Transcript EXPERIMENT B6 Power point

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
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Before you use this presentation
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:
For teacher:
EXPERIMENT B- TEACHER DOCUMENT
NANOYOU_VIDEO 1_LIQUID CRYSTALS
For students*:
EXPERIMENT B-STUDENT BACKGROUND READING
EXPERIMENT B-STUDENT LABORATORY WORKSHEET
LEVEL OF EXPERIMENT: Medium
•Available in different languages
DOCUMENTS CAN BE FOUND AT WWW.NANOYOU.EU
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.
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.
Liquid Crystals
EXPERIMENT B
AGE LEVEL: 14-18 YEARS
Experiment B- Liquid Crystals
Fundamental concept of nanoscience
1
Liquid crystals as an example of selfassembled molecules
LCs are a fourth state of matter: LC
flows like a liquid and has long-range
order like solids (”soft matter”)
→ A LC is formed by the self-assembly
of molecules in ordered structures,
called phases.
→ The molecules in a LC are shaped
like rods or plates.
→ Liquid crystals are abundant in living
systems (lipid bilayer).
Figure 1. Schematic representation of liquid crystal molecules.
Image credit see slide 15
Experiment B- Liquid Crystals
Fundamental concept of nanoscience
2
External perturbations modify the self-assembly of the molecules
→LC change their molecular and supermolecular organization
drastically as an effect of very small external perturbations (e.g.,
electric field, temperature)
→ An external perturbation can induce the LC to assume a different
phase. Different phases can be distinguished by their different optical
properties
→ Three groups of LC: Lyotropic, metallotropic and thermotropic
→ In this experiment we study thermotropic liquid crystals: change
structure with temperature.
Experiment B- Liquid Crystals
Fundamental concepts of nanoscience
→ Thermotropic LCs exhibit a variety
of phases as temperature is
changed
→ Very high temperature order is lost
(“liquid”).
→ Very low temperature gives a semicrystalline (anisotropic) structure
→ Ordering inside a thermotropic
liquid crystal exists in a specific
temperature range
→ Intermediate phases have some
level of order, which is progressively
loss as the temperature is increased
Figure 2. Smectic and nematic phase. Image credit see slide 15
Experiment B- Liquid Crystals
Fundamental concepts of nanoscience
Figure 3. Smectic and nematic phase. Image credit see slide 15
→ Smectic phases molecules positionally
ordered along one direction.
→ 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).
Experiment B- Liquid crystals
Fundamental concepts of nanoscience
→ 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.
→ 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”
→ The chiral pitch, p, refers to the distance over
which the LC molecules undergo a full 360° twist
Figure 4 Schematic representation of the chiral twisting in a
chiral nematic liquid crystal. Image credit see slide 15
Colour generation in thermotropic LCs
→ When light strikes a liquid crystal, some of the light is reflected. What we see
is the reflected light
→ The colour (i.e. the wavelength) of the reflected light depends on how tightly
twisted the helix is. It depends on the pitch
→ 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
→ 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).
→ 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
Figure 4 Change in colour in a thermotropic LC.
Image credit see slide 15
Experiment B- How is this “nano”?
→ 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.
→ 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.
→ In nanotechnology scientist take advantage of the peculiar properties of
materials at the nanoscale to engineer new materials with tailored properties.
Experiment B- Protocol
→ Make four different cholesteric liquid crystals sensitive to four different
temperature ranges (overall from 17 to °40 C)
→ Prepare four diffent liquid crystal sheets and test them (heating with fingers
fingers; water bath). See Video 1- Liquid Crystals (3 min 30 sec).
Liquid crystal
Type 1
Type 2
Type 3
Type 4
Cholesteryl oleyl
carbonate
0.65
0.45
0.40
0.30
Cholesteryl
pelargonate
0.25
0.45
0.50
0.60
Cholesteryl
benzoate
0.10
0.10
0.10
0.10
Temperature
(°C)
17-23
26.5-30.5
32-35
37-40
NANO LC Thermometer
→ Make a liquid crystal thermometer: 4 letters
(“NANO”) each containing a different liquid
crystals
N – Type 1
A – Type 2
N – Type 3
O – Type 4
→ 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.
→ Now you have a room thermometer!
NANO LC Thermometer
→ If you are in a room with a heater,
you can place the thermometer over
it…and see what happens!
→ place it over a working laptop
computer….it will show what we all
know, they heat up!
→ You can use it as a room
thermometer throughout the year
→ Thermometer will last 3-6 months
Running Experiment B in class
1. Start with a discussion on self-assembly. What other molecules selfassemble 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.
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 ("director"). 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).