Liquid Crystal Sheet Demos [W/Video]

Liquid Crystal Sheet Demos [W/Video]

Safely demonstrating the principles and properties of heat can sometimes be a risky proposition. But it needn’t be. Watch as Physics Teacher James Lincoln uses the safe, simple, and inexpensive Liquid Crystal Sheet to visually display some very cool things about heat we usually only TELL our students about.

Introduction
These Liquid Crystal Sheets are heat sensitive and offer a wide-range of possibility for experiments. Because they change color based on temperature, they can be read visually and quickly, at a distance, allowing the whole class to enjoy your demonstrations. The sheets easily warm to the touch and as they do, will display the visible spectrum ROYGBIV – with blues and purple signifying the warmest temperatures. If they get too hot, they will become dark again. The color change is caused by tiny crystal layers – on the micron scale – twisting as they warm.

heat_article-1 hand on sheet

The thin layer responds rapidly to the touch. These sheets offer a wide range of possibilities.

 

 

 

 

 

 

 

 

Fun and Easy Explorations
After warming a sheet with your hand, try putting some cold water or ice on it. Then, you can put some hot water on it and demonstrate evaporative cooling (blow air on the hot water and it cools rapidly). The thin design of the sheet allows for rapid color changes. Another cool thing to try is to compare the hands of male students to female students. Male hands are usually warmer because of higher blood pressure and surface area-to-volume ratios. This is plain to see using the Liquid Crystal Sheet.

heat_article-2

A streak of warm water beautifully demonstrates the color changes. Notice the far end is already demonstrating evaporative cooling.

 

 

 

 

 

 

 

 

Efficiency of Light Sources
One experiment you should conduct is to compare the heat output of various light sources. Of course the candle puts out an enormous amount of heat. This is easily tested by holding the Liquid Crystal Sheet above the light source, since heated air rises by convection. The classic incandescent light bulb is next and it puts out a medium amount of heat, because it has to heat up to glow. But the fluorescent bulbs barely put out any heat at all compared to these other sources – they are much more efficient. This topic is appropriate for Physics and Environmental Science. The more modern light sources are light-emitting diodes (LEDs), and they are the most efficient of all, putting out almost no heat. You can get one of the new, futuristic diode bulbs at your local hardware store.

Insulation and Conduction
You can demonstrate insulation using a piece of Styrofoam. Place the Styrofoam below the sheet to delay the transfer of heat to the tabletop and hold the thermal images longer. Once you have your image, use a piece of metal to demonstrate thermal conductivity. The metal takes the heat away faster. You can also demonstrate that friction generates heat when the metal is applied to the surface.

heat_article-4

Placing the sheet on Styrofoam will lengthen the duration of the thermal image.

Thermocline
As an example of how well the sheets can display ‘what’s hot & what’s not”, I offer the thermocline. Cold water and warm water will sort themselves based on temperature due to density differences, called a thermocline. Thermoclines occur in swimming pools, lakes, and the ocean. Generally, the warm water rises and the cold water sinks. However, in the ocean there are also haloclines, which are density differences caused by salt content. This is a good chance to probe the water line and determine which color corresponds to which temperature.

Measuring the Microwave’s Wavelength
In this experiment, we are going to measure the wavelength of a microwave with a ruler, a piece of Styrofoam, and the Liquid Crystal Sheets. Insert two sheets on a plate into the microwave; make sure that they do not rotate by using a tube or rack to hold the plate up, then let it cook for only a few seconds. When you take the sheets out the pattern you will see patterns in the shape of the microwaves. You can measure the wavelength of the microwave by measuring the distance between these hot spots. You will probably notice two different distances between spots. Between two close spots measure only half the wavelength (antinode to antinode distance) but when spots are far apart, it is a full wavelength (think cosine). The full wavelength is about 12 cm. Using the microwave oven’s frequency (usually stamped on the back or the inside), you can calculate the speed of light. My microwave oven is 2450 MHz. Multiplying this by .12 meters, and using v = λ f gives us the speed of light: 3 x 108 m/s.

Lasers
I have found that these sheets absorb heat well from red, and sometimes blue, lasers. Though I haven’t thoroughly tested the effect of different colored lasers, my belief is that the red is absorbed more readily than the green, for example, and so it can be used to write messages. Awesome!

heat_article-7

With a red laser you can write not-so-secret messages. Light waves transfer energy.

 


  James Lincoln

Tarbut V’ Torah High School

Irvine, CA, USA

James Lincoln teaches Physics in Southern California and has won several science video contests and worked on various projects in the past few years.  James has consulted on TV’s “The Big Bang Theory” and WebTV’s “This vs. That”  and  the UCLA Physics Video Project.

Contact: [email protected]


 

Recommended Tools

Liquid Crystal Sheet

In Stock SKU: P1-2100
$11.95

Strip Thermometer

In Stock SKU: P1-2060-02
$1.25

Giant Drinking Bird

In Stock SKU: P3-5014
$35.00

Dye Sensitized Solar Cell Kit

In Stock SKU: P6-2110
$110.00

Ice Melting Blocks - Thermal Conductivity

In Stock SKU: P6-7060
$17.00

Share this post

Comments (2)

  • Don Brott Reply

    Really like the demos with the liquid crystal sheets. The differences of heat emission from the various light sources was especially effective. Nice work, thanks for sharing.

    January 15, 2015 at 4:10 pm
  • Ralph McGrew Reply

    Two more:
    This demonstrates that it is work as such, not necessarily frictional work, that can add to the internal energy of a system. Equivalently, it shows the conversion of kinetic energy into internal energy in an inelastic collision. A large rubber mallet has been sitting for some time on the tabletop, as has the liquid crystal sensor sheet. Ask a student to set the mallet on the sheet. Do you expect a temperature change? There is none, because everything is at thermal equilibrium. The student pushes the stationary head of the mallet down onto the sheet hard. Is pressure accompanied by temperature change? No, fundamentally because there is no work input. Now the student takes a swing and hits the sheet with the head of the mallet. Immediately there is a little crescent of color on the sheet where energy of organized translational motion has been degraded into energy of random molecular vibration. No heat is involved because everything was at room temperature. But then heat leaves the sheet into the environment as the color fades away. Practice in advance–the sheet may need some backing of plastic or cardboard.
    To show the effect that the angle of incident rays of light has on the power reaching each square centimeter of an absorbing surface: Bend the sheet to form a convex cylindrical surface. Hold it with its nearest point 10 or 20 cm, measured horizontally, from an incandescent light bulb. The section of the cylinder closest to the bulb warms up the most or the soonest. Cool the sheet back to room temperature and bend it to be concave. What do you expect when we hold it at the same location? This time the section farthest from the bulb warms up most. The rays of light are “most direct” there, being perpendicular to the absorbing surface. The same phenomenon can be displayed again, this time with multiple trials: Lay the sheet flat on the table, starting each trial at room temperature. Hold an incandescent bulb as a spotlight at the same distance from the sheet on each trial, so that the light’s angle of incidence is 0 degrees, 20 degrees, 40 degrees, 50 degrees, 70 degrees, close to 90 degrees, and the same values again on the other side, as the light source is moved among points on a vertical semicircle centered on the sheet. On each trial measure the time required for the sheet to reach some particular color. You can relate this phenomenon to how the elevation angle of the noon sun is responsible for the difference between summer and winter in temperate regions of the earth.

    January 23, 2015 at 3:27 pm

Leave a Reply

Your email address will not be published. Required fields are marked *

*