Nanotechnology Activity Guides: Liquid Crystal Sensors

Nanotechnology Activity Guides: Liquid Crystal Sensors Audience: Middle-school Time Needed: 50 minutes Description During this activity, students will...
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Nanotechnology Activity Guides: Liquid Crystal Sensors Audience: Middle-school Time Needed: 50 minutes Description During this activity, students will: • • • •

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: • • •

• • •

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 • One sealed vial of heat-sensitive liquid crystals per group (For instructions to make these vials, see • 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,, 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 • Student worksheets (one worksheet per student): o “Brainstorming the Properties of Liquid Crystals” (pdf), o “Temperature-Sensitive Liquid Crystals” (pdf), o “Liquid Crystals as Temperature Sensors” (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 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


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


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


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 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


Supplemental Materials • Teacher's Guide: “Materials and Where to Find Them” (pdf), • Transparencies: “Background for the Teacher” (pdf), • Transparencies: “Teaching Slides” (pdf), • Worksheet: “Brainstorming the Properties of Liquid Crystals” (pdf), • Worksheet: “Liquid Crystals as Sensors” (pdf), • Worksheet: “Temperature-Sensitive Liquid Crystals” (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: • How Liquid Crystal Displays Work. URL: • PLC Virtual Laboratory. URL: • Soap Basics. URL: • What Are Liquid Crystals. URL: ml 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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