2. Liquid crystals : The fourth state of matter Thierry Chuard, LLB, Universität Bern

3. Abstract  What is a liquid crystal ?  Some applications of liquid crystals being a liquid crystal ?  How can we identify a compound

4. What is a liquid crystal ? Crystal Highly organized solid Liquid Non-organized fluid Liquid crystal Organized fluid (mesophase) heat

5. The liquid crystalline state is a hybrid state between the liquid and and the solid phase. Liquid crystals possess both the fluidity of liquids and organization found in solids. This is an anisotropic phase.

6. Liquid crystals Depends on - the temperature - the solvent - the concentration Depends only on the temperature Thermotropic Lyotropic

7. Which kind of molecules can show liquid crystalline state ? Calamitics Rigid and rod-shaped Rigid and disc-shaped Discotics

9. A calamitic compound 4-methoxybenzilidene-4'-butylaniline CrystalLiquid crystal 20 °C 47 °C

10. How do calamitic molecules organize ?

11. The nematic phase Director + + + + + + + + + - - - -- - -- - + + + - - + + + + + + + - - - - - - - - Random orientation of the dipoles No polarity of the mesophase

12. The nematic phase The less organized mesophase is the most fluid 

13. The cholesteric phase A cholesteric phase is a chiral nematic phase

14. The cholesteric phase p The wavelength of the light reflected by a cholesteric material is proportional to the helical pitch p and to the mean refractive index n :  = p ∙ n Bragg’s law

15. Smectic ASmectic C The smectic (lamellar) phases

16. Smectic B The smectic (lamellar) phases hexagonal phase

17. N SASA SCSC The same compound can show different mesophases Crystal 63 °C60 °C80 °C Liquid 86 °C

18. A discotic compound Liquid crystal CrystalLiquid 69 °C 122 °C

19. How do discotic molecules organize ? HexagonalTetragonalRectangular  Nematic phase  Columnar phases

20. How can we identify a compound being a liquid crystal ?  X-Ray diffraction  Miscibility  Differential scanning calorimetry  Polarized transmitted-light microscopy

21. 20406080100 Temperature (°C) 20406080100 Temperature (°C) Differential scanning calorimetry Temperatur e Control Heat flow

22. Differential scanning calorimetry 20406080100 Temperature (°C) Thermogram = difference between heat flows

23. 20406080100 20406080100 Temperature (°C) Differential scanning calorimetry Temperatur e Control 1-2 mg Heat flow

24. Differential scanning calorimetry 20406080100 Temperature (°C) Melting point Thermogram = difference between heat flows

25. Thermogram of a non liquid-crystalline compound LiquidSolid

26. Thermogram of a liquid-crystalline compound Solid Liquid Crystal Liquid

27. X-Ray diffraction  Determination of the layer thickness in smectic (lamellar) phases  50 - 100 mg samples  Identification of the mesophase  Determination of the angle in non orthogonal smectic phases

28. polarized light Polarized transmitted-light microscopy

29. The Schlieren texture Nematic phase

30. The focal conic fan texture Smectic A phase

31. The focal conic fan texture Columnar hexagonal phase

33. Some applications of liquid crystals  Displays  Materials (polymers)  Thermometry  Pigments (cars)  Specific oils  Adjustable Tinted Window

34. LC displays  different technologies  Switching time always faster  Switching voltage always lower  Screen size  Resolution  Colors quality and greyscale

35. Twisted nematic (TN) cell Front glass Rear glass Contacts Electrode Counter-electrode Alignment layers LC

37. Twisted nematic (TN) cell Front glass Rear glass LC Polarizer

38. TN cells switching Front glass Rear glass Electrode Counter-Electrode Alignment layer Polarizer

39. The plane of the polarized light is turned by 90° by the LC The light can go through the 2nd polarizer Only the polarized Light in the plane of the polarizer goes through

40. The plane of the polarized light is not turned by the LC The light cannot go through the 2nd polarizer 

41. How can we obtain a white pixel on a black background with the same disposal ?

42. The plane of the polarized light is turned by 90° by the LC The light cannot go through the 2nd polarizer

43. The plane of the polarized light is not turned by the LC The light can go through the 2nd polarizer 

44. ITO electrodes

46. Conclusion