How Mood Rings Work With Thermochromic Liquid Crystals

What Are Mood Rings Made Of?

Mood rings contain liquid crystals that orient according to temperature.
Minutemen, Creative Commons License

Mood rings are rings which have a stone or band that changes color in response to temperature. Have you ever wondered how they work or what is inside one of them? Here's a look at the liquid crystals found in mood rings and how they change color.

What Are Mood Rings Made Of?

A mood ring is sort of a sandwich. The bottom layer is the ring itself, which could be sterling silver, but usually is plated silver or gold over brass.

A strip of liquid crystals is glued onto the ring. A plastic or glass dome or coating is placed over the liquid crystals. Higher-quality mood rings are sealed to prevent water or other liquids from seeping into the liquid crystals, since moisture or high humidity will damage the ring irreversibly.

Thermochromic Liquid Crystals

Mood rings change color in response to temperature because they contain thermochromic liquid crystals. There are several natural and synthetic liquid crystals that change color according to temperature, so the exact composition of a mood ring depends on its manufacturer, but most rings contain crystals made from organic polymers. The most common polymer is based on cholesterol. As the ring becomes warmer, more energy is available to the crystals. The molecules absorb the energy and essentially twist, altering the way light passes through them.

Two Phases of Liquid Crystals

Mood rings and colored liquid crystal thermometers employ two phases of liquid crystals: the nematic phase and the smectic phase.

The nematic phase is characterized by the rod-shaped molecules pointing in the same direction, but with little lateral order. In the smectic phase, the components of the crystal are both aligned and display some degree of lateral order. The liquid crystals in mood rings tend to shift between these phases, with the less-ordered or "hot" nematic phase occurring at the warmer temperature and the more-ordered or "cold" smectic phase occurring at the cooler temperature.

The liquid crystal becomes liquid above the nematic phase temperature and solid below the smectic phase temperature.

How Do Mood Rings Work?