DCMST May 23 rd , 2008

1. Liquid Crystals Gavin Lawes Wayne State University DCMST May 23rd, 2008

2. Liquid Crystal Displays LGE.com Amazon.com casio.com Apple.com LCI at Kent State DCMST May 23rd, 2008

3. Outline Liquid Crystals What are liquid crystals? Light and polarization filters Demonstration Types of liquid crystals Nematic, smetic, chiral, and more Controlling the liquid crystal transition Demonstration Twisted nematic displays DCMST May 23rd, 2008

4. Why does “liquid crystal” sound like an oxymoron? Highly regular array of atoms with fixed positions Short length scales (nm) Crystals Long length scales (mm+) Self-supporting, often hard DCMST May 23rd, 2008

5. Random, disordered array of atoms that move around. Short length scales (nm) Liquids Flow (unable to support shear forces) Long length scales (mm+) DCMST May 23rd, 2008

6. So what are liquid crystals? Liquid crystals have properties associated with both liquids and crystals. Anisotropic rod-like particles in liquid crystals develop orientational order (like crystals) BUT… …these particles can move around (like liquids). DCMST May 23rd, 2008

7. Example of a liquid crystal 5CB liquid crystal (4-cyano-4′-n-pentyl-biphenyl) liqcryst.chemie.uni-hamburg.de T. Bouchouar et al, Polymer 42, 1663 (2000) DCMST May 23rd, 2008

8. Phase diagram for 5CB liquid crystal director Crystal Liquid crystal (nematic) Isotropic liquid 23 oC 35 oC DCMST May 23rd, 2008

9. The amount of order is given by: S=½<3cos2q-1> q Crystals: S=1 Isotropic liquids: S=0 Liquid crystals: 0.3<S<0.9 DCMST May 23rd, 2008

10. Light polarization Light is an electromagnetic wave consisting of perpendicular oscillating magnetic and electric field. Because light interacts with matter primarily through the electric field component, the direction of the electric field defines the polarization of the wave. polarization nrc.gc.ca DCMST May 23rd, 2008

11. Unpolarized light Polarized light DCMST May 23rd, 2008

12. High intensity Low intensity “normal” filter DCMST May 23rd, 2008

13. ½I0 I0 polarizing filter DCMST May 23rd, 2008

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16. Liquid crystals act like polarizing filters. The orientationally ordered rod-like molecules in liquid crystals affect the polarization of the transmitted light. Because the director of liquid crystals can be changed, these materials can be used as switchable light filters. DCMST May 23rd, 2008

17. Image of a liquid crystal under crossed polarizers Liquid crystal ccmr.cornell.edu DCMST May 23rd, 2008

18. Type of liquid crystals Orientational order along director, but no positional order. Nematic DCMST May 23rd, 2008

19. Orientational order along director, molecules form planes, but no positional order within each plane. Smectic A DCMST May 23rd, 2008

20. Orientational order along director, molecules form planes, but stacking direction is not parallel to director. Smectic C DCMST May 23rd, 2008

21. pitch Orientational order along director, but direction of director rotates (in xy plane as you move along z direction). Cholesteric (chiralnematic) DCMST May 23rd, 2008

22. Controlling the liquid crystal transition Lyotropic liquid crystals Liquid crystalline properties depend on concentration of molecules in solution. Often associated with amphiphilic molecules. Hydrophilic Hydrophobic DCMST May 23rd, 2008

23. Amphilic molecules dissolved in water form different liquid crystal structures depending on concentration. Micelle (low concentration) Bilayer (high concentration) DCMST May 23rd, 2008

24. Thermotropic liquid crystals Show liquid crystal phase changes as a function of temperature. Temperature Crystalline Liquid crystal Isotropic liquid More crystalline More isotropic DCMST May 23rd, 2008

25. Thermochromic liquid crystals Thermotropic liquid crystals can show a colour change as a function of temperature (caused by changes in the pitch). Temperature Red Blue DCMST May 23rd, 2008

26. Boundary constraints The director in a liquid crystal system can be oriented by textures at the interface. Nematic liquid crystal Polymer coating on surface Director of nematic is lined up with polymer coating on surface. DCMST May 23rd, 2008

27. Electric field control The director of a liquid crystal tends to align with an applied electric field. E E DCMST May 23rd, 2008

28. Twisted nematic displays Combine boundary effect control of director with electric field effect control of director. 0o textured surface 90o textured surface Glass Glass Unpolarized light Transparent electrode Transparent electrode Nematic liquid crystal 90o polarizer 0o polarizer DCMST May 23rd, 2008

29. Zero bias response Director in the nematic twists between bottom and top of the liquid crystal. Light polarization follows this twist when propagating through LC. LIGHT DCMST May 23rd, 2008

30. Biased response Director in the nematic aligns with electric field. Light polarization does not change when propagating through LC. E DARK DCMST May 23rd, 2008

31. Summary Liquid crystals exhibit properties intermediate between a liquid and a crystal. Rod-like molecules in liquid crystals show different types of orientational order. Because the transmission of light through a liquid crystal depends on the polarization and director of the order, liquid crystals can be used for making switchable optical devices, including displays. DCMST May 23rd, 2008

32. On-line resources for liquid crystals Polymers and liquid crystals page at Case Western (http://plc.cwru.edu/) 2. Liquid Crystal Institute at Kent State (http://www.lci.kent.edu/) 3. University of Cambridge page on liquid crystals (http://www.doitpoms.ac.uk/tlplib/liquid_crystals/) DCMST May 23rd, 2008

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