Wayne State University

Transcription

1 Liquid Crystals Gavin Lawes Wayne State University

2 Liquid Crystal Displays LGE.com Amazon.com casio.com Apple.com LCI at Kent State

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

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

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

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 i order (like crystals) BUT these particles can move around (like liquids).

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)

8 Phase diagram for 5CB liquid crystal or directo Crystal Liquid crystal Isotropic liquid (nematic) 23 o C 35 o C

9 The amount of order is given by: S=½<3cos 2 θ-1> 1 θ Crystals: S=1 Isotropic liquids: S=0 Liquid crystals: 0.3<S<

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

11 Unpolarized light Polarized light

12 High intensity Low intensity normal filter

13 I 0 ½I 0 polarizing filter

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16 Liquid id crystals act like polarizing i 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.

17 Image of a liquid crystal under crossed polarizers Liquid crystal ccmr.cornell.edu

18 Type of liquid crystals Orientational order along director,, but no positional order. Nematic

19 Orientational order along director, molecules l form planes, but no positional order within each plane. Smectic A

20 Orientational order along director,, molecules form planes, but stacking direction is not parallel to director. Smectic C

21 pitch Orientational order along director, but direction of director rotates (in xy plane as you move along z direction). Cholesteric (chiral nematic)

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

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

24 Thermotropic liquid crystals Show liquid crystal phase changes as a function of temperature. Temperature Crystalline Liquid crystal More crystalline More isotropic Isotropic liquid

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

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.

27 Electric field control The director of a liquid crystal tends to align with an applied electric field. E E

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

29 Zero bias response Director in the nematic twists between bottom and top of the liquid crystal. Light polarization follows this twist t when propagating through h LC. LIGHT

30 Biased response Director in the nematic aligns with electric field. Light polarization does not change when propagating through LC. E DARK

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.

32 On-line resources for liquid crystals 1. Polymers and liquid crystals page at Case Western ( /) 2. Liquid Crystal Institute at Kent State ( 3. University of Cambridge page on liquid crystals (

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