Liquid Crystals—How Do They Do That?

Introduction

Combine two clear liquids and heat them, then watch as the mixture produces vivid color changes from blue to green to red as it cools. From skin mapping to circuit board testing to mood rings, liquid crystals have amazing modern applications. Liquid crystals have properties that are in-between those of solids and liquids—molecules move independently as in a liquid, but they also tend to orient or align themselves like a crystalline solid. The alignment of the molecules changes with temperature and produces fascinating color changes. A simple introduction to the world of nanotechnology!

Concepts

• Liquid crystals
• Nanotechnology
• Diffraction

Materials

Safety Precautions

Cholesteryl oleyl carbonate and cholesteryl pelargonate are skin and eye irritants and may cause respiratory and digestive tract irritation. Avoid contact of all chemicals with skin and eyes. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Cholesteryl oleyl carbonate and cholesteryl pelargonate may be disposed of according to Flinn Suggested Disposal Method #18b.

Prelab Preparation

Low Temperature Liquid Crystal

1. Weigh out 0.6 g of cholesteryl oleyl carbonate (COC) and transfer to a labeled glass vial.
2. Weigh out 0.4 g of cholesteryl pelargonate (CP) and transfer to the same glass vial.
3. Cap the vial and gently heat the vial in a hot water bath or with a hair dryer at a medium setting until the mixture melts.

High Temperature Liquid Crystal

1. Weigh out 0.5 g of cholesteryl oleyl carbonate (COC) and transfer to a second labeled glass vial.
2. Weigh out 0.5 g of cholesteryl pelargonate (CP) and transfer to the same glass vial.
3. Cap the vial and gently heat the vial in a hot water bath or with a hair dryer at a medium setting until the mixture completely melts.

Liquid Crystal “Sandwiches”

1. Cut four 10 x 10 cm squares of clear contact paper.
2. Peel off the backing of one of the squares of contact paper.
3. Use a wood splint to spread the low temperature liquid crystal mixture on the tacky side of the clear contact paper. Spread the mixture into an approximate 2-inch-diameter circle.
4. Peel the backing off of one other square. With the tacky side down, place the square over the tacky side of the square containing the liquid crystal mixture. Seal all the edges of the contact paper “sandwich,” making sure not to squeeze out the mixture. Label the square 1.
5. Repeat steps 1–4 for the other liquid crystal mixture. Label it square 2.

Procedure

Light Reflection and Transmission

1. Take the liquid crystal sandwich 2 (the high temperature liquid crystal) and show it to the students. Press the square in your hands to heat the mixture.
2. Place the square in front of or on top of a dark surface. Observe the reflected colors that are seen, and record all observation, including how the colors change over time, on the demonstration worksheet.
3. Heat the square again, then hold the square in front of a white light source and view the transmitted colors that are seen. Record the colors and color changes over time on the demonstration worksheet.
4. Repeat steps 1–3 using the other liquid crystal mixture.

Liquid Crystals as Temperature Indicators

1. Place a strip of clear tape on the top center of each liquid crystal sandwich.
2. Tape the squares next to each other (side by side) on a dark surface.
3. Gently heat the “sandwiches” with a hair dryer until the liquid crystal mixtures are clear and colorless.
4. Observe and record the sequences of color changes for the two liquid crystal mixtures.

Transition Temperature

1. Remove the backing from the thermometer strip and attach it to the contact paper in sandwich 2, next to the liquid crystal mixture.
2. Pass the liquid crystal sandwich around the class. Tell the students to record the transition temperatures of the liquid crystal for the appearance of blue reflected color and the green reflected color.

Student Worksheet PDF

12641_Student1.pdf

Teacher Tips

• This kit contains enough chemicals and materials to create two liquid crystal “sandwiches” that can be stored and reused multiple times: 1.2 g of cholesteryl oleyl carbonate, 1.1 g of cholesteryl pelargonate, a 20 x 20 cm sheet of contact paper, an aquarium thermometer, and two vials.
• Prepare a hot water bath for use in the Preparation section. The water does not need to be boiling—the melting point of cholesteryl oleyl carbonate is ~ 20 °C, that of cholesteryl pelargonate 74–77 °C.
• Below is a graph of the reflected color transition temperatures for liquid crystal mixtures as a function of the percent composition of cholesteryl oleyl carbonate (COC).
  
  {12641_Tips_Figure_1}
  Cholesteryl oleyl carbonate has a lower transition temperature than cholesteryl pelargonate.
• The liquid crystal sandwiches can be placed on an overhead projector to view the transmitted colors. If the projector surface heats the liquid crystals above their transition temperature, place the square on top of a Petri dish to prevent the square from being heated by the projector.
• The two vials each contain a small portion of liquid crystal. Pass the vials around the classroom. Have each student use his or her hands to heat up both vials simultaneously, then set them on their desks to view the transition colors for each mixture.

Answers to Questions

1. Describe the reflected colors and color changes that were observed when the liquid crystal mixtures were placed against a black background.

For reflected light, the color transitions are violet→blue→green→yellow→orange→red.

For transmitted light, the color transitions are yellow→orange→pink→blue→violet.

2. Compare and contrast with the observed transmitted color changes when the mixtures were viewed in front of a light source.

Instead of the violet to green to red sequence of colors as the mixture cools, the color changes are yellow to pink to violet.

3. List the complementary colors of transmitted light and reflected light that would be observed simultaneously at the same temperature for a liquid crystal.

Violet/yellow, blue/orange, green/pink, orange/blue, red/violet

4. For the liquid crystal mixture passed around the class, what are the transition temperatures for the appearance of blue reflected light and green reflected light?

The transition temperature for the appearance of blue color was 29 ºC and for the green color it was 27 ºC.

Discussion

Nanotechnology involves the preparation, characterization, and uses of nano-sized particles having dimensions in the 1–100 nm range (1 nm = 1 x 10^–9 m). Nanoparticles have unique physical and chemical properties that differ from the macroscopic properties of traditional or “bulk” solids. The electronic, magnetic, and optical properties of nanoparticles have proven to be very useful in the creation of new products using nanotechnology. Liquid crystals consist of nano-sized organic compounds that are in a state between liquid and solid compounds.

Liquid crystals are partially ordered compounds that float around as in a liquid, but align themselves, to a degree, as in a crystalline solid. Cholesteryl esters are long, cylindrical or rod-like molecules that arrange themselves in a layered helical pattern, similar to a spiral staircase (see Figure 1).

The molecules in each layer line up in a parallel pattern, with each adjacent layer having this parallel pattern slightly rotated. After a certain number of layers and rotations, the molecules in the top and bottom layers are aligned in the same direction. The distance between these layers is called the pitch of the liquid crystal (see Figure 2).

As the liquid crystal heats up, the rotational angle between adjacent layers increases. Since fewer layers are required to realign the top and bottom, the pitch decreases with increasing temperature. These pitch distances are on the order of magnitude corresponding to visible light wavelengths, that is, 300 nm to 400 nm. Visible light is selectively diffracted by the liquid crystal according to Snell’s Law (Equation 1).
where λ is the reflected wavelength, p is the pitch, θ is the angle with respect to the surface, and n is the mean refractive index. As the temperature increases, the wavelength of visible light decreases. The reflected light changes from yellow (longer wavelength) to green to blue (shorter wavelength) as the liquid crystal is heated, and blue to green to red as it is cooled. The temperature range for these color transitions is different for each liquid crystal compound and mixture of compounds.

If a specific wavelength of light is reflected by the crystal, then all other wavelengths pass through the crystal. If blue is the reflected light, then light transmitted through the crystal is white light minus violet light, which is perceived as yellow. If orange light is reflected, then white light minus orange light, which is seen as blue light, is transmitted. When a liquid crystal square is viewed against a black background and then in front of a light source, the reflected color, followed by its complementary color, is observed.

References

Lisensky, G. and Boatman, E., Colors in Liquid Crystals. J. Chem. Educ., 2005, 82, 1360A.