Surface Anchoring

Goals Surface Alignment Homeotropic Alignment Homogeneous Alignment Weak Anchoring Deposition Methods Non-Rubbed Methods

Surface Anchoring There is really no rule of thumb when it comes to surfaces ! It is a very empirical area

Surface: Microscope Slide Courtesy of the NSF ALCOM Center: Presented at 1992 ALCOM Symposium by H. Vithana and D. Johnson

Surface: Optically Flat Glass Courtesy of the NSF ALCOM Center: Presented at 1992 ALCOM Symposium by H. Vithana and D. Johnson

Surface: ITO coated display glass Courtesy of the NSF ALCOM Center: Presented at 1992 ALCOM Symposium by H. Vithana and D. Johnson

Surface Anchoring Alignment at surfaces propagates over macroscopic distances microgrooved surface - homogeneous alignment (//) rubbed polyimide ensemble of chains - homeotropic alignment () surfactant or silane

Homeotropic Anchoring Varying the pretilt angle for homeotropic anchoring n n=16 Aliphatic Chain Tilt Angle from Length (n=) Normal (qo) 16 0o 14 0o 12 0o 10 12o 9 22o 8 30o 7 55o 6 70o n=7 N 55o n

Surface: Silane Surfactant on Glass Courtesy of the NSF ALCOM Center: Presented at 1992 ALCOM Symposium by H. Vithana and D. Johnson

Surface: DMOAP on ITO Courtesy of the NSF ALCOM Center: Presented at 1992 ALCOM Symposium by H. Vithana and D. Johnson

Surface: Lecithin Surfactant on Glass Courtesy of the NSF ALCOM Center: Presented at 1992 ALCOM Symposium by H. Vithana and D. Johnson

Surface Rubbing Technique Glass plate coated with an organic layer Cloth

Surface Rubbing Technique: Groovy director field profile  to ridges z Scenario #1:  to ridges Scenario #2: // to ridges

Surface Rubbing Technique: Groovy Shape of profile Minimum elastic energy achieved under this condition Solution to above equation

Surface Rubbing Technique: Groovy

Surface Anchoring q f N polar anchoring Wq n surface azimuthal Wf Wq,f is energy needed to move director n from its easy axis Strong anchoring 10-4 J/m2 Weak anchoring 10-7 J/m2

Surface Anchoring Measurement VSAT Threshold Condition Voltage Intensity VTH time Saturation Condition

Effects of Anchoring Energy Consider electrically controllable birefringence Weak anchoring results in: Lower threshold Rise time decreases Fall time increases Parameter Threshold Fall time Rise time Strong Anchoring Weak Anchoring

Weak Anchoring Energy Example: Threshold d=5 mm, K=10-11 N, W=5x10-6 J/m2, De=10

Weak Anchoring Energy Example: Fall Time d=5 mm, K=10-11 N, W=5x10-6 J/m2, De=10

Weak Anchoring Energy Example: Rise Time d=5 mm, K=10-11 N, W=5x10-6 J/m2, De=10

Tilt Angle Measurement photodiode with I/U converter HeNe - laser pinhole pinhole polarizer polarizer rotation stage with sample holder Q Intensity qo Io q1 I1 q2 I2 qn In . . IEEE-bus rotation stage controller digital multimeter

Tilt Angle: Crystal Rotation Method Z j d a Z y x

Example Tilt Angle Transmission -60 -17 30 Angle of incidence y

Why Study Tilt Angle E Tilt angle stops reverse tilt disinclination Simplest picture - disclination E Can influence your contrast Simple ECB Cell where qo is the pretilt angle

Surface Rubbing Technique Rotation speed e.g. 405 mm/sec Rubbing Cloth Rotor Substrate Stage Moving speed e.g. 6 mm/sec

Surface Rubbing Technique: Rubbing Length The rubbing length L is the total length of the rubbing fiber in contact with a certain point of the substrate N = 2 (number of rubbings) L = 11 mm (contact length); pile impression = 0.3mm r = 50 mm (radius of the rubbing wheel) n = 190 rpm (rotation speed) V = 25 mm/s (speed of the glass plate) L Rubbing length

Surface Rubbing Technique: Tilt Angle vs. Rubbing Length 8 Example plot to demonstrate general dependence Licralign SN-7200 (STN LCD) 6 Tilt Angle (degrees) 4 Licralign 4200 (for AM LCD) 2 1 2 3 4 5 6 7 8 (mm) Rubbing Length L 103

Surface Rubbing Technique 4 Increasing pressure

Surface: Unrubbed Poyimide on ITO Display Glass Courtesy of the NSF ALCOM Center: Presented at 1992 ALCOM Symposium by H. Vithana and D. Johnson

Surface: Rubbed Polyimde on ITO Display Glass Courtesy of the NSF ALCOM Center: Presented at 1992 ALCOM Symposium by H. Vithana and D. Johnson

SiO Deposition Technique

Surface: SiOx at 60o on Glass Courtesy of the NSF ALCOM Center: Presented at 1992 ALCOM Symposium by H. Vithana and D. Johnson

Surface: SiOx at 70o on Glass Courtesy of the NSF ALCOM Center: Presented at 1992 ALCOM Symposium by H. Vithana and D. Johnson

Surface: SiOx at 80o on Glass Courtesy of the NSF ALCOM Center: Presented at 1992 ALCOM Symposium by H. Vithana and D. Johnson

( )n ( )n ( )n Non-Rubbed Alignment O O UV O O O Schadt (Jap. J. App. Phys. 31, 2155 (1992)) showed that Poly Vinyl 4-MethoxyCinnamate that has been exposed polymerized with polarized UV light, will align liquid crystals perpendicular to the UV polarization axis O ( )n ( )n O UV O O O ( )n 2+2 cycloaddition of cinnomoyle groups of the cinnamic acid side chains

Non-Rubbed Alignment: Top View UV LC Alignment

Non-Rubbed Alignment: Cis-Trans Gibbons et al, Nature Vol. 351, page 49 (1991).

Summary Surface Alignment Homeotropic Alignment Homogeneous Alignment Weak Anchoring Deposition Methods Non-Rubbed Methods