Isotropy: Definition & Materials

Instructor: Nichole Miller

Nichole is a research scientist with a PhD in Materials Science & Engineering.

This lesson defines isotropy and explains its importance. It also discusses both isotropic materials and anisotropic materials and gives several examples of each.

Why Do Pencils Write?

Have you ever wondered why pencils write so well? If you try to write on a piece of paper with a computer stylus, the stylus won't write. So why does a pencil write?

The reason is that pencil lead is made of a layered material called graphite. Each graphite layer has carbon atoms that are strongly bonded to each other. These strong bonds are very hard to break. On the other hand, the bonds between the graphite layers themselves are very weak, so it is very easy for the layers to slide past each other.

A pencil writes so well because the graphite layers can easily slide past each other and leave a trail of graphite on the paper.

Carbon atoms strongly bonded to each other in a layer of graphite
Graphite layer

Isotropy

We've just discussed how graphite is strong in some directions and weak in others. Other materials like glass have the same properties in all directions. We can therefore classify materials by their isotropy.

Isotropy comes from the Greek words isos (equal) and tropos (way) and means uniform in all directions. Isotropic materials like glass exhibit the same material properties in all directions, whereas anisotropic materials like graphite exhibit different material properties depending on the direction.

For graphite, the material property we've discussed is its strength, but there are many other material properties we can consider. A few other examples of material properties are thermal conductivity, electrical resistance, and absorptivity (a measure of how much light a material absorbs).

Isotropic Materials

Two common types of isotropic materials are metals and glasses. In metals, the electrons are shared by many atoms in all directions, so metallic bonds are nondirectional. As a result, the properties of metals are often very similar in all directions, meaning that metals tend to be isotropic.

Glassy materials are also isotropic. The atoms that make up a glass are not well organized in any direction, so the material properties of glasses tend to be the same in all directions.

One important thing to remember is that even if a material has anisotropic regions, the material as a whole can be isotropic if the anisotropic regions are randomly oriented so that can cancel each other out.

Several randomly oriented anisotropic regions can cancel out, making the material, as a whole, isotropic
Randomly oriented regions

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