Switching dynamics and bistability in blue phase devices

Transcription

1 II Theory Xmas Workshop, Department of Physics University of Bari Switching dynamics and bistability in blue phase devices Adriano Tiribocchi SUPA School of Physics and Astronomy, UK

2 Outline Introduction: Liquid crystals Motivations Dynamical equations Switching dynamics, anchoring conditions, bistability Conclusions

3 Liquid crystals A liquid crystal is a phase of matter with properties in common between liquids and solids. P.G. de Gennes and J. Prost, The Physics of Liquid Crystals, 2 ed., Oxford (2003) S. Chandrasekhar, Liquid Crystals, Camb.Univ.Press, (1992)

4 Blue phases Double twist BP I BP II Coles et al., Nature 436 (2005), Casltes et al., Nat. Mat. (2012)

5 Liquid crystal device Twisted nematic displays (used for the construction of flat panel monitors) Technological applications: Screens, digital clocks, mobiles, e-books... Improvements: viewing angle, bistable behaviour, decrease in switching times Hydrodynamic and electric field effects in blue phase devices

6 Dynamic equations for liquid crystals t ρ + α ρ u α = 0 ρ t u α +ρu β β u α= β Pαβ [ 2 /d δ αβ γ u γ α u β β u α ] Q S W, Q = H t u ρ r,t Total density u r,t Velocity field P Q H Q Pressure tensor Molecular field 1 Qα β = n α n β δ α β 3 W = u Equations are solved by using a hybrid lattice Boltzmann method * Order parameter Velocity gradient tensor * Cates et al, Soft Matter 5, 3791 (2009). A. N. Beris and B. J. Edwards, Thermodynamics of Flowing Systems, Oxf. Univ. Press, Oxford, (1994)

7 Equilibrium properties A0 A0 γ 2 2 γ 2 γ f= 1 Qα β A 0 Qαβ Qβ γ Qγ α + Q α β) ( K K + ( α Qβ γ ) 2+ (εα ςδ ς Q δβ +2q 0 Q α β )2 2 2 ε Q αβ Eα E β +εbfl Qαβ (E α γ E γ α )Qβ γ 12π 1 s 0 2 +εfl ( β Qα β )E α + W 0 ( Qα β Q αβ ) 2 ( ) 1 1 Pαβ = P 0 δ αβ ξh αγ Q γβ + δ γβ ξ Q αγ + δ αγ H γβ δf 2ξ Qαβ + δ αβ Q γε H γε β Qγν +Q αγ H γβ H αγ Q γβ 3 δ α Q γν ( ( ) ) ( ) Bulk Distortion Electric field Surface F δf H= I/3 Tr δq δq

8 Defect dynamics in a BPI cell E = E z z y x A. T., G. Gonnella, D. Marenduzzo, E. Orlandini, Soft Matter 7, 3295 (2011). Mukherjee, A. et al., Liquid Crystals 39, (2012).

9 Anchoring effects Homogeneous anchoring Homeotropic anchoring Confined blue phases Periodic Homeotropic Homogeneous A. T., G. Gonnella, D. Marenduzzo, E. Orlandini, Soft Matter 7, 3295 (2011).

10 Bistable liquid crystal device Existence of two stable states in absence of an applied electric field: each of the bistable states is metastable and is retained after the field is switched off. This fact eliminates the need of having a constant electric field to keep the system in the on state, hence sharply reducing energy consumption. Energy saving devices (no voltage for fixed images) : e book, e newspapers... Portable devices: less electronics, less weight, small size Experimentally realized bistable devices: Zenithal bistable device (DERA), CNRS laboratories, Hewlett Packard laboratories (Bristol), surface stabilazed Cholesteric textures (SSCT) and many others.

11 Bistabiliy in a BPI cell A. T., G. Gonnella, D. Marenduzzo, E. Orlandini, F. Salvadore, Phys. Rev. Lett. 107, (2011).

12 Conclusions BPI device: Disclinations twist up and pin to the boundary. Upon switching off the field part of the network deforms and the zero field configuration is not recovered. A device that show a bistable behaviour has been proposed. The dyamical schedule of the applied electric field is fundamental to observe bistability. Twisted rings and double helix disclinations can be selected by varying the electric field. Perspectives What if a flexo electric field is considered? Active systems: droplets, cells, bacteria, dna, etc...

13 Flexoelectricity Flexoelectricity is the generation of a spontaneous polarization in a liquid crystal due to a deformation of the director, or conversely, the deformation of the director due to an applied electric field (R. B. Meyer Phys. Rev. Lett. 22, 918 (1969)). An applied field will induce a splay distortion as the molecular dipole aligns along the field lines. An electric field in the opposite direction will induce a different distortion A. J. Davisson, N. J. Mottram, Phys. Rev E 65, (2002)

14 Flexoelectric blue phase Bulk flexoelectric Surface flexoelectric A.T., M.E. Cates, G. Gonnella, D. Marenduzzo, E. Orlandini, Flexoelectric switching in cholesteric blue phases, in preparation.