The key difference between anisotropy and isotropy is that anisotropy is directionally dependent whereas isotropy is directionally independent.
The words isotropy and anisotropy are useful in various fields. According to where we use it, the meaning may be slightly different. However, the fundamental concept behind these two words is similar and independent from where we use them. Above all, we use the terms isotropy and anisotropy often to describe properties of macroscopic bodies. There, they depend on the scale of the macroscopic body. For instance, one crystal can be anisotropic, but when many crystals are together, they can be isotropic.
What is Anisotropy?
Anisotropy is the property of being dependent on the direction. It is the opposite of isotropy. There, the measured properties of a material differ in various directions in anisotropy. Furthermore, these properties fall under two categories; physical or mechanical properties such as conductivity and tensile strength or absorbance. Also, this property has slightly different meanings in different subjects where we use it.
Usually, liquids have no order in molecules. However, anisotropic liquids are liquid with a structural order in contrast to other common liquids. The sedimentary materials can have electrical anisotropy, where the electrical conductivity differs from one direction to another direction. Moreover, the rock-forming minerals are anisotropic in relative to their optical properties.
The orientation of nuclei of a molecule differs with the strength of the applied magnetic field in NMR spectroscopy. In this case, anisotropic systems refer to the molecules with high electron density. Because of the anisotropic effect (in molecules with high electron density), the molecule feels the applied magnetic field differently (most often less than the real value); therefore, the chemical shift varies.
Furthermore, in fluorescence spectroscopy too, we use the anisotropic measurement of the fluorescence polarization to determine the molecular structures. Moreover, anisotropy is a common concept in medicine when talking about ultrasound imaging.
What is Isotropy?
The word “isotropy” relates to the uniformity. The meaning of the word itself is “uniformity in all directions.” As stated in the introduction, the meaning may slightly differ according to the subject area. For instance, when talking about the isotropy of a material or a mineral, it means having the same properties in all directions.
Furthermore, in industrial processes, isotropy means having the same rate in all the steps regardless of the direction. There, we say the molecules having kinetic energy moves randomly in any direction. Therefore, in a given time, there will be many molecules moving in the same direction. Hence, it shows isotropy. Likewise, the materials having this property will have the same properties in all the directions (ex: Amorphous solids). For instance, when we apply heat, if a solid expands in a similar manner, in all directions, it is an isotropic material.
What is the Difference Between Anisotropy and Isotropy?
Anisotropy is the property of being dependent on the direction and isotropy is the property of being independent on the direction. This is the key difference between anisotropy and isotropy. Therefore, isotropic means having the same property in all directions. If the properties of a material are different in different directions, we name it as anisotropic.
As another important difference between anisotropy and isotropy, anisotropic materials have more than one refractive index while isotropic materials have a single refractive index (the ratio of the velocity of light in a vacuum to its velocity in a specified medium is refractive index).
Summary – Anisotropy vs Isotropy
We use the terms isotropy and anisotropy often to describe properties of macroscopic bodies. Hence, the key difference between anisotropy and isotropy is that anisotropy is directionally dependent whereas isotropy is directionally independent.
1. A. Gambacorta, C.D. Barnet, in Comprehensive Remote Sensing, 2018.
2. Britannica, The Editors of Encyclopaedia. “Anisotropy.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 26 Dec. 2017. Available here