Nanotechnology
Activity Guides:
Liquid Crystal
Sensors
Audience: Middle-school
Time Needed: 50 minutes
Description
During this activity, students will:
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Learn about liquid crystals (LCs), a fourth phase of matter
Learn about and experiment with different kinds of LCs, including bubbles and
temperature-sensitive LCs
Explore how LC sensors can be used to detect changes that cannot be seen with the naked
eye
Brainstorm applications of LC sensors
Objectives
After participating in this activity, students will:
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Understand how liquid crystals (LCs) relate to other phases of matter (liquid, solid, gas)
Understand that some liquid crystals reflect many different colors (wavelengths) of light
NOTE: more generally, liquid crystals interact with light. Some reflect light, others
merely allow light to pass or not (like a shutter). The latter approach is how LCD
displays work.
Understand that LCs may react to temperature changes, electric fields, or chemical and
biological agents
Understand that some thermally sensitive LCs twist and turn (change pitch & orientation)
in response to temperature, which changes the color of light that they reflect
Understand that LCs can be used as sensors to detect changes that cannot be seen with the
naked eye
Related Wisconsin Model Academic Science Standards (These are some things students
should know by the end of 8th grade.
• A.8.6 Use models and explanations to predict actions and events in the natural world
• C 8.1 Identify questions they can investigate using resources and equipment they have
available
• D.8.2 Use the major ideas of atomic theory and molecular theory to describe physical and
chemical interactions among substances, including solids, liquids, and gases
• D.8.3 Understand how chemical interactions and behaviors lead to new substances with
different properties
• ELA C.8.3 Participate effectively in discussion
ACTIVITY MATERIALS
• One bottle of bubbles per group of 2-4 students (To make your own bubbles, see
http://www.zurqui.co.cr/crinfocus/bubble/bubble.html.)
• One sealed vial of heat-sensitive liquid crystals per group (For instructions to make these
vials, see http://mrsec.wisc.edu/Edetc/nanolab/LC_prep/index.html.)
• Liquid crystal thermal sheets for four different temperature ranges, cut into 3” x 6”
rectangles (each group should get four sheets total – one sheet of each kind). 12” x 12”
sheets are available at Edmund Scientific,
http://scientificsonline.com/product.asp_Q_pn_E_3072375, for $22.50 each. (Each sheet
can be cut into 8 smaller 3” x 6” rectangles.) The sheets can be reused. If cost becomes
an issue, the experiment can be done with three different kinds instead of four; just
modify the instructions and worksheet accordingly.
• An electric hot plate or griddle – something warm enough to cause a color change in your
highest temperature LC sheet (one or two for the whole class)
• Ice packs or ice in plastic bags (one per group)
• Refrigerator magnets (or another material that will hold heat well and can be cut into
shapes) in the shape of words and/or other geometric shapes, placed inside opaque
envelopes (several per group, but groups could also share)
• Overhead transparencies (pdf), available at
http://mrsec.wisc.edu/Edetc/IPSE/educators/activities/supplements/lcSensorsTransparencies.pdf
• Student worksheets (one worksheet per student):
o “Brainstorming the Properties of Liquid Crystals” (pdf),
http://mrsec.wisc.edu/Edetc/IPSE/educators/activities/supplements/lcSensorsProperties.pdf
o “Temperature-Sensitive Liquid Crystals” (pdf),
http://mrsec.wisc.edu/Edetc/IPSE/educators/activities/supplements/lcSensorsThermal.pdf
o “Liquid Crystals as Temperature Sensors” (pdf),
http://mrsec.wisc.edu/Edetc/IPSE/educators/activities/supplements/lcSensorsSensors.pdf
ACTIVITY INSTRUCTIONS
Intro to Nano (optional)
Introduce the concept of “nano,” one billionth of something. A nanometer is one billionth of a
meter. For activity ideas, see “Nanoscale Activities” at
http://www.mrsec.wisc.edu/edetc/IPSE/educators. It is not essential for the liquid crystal
activities that the students know about the nanoscale. However, if you wish to include the
nanoscale, the following ideas are relevant and important: liquid crystal molecules are on the
nanoscale, and liquid crystals can detect changes in their environment that are on the nanoscale.
It is this extreme sensitivity and resulting ability to magnify the extremely small that makes
liquid crystals such good sensors.
University of Wisconsin-Madison
IPSE Liquid Crystal Activity Guide
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Introduction: Liquid Crystal (LC) is a Phase of Matter (5-10 min)
Have students brainstorm the different properties of liquids, solids (crystals), and gases. Note
that liquid crystal (LC) is a special phase of matter, and have students brainstorm its properties
(e.g., liquid-solid hybrid) based on the discussion of liquids and solids. If desired, students can
use the “Brainstorming the Properties of Liquid Crystals” worksheet for this activity.
Optional: Have students arrange the phases of matter based on randomness at the atomic or
molecular level. Note that molecules in a liquid crystal phase are less random than atoms or
molecules in a liquid phase and more random than atoms or molecules in a solid phase.
Bubble Activity: LCs Reflect Different Colors of Light (10 min)
Have students blow bubbles, and explain to them that bubbles are a
particular type of LC. (For more about the different kinds of LCs, see
the “Phases of Matter” portion of the “Background Information”
section.) Ask the students to observe how the bubbles change colors
from when they were brand new to right before they pop.
Bubbles are colorful because of the way light is reflected through the
bubble walls. When a bubble is first blown, the walls are thicker and
thus reflect more light. As the bubble ages and gets closer to popping,
the walls become thinner and less colorful. (For more information on
this topic, visit the Bubblesphere web-site mentioned in the
“References” section.) Note that bubbles reflect all the full spectrum of
colors in a rainbow.
Fig. 1: Bubbles are an
example of a liquid
crystal.
Introduce Temperature-Sensitive LCs (10 min)
Note that there are many different types of LCs, including bubbles and LCs
that respond to temperature changes. Hand out the LC vials and ask students
to rub and hold them and notice what happens.
CAUTION: Do not allow students to open the vials. This type of liquid
crystal is toxic.
Fig. 2: The vials
of temperaturesensitive liquid
crystals, before
(L) and after (R)
heating.
The students should see the vials turn various shades of blue, green, pink, and
purple. (If they are having trouble getting the vials to change colors, ask the
students to breathe on them or use a hair dryer or other heat source.) Ask
students why they think the LC material changed color (Answer: because of
the heat from their hands) and what actually caused the color change
(Answer: the LC molecules change orientation, thus reflecting light in
different ways). Ask the students if they have seen something similar in their
everyday life to this phenomenon (Possible answers: mood rings and
thermometers).
Experiment: Compare Different LCs at Different Temperatures (15 min)
Give the students the sets of liquid crystal thermal sheets and the worksheet, “TemperatureSensitive Liquid Crystals.” Ask the students to use their hands, the ice, and/or the griddle to
figure out the order of the thermal sheets, ranging from low-temperature sensitive to hightemperature sensitive.
University of Wisconsin-Madison
IPSE Liquid Crystal Activity Guide
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LC Sensor Activity: LC Sensors Can "See" What Our
Eyes Can't (15 min)
Once the student have finished ordering the LC sheets, ask
them to use the one that responds to body temperature to see
the invisible thermal print that your hand leaves on a tabletop
(see Fig. 3). Explain that LCs can be useful to help us “see”
what we can’t see with our eyes. LCs can sense changes in
their surroundings that our eyes can’t see. The students can
use the “Liquid Crystals as Temperature Sensors” worksheet
to work through this and the following activities. The
second page of this worksheet includes additional activities
not described here.
For an additional demonstration, place an envelope
containing a magnetic shape/message on the griddle for a
couple seconds to heat it up. Remove the envelope from the
griddle and wait a few seconds before placing a hightemperature sensitive LC sheet on the envelope. Note the
appearance of the message on the LC sheet.
Have the students use their LC thermal sheets to decode
other magnetic shapes and messages hidden inside the
envelopes.
Fig. 3: Liquid crystal thermal sheets can
sometimes detect the invisible, including
the handprint left by the heat of our hand.
Brainstorm Potential Uses for LCs and LC Sensors (optional)
Have students think of their own ideas for how LC products and sensors might be used in the real
world. The second page of the “Liquid Crystals as Temperature Sensors” worksheet includes a
question related to this topic.
Fig. 4: Liquid crystals can make something visible that we otherwise could not see with our naked eyes.
University of Wisconsin-Madison
IPSE Liquid Crystal Activity Guide
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BACKGROUND INFORMATION
Phases of Matter
The atoms or molecules in a solid are compact with fixed positions and orientations. In contrast,
atoms or molecules in a liquid can move around freely and assume the shape of the container the
liquid is in. Liquid crystals are a phase for some organic materials someplace in between a solid
and a liquid: molecules in a liquid crystal have the fluidity of a liquid, but their movement is
coordinated in the same direction. [They have fixed orientations, but random positions.]
Therefore, one molecule in the liquid crystal phase impacts the orientation of other atoms nearby.
This property of liquid crystals is what allows them to be such good magnifiers of events on the
nanoscale. If just a couple liquid crystal molecules detect a change in their environment and
shift orientation, the molecules all around them shift, too, creating a larger effect that is visible to
the eye.
There are different types of liquid crystals. The most general classification is that of
thermotropic and lyotropic liquid crystals. Thermotropic liquid crystals are a phase of matter
intermediate between solids and liquids. Thermotropic liquid crystals are made of rod- or diskshaped polar molecules. Lyotropic liquid crystals are made of amphiphilic molecules, which
have a water-loving part and a water-hating part, mixed with water. Mood rings are examples of
thermotropic LCs, while soap bubbles are an example of lyotropic LCs. Thermotropic liquid
crystals are further classified into smectic, nematic, and cholesteric (also known as chiral
nematic), among other types. These subcategories are based on the level and type of ordering in
the liquid crystal. The molecules in a smectic liquid crystal are in a fairly ordered (but not
perfect) pattern and align in one direction. The molecules in a nematic liquid crystal also align in
one direction, but are randomly arranged and not in an ordered pattern. The molecules of
cholesteric liquid crystals are randomly arranged, but the molecules arrange as spirals rather than
pointing in one direction.
Liquid crystals are made by boiling certain kinds of organic compounds and then cooling them.
For a more in-depth look at the structure of liquid crystals and for diagrams, see the Background
for the Teacher transparencies, available at
http://mrsec.wisc.edu/Edetc/IPSE/educators/activities/supplements/lcSensors-TeacherSlides.pdf.
Nano
Nano is a prefix that means one billionth of something, so a nanometer is one billionth of one
meter. Things on the nanometer scale are on the nanoscale, a useful term for describing the size
of atoms. Ten hydrogen atoms or four iron atoms laid side by side are approximately one
nanometer long. Nanotechnology is the study and design of systems at the nanoscale and tools
to visualize the nanoscale, including atoms. The ability to manipulate materials on the nanoscale
could revolutionize the way that almost everything is designed and made.
Light
Light is both a wave and particle. Different colors of light are characterized by different
wavelengths, which range from 400 to 700 hundred nanometers in the visible spectrum. White
light consists of all the colors of the rainbow. When white light hits the molecules of a liquid
crystal, the orientation of these molecules may allow only certain wavelengths or colors of light
to be reflected back to our eyes. When the molecules shift orientations, like when they sense a
change in temperature, the colors that are reflected change.
University of Wisconsin-Madison
IPSE Liquid Crystal Activity Guide
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Supplemental Materials
•
Teacher's Guide: “Materials and Where to Find Them” (pdf),
http://mrsec.wisc.edu/Edetc/IPSE/educators/activities/supplements/lcSensors-Materials.pdf
•
Transparencies: “Background for the Teacher” (pdf),
http://mrsec.wisc.edu/Edetc/IPSE/educators/activities/supplements/lcSensorsTeacherSlides.pdf
•
Transparencies: “Teaching Slides” (pdf),
http://mrsec.wisc.edu/Edetc/IPSE/educators/activities/supplements/lcSensorsTeacherSlides.pdf
•
Worksheet: “Brainstorming the Properties of Liquid Crystals” (pdf),
http://mrsec.wisc.edu/Edetc/IPSE/educators/activities/supplements/lcSensors-Properties.pdf
•
Worksheet: “Liquid Crystals as Sensors” (pdf),
http://mrsec.wisc.edu/Edetc/IPSE/educators/activities/supplements/lcSensors-Sensors.pdf
•
Worksheet: “Temperature-Sensitive Liquid Crystals” (pdf),
http://mrsec.wisc.edu/Edetc/IPSE/educators/activities/supplements/lcSensors-Thermal.pdf
References
• Elston, S. and Sambles, R. 1998. The Optics of Thermotropic Liquid Crystals.
• Sonin, Andrei, A. 1998. Freely Suspended Liquid Crystalline Films.
• Bubblesphere. URL: http://bubbles.org
• How Liquid Crystal Displays Work. URL: http://www.howstuffworks.com
• PLC Virtual Laboratory. URL: http://plc.cwru.edu/tutorial/enhanced/lab/lab.htm
• Soap Basics. URL: http://home.earthlink.net/~marutgers/science/soapbasics/soapbasics.html
• What Are Liquid Crystals. URL:
http://www.lassp.cornell.edu/sethna/LiquidCrystals/BluePhase/What_Are_Liquid_Crystals.ht
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Authors
IPSE Interns: Julie He, Jeffrey Maxwell
IPSE Leadership Team: Wendy C. Crone, J. Aura Gimm, Wendy deProphetis, Greta Zenner, and
Tom Derenne
Thanks also to MRSEC postdoc Nathan Lockwood for providing expert topic consulting.
The Nanotechnology Activity Guides are a product of the Materials Research Science and
Engineering Center and the Internships in Public Science Education Project of the University of
Wisconsin - Madison. Funding provided by the National Science Foundation.
University of Wisconsin-Madison
IPSE Liquid Crystal Activity Guide
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