Experiment with Liquid Crystals - Nanotechnology


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In this experiment developed within the NANOYOU project you will discover Liquid crystals, an example of self-assembled molecules that are sensitive to external factors, such as temperature, and that change their assembly as a consequence of variations in these factors.

In this experiment you will see how some liquid crystals change colour as their temperature is changed!

To download the protocol and background information, and to watch the videos available, visit www.nanoyou.eu

Translations to several languages are also availabe in the NANOYOU website.

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Experiment with Liquid Crystals - Nanotechnology

  1. 1. Teachers Training Kit in Nanotechnologies Experiment Module A comprehensive training kit for teachers Experiment B Luisa Filipponi, iNANO, Aarhus University This document has been created in the context of the NANOYOU project. (WP4, Task 4.1) All information is provided “as is” and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and liability. The document reflects solely the views of its authors. The European Commission is not liable for any use that may be made of the information contained therein.
  2. 2. Before you use this presentation <ul><li>This Power Point Presentation is part of the Experiment Module of the NANOYOU Teachers Training Kit in Nanotechnologies. MATERIAL INCLUDED IN THIS EXPERIMENT B PACKAGE : </li></ul><ul><li>For teacher: </li></ul><ul><li>EXPERIMENT B- TEACHER DOCUMENT </li></ul><ul><li>NANOYOU_VIDEO 1_LIQUID CRYSTALS </li></ul><ul><li>For students * : </li></ul><ul><li>EXPERIMENT B-STUDENT BACKGROUND READING </li></ul><ul><li>EXPERIMENT B-STUDENT LABORATORY WORKSHEET </li></ul><ul><li>  </li></ul><ul><li>LEVEL OF EXPERIMENT : Medium </li></ul><ul><li>Available in different languages </li></ul><ul><li>DOCUMENTS CAN BE FOUND AT WWW.NANOYOU.EU </li></ul><ul><li>This NANOYOU documents is distributed with Creative Commons Non-Commercial Share Alike Attribution, except where indicated differently. Please not that some images contained in this PPT are copyright protected, and to re-use them outside this document requires permission from original copyright holder. See slide 14 for details. </li></ul>DISCLAIMER : The experiments described in the following training kit use chemicals which need to be used according to MSDS specifications and according to specific school safety rules. Personal protection must be taken as indicated. As with all chemicals, use precautions. Solids should not be inhaled and contact with skin, eyes or clothing should be avoided. Wash hands thoroughly after handling. Dispose as indicated. All experiments must be conducted in the presence of an educator trained for science teaching. All experiments will be carried out at your own risk. Aarhus University (iNANO) and the entire NANOYOU consortium assume no liability for damage or consequential losses sustained as a result of the carrying out of the experiments described.
  3. 3. EXPERIMENT B AGE LEVEL: 14-18 YEARS <ul><li>Liquid Crystals </li></ul>
  4. 4. Experiment B- Liquid Crystals Fundamental concept of nanoscience <ul><li>Liquid crystals as an example of self-assembled molecules </li></ul><ul><li>LCs are a fourth state of matter: LC flows like a liquid and has long-range order like solids (”soft matter”) </li></ul><ul><li>A LC is formed by the self-assembly of molecules in ordered structures, called phases . </li></ul><ul><li>The molecules in a LC are shaped like rods or plates . </li></ul><ul><li>Liquid crystals are abundant in living systems (lipid bilayer). </li></ul>Figure 1. Schematic representation of liquid crystal molecules. Image credit see slide 15 1
  5. 5. Experiment B- Liquid Crystals Fundamental concept of nanoscience <ul><li>External perturbations modify the self-assembly of the molecules </li></ul><ul><li>LC change their molecular and supermolecular organization drastically as an effect of very small external perturbations (e.g., electric field, temperature) </li></ul><ul><li>An external perturbation can induce the LC to assume a different phase . Different phases can be distinguished by their different optical properties </li></ul><ul><li>Three groups of LC: Lyotropic, metallotropic and thermotropic </li></ul><ul><li>In this experiment we study thermotropic liquid crystals: change structure with temperature. </li></ul>2
  6. 6. Experiment B- Liquid Crystals Fundamental concepts of nanoscience <ul><li>Thermotropic LCs exhibit a variety of phases as temperature is changed </li></ul><ul><li>Very high temperature order is lost (“liquid”). </li></ul><ul><li>Very low temperature gives a semi-crystalline (anisotropic) structure </li></ul><ul><li>Ordering inside a thermotropic liquid crystal exists in a specific temperature range </li></ul><ul><li>Intermediate phases have some level of order, which is progressively loss as the temperature is increased </li></ul>Figure 2. Smectic and nematic phase. Image credit see slide 15
  7. 7. Experiment B- Liquid Crystals Fundamental concepts of nanoscience <ul><li>Smectic phases molecules positionally ordered along one direction. </li></ul><ul><li>Nematic phase , molecules have no positional order, but they have long-range orientational order. This means that the molecules move quite randomly but they all point in the same direction (within each domain). </li></ul>Figure 3. Smectic and nematic phase. Image credit see slide 15
  8. 8. Experiment B- Liquid crystals Fundamental concepts of nanoscience <ul><li>A particular type of liquid crystal phase is the chiral nematic phase . The chiral nematic phase exhibits chirality (handedness). This phase is often called the cholesteric phase because it was first observed for cholesterol derivatives. </li></ul><ul><li>the molecules are stacked in rotating layers, like a spiral staircase (helix) . In each “step” of the staircase the molecules are arranged in a specific order, but there is a finite angle between each “step” </li></ul><ul><li>The chiral pitch, p , refers to the distance over which the LC molecules undergo a full 360° twist </li></ul>Figure 4 Schematic representation of the chiral twisting in a chiral nematic liquid crystal. Image credit see slide 15
  9. 9. Colour generation in thermotropic LCs <ul><li>When light strikes a liquid crystal, some of the light is reflected . What we see is the reflected light </li></ul><ul><li>The colour (i.e. the wavelength) of the reflected light depends on how tightly twisted the helix is. It depends on the pitch </li></ul><ul><li>If the pitch in the liquid crystal is of the same order of the wavelength of visible light (400-700 nm) , interesting effects are seen </li></ul><ul><li>When the helix is tightly twisted , the pitch is smaller, so it reflects smaller wavelengths ( blue end of the spectrum ); when the liquid crystal is less twisted , it has a larger pitch, so it reflects larger wavelengths ( red end of the spectrum ). </li></ul><ul><li>An increase in temperature leads to a decrease of the pitch , therefore by increasing the temperature of the liquid crystal one should expect a color change from the red end of the spectrum to the blue end of the spectrum , so from orange, to yellow, green, blue and violet </li></ul>Figure 4 Change in colour in a thermotropic LC. Image credit see slide 15
  10. 10. Experiment B- How is this “nano”? <ul><li>Liquid crystals are an example of self-assembled molecules which change their spatial organization in dependence of external factors such as temperature. Self-assembly is a fundamental concept in nanoscience . </li></ul><ul><li>The properties of materials at the macroscale are affected by the structure of the material at the nanoscale . Changes in a material’s molecular structures are often too small to see directly, but sometimes we can see changes in the materials properties. Thermotropic liquid crystals are an example. </li></ul><ul><li>In nanotechnology scientist take advantage of the peculiar properties of materials at the nanoscale to engineer new materials with tailored properties. </li></ul>
  11. 11. Experiment B- Protocol <ul><li>Make four different cholesteric liquid crystals sensitive to four different temperature ranges (overall from 17 to °40 C) </li></ul><ul><li>Prepare four diffent liquid crystal sheets and test them (heating with fingers fingers; water bath). See Video 1- Liquid Crystals (3 min 30 sec). </li></ul>Liquid crystal Cholesteryl oleyl carbonate Cholesteryl pelargonate Cholesteryl benzoate Temperature (°C) Type 1 0.65 0.25 0.10 17-23 Type 2 0.45 0.45 0.10 26.5-30.5 Type 3 0.40 0.50 0.10 32-35 Type 4 0.30 0.60 0.10 37-40
  12. 12. NANO LC Thermometer <ul><li>Make a liquid crystal thermometer: 4 letters (“NANO”) each containing a different liquid crystals </li></ul><ul><li>N – Type 1 </li></ul><ul><li>A – Type 2 </li></ul><ul><li>N – Type 3 </li></ul><ul><li>O – Type 4 </li></ul><ul><li>Make sure that the liquid crystal sheets do no overlap inside the letters. The idea is that each letter should contain only one liquid crystal sheet . </li></ul><ul><li>Now you have a room thermometer! </li></ul>
  13. 13. NANO LC Thermometer <ul><li>If you are in a room with a heater, you can place the thermometer over it…and see what happens! </li></ul><ul><li>place it over a working laptop computer ….it will show what we all know, they heat up! </li></ul><ul><li>You can use it as a room thermometer throughout the year </li></ul><ul><li>Thermometer will last 3-6 months </li></ul>
  14. 14. Running Experiment B in class 1. Start with a discussion on self-assembly . What other molecules self-assemble in organized structures? (e.g., proteins, DNA). Discuss how the structure is fundamental for the function of the macromolecule.   2. Link structure to function . Discuss that the way a material behaves at the macroscale is affected by its nanostructure. Although we cannot see this with our eyes, we can observe changes in the material properties, such as its colour. Liquid crystals are such an example: they are self-assembled molecules which have specific properties, like colour, depending on the structure they have.   4. Liquid crystals that change colour with temperature . Discuss how they work. Give examples (small flat thermometers, sensors etc.)   5. Run the experiment in the lab diving students in groups of 2 or more depending on need. Each couple should make four different liquid crystals sheets and test them with their fingers and in a water bath. One or more room thermometers can be made in the class.
  15. 15. Images credit Figure 1 : Example of the self-organisation of anisometric (i.e., with asymmetrical parts) molecules in liquid-crystalline phases. On the left: rod-like molecules form a nematic liquid, in which the longitudinal axes of the molecules are aligned parallel to a common preferred direction (&quot;director&quot;). On the right: disc-like (discotic) molecules arrange to molecule-stacks (columns), in which the longitudinal axes are also aligned parallel to the director. As a result of their orientational order, liquid crystals exhibit anisotropic physical properties, just like crystals. (Image Credit: http://www.ipc.uni-stuttgart.de/~giesselm/AG_Giesselmann/Forschung/Fluessigkristalle/Fluessigkristalle.html ) Figure 2 : Schematic representation of the structure transition of a thermotropic LC from the smetic to the nematic phase as the temperature is increased. (Image credit: own work adapted from Wiki commons, Creative Commons Attribution ShareAlike 3.0 and from IPSE Educational resources “liquid crystals”, University of Wisconsin-Madison.) Figure 3 : see Figure 2. Figure 4 (top, left): Schematic representation of ordering in chiral liquid crystal phases: a chiral nematic phase (also called the cholesteric phase) in an LC (Image credit: Wiki Commons, Creative Commons Attribution ShareAlike 3.0) Figure 4 (top, right): Schematic representations of stacked rotating layers in a chiral LC forming a “spiral staircase” having a pitch p . (Image credit: Wiki Commons, Creative Commons Attribution ShareAlike 3.0) Figure 4 (bottom): Schematic representation of the pitch in a chiral LC (Images credit: Wiki commons, Creative Commons Attribution ShareAlike 3.0). Figure 5 : Representation of a pitch change in a chiral LC as the temperature is changed. Image credit: Image adapted from IPSE Educational Resources (Liquid crystals), University of Wisconsin Madison).