crystals affect the birefringence, optical activity, dichroism and dispersion of crystals. Birefringence is known as the difference in the light-transmission of a crystal. Certain crystals such asquarts rotate when thepolarized light passing through them. Such crystals are called optically active crystals. The ability to absorb electromagnetic radiation along two different vibrational axes is called dichroism. When the same crystal has differential refraction of a different wavelength of light, it is called dispersion. Anisotropic crystals are used for many optical applications, such as polarizers, optical waveplates, wedges etc. Wood and composites are the common examples of anisotropic materials. In plant cells, the interior part or cytoplasm is considered as anisotropic due to the presence of intracellular organelles.

Figure 02: Wood is an example of an anisotropic material.

Difference Between Isotropic and Anisotropic

Definition

Isotropic Materials: Certain properties of material do not change along its atomic arrangements

Anisotropic Materials: Properties of material vary along its atomic arrangements.

Properties 

Isotropic Materials: The properties of isotropic materials are direction independent.

Anisotropic Materials: The properties of anisotropic materials are direction dependent.

Examples

Isotropic Materials: Crystals with cubic symmetry and amorphous materials such as glasses are examples.

Anisotropic Materials: All crystals except cubic crystals, wood, and composite materials are examples of anisotropic materials. 

RI Index

Isotropic Materials: Isotropic materials have a single refractive index.

Anisotropic Materials: Anisotropic materials have more than one refractive indices.

Characteristics 

Isotropic Materials: Isotropic crystals don’t show characteristics such as birefringence, optical activity, dichroism and dispersion due to different refractive indices.

Anisotropic Materials: Anisotropic crystals show birefringence, optical activity, dichroism and dispersion due to different refractive indices.

Applications in Optical Field

Isotropic Materials: Isotropic crystals are used for windows and lenses.

Anisotropic Materials: Anisotropic crystals are used for polarizers, optical waveplates, and wedges.

Summary

Isotropic and anisotropic are two terms that are widely used in material science and crystallography to explain the atomic orientation, structure, and morphology of materials. In isotropic materials such as cubic crystals and amorphous materials (ex: glass), the properties do not change along the direction of the material. In anisotropic materials such as wood and composites, the properties vary along the directions of the material. This is the main difference between isotropic and anisotropic.

References:
1. Hammond, C., & Hammond, C. (2009). The basics of crystallography and diffraction (Vol. 12). Oxford: Oxford University Press.
2. Furukawa, Y., & Nakajima, K. (2001). Advances in crystal growth research. Elsevier.
3. Bell, S., & Morris, K. (2009). An introduction to microscopy. CRC Press.
4. Sivasankar, B. (2008). Engineering chemistry (p. 499). New Delhi: Tata McGraw-Hill.

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