Controlled Generation of Nematic Twist Disclinationsand capacitance sensor provide precise closed...
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Controlled Generation of Nematic Twist Disclinations Liquid crystal configuration Experimental setup Experimental results Related and future work Abstract Christopher Culbreath Nik Glazar Hiroshi Yokoyama Liquid Crystal Institute. Kent State University. Kent, Ohio. [1] J. Fukuda and H. Yokoyama, Phys. Rev. E 66, 012703 (2002). [2] De Gennes and Prost. The Physics of Liquid Crystals Second Edition. Oxford Sci Pub. 1993. [1] J. Fukuda and H. Yokoyama, Phys. Rev. E 66, 012703 (2002). [2] De Gennes and Prost. The Physics of Liquid Crystals Second Edition. Oxford Sci Pub. 1993. [3] F M Leslie. Pramana Suppl. No 1. 1975 [4] T J Scheffer. J. Appl Phys. 49(12) 1978. [5] T. Nagaya, H. Orihara, and Y. Ishibashi. J. Phys. Soc. of Japan. 56 (5) 1987 Cell Thickness (d) Temperature (T) Twist Angle (θ) Schematic overview Bulk Dynamic control variables Defect generation Bulk vs surface Nucleation twist angle System description Implementation details Transition Mechanism Halogen Lamp Lock-in amplifier Rotating bottom substrate and thermal chamber Fixed top substrate and thermal chamber Stepper motor coarse thickness adjustment Piezo actuator Temperature controlled recirculating water bath Objective Analyzer Liquid Crystal Camera Capacitance distance sensor 632nm Filter Polarizer _ + Labview control Mechanical rendering c . Upper thermal chamber a. Convex bottom substrate Disclination lines play a decisive role in determining the equilibrium structures of topologically constrained liquid crystal systems [1] including cholesteric blue phases, twist grain boundary phases and liquid crystal colloids [2]. The extra energy associated with disclinations is key to stabilizing one particular director structure over another, yet our knowledge of disclination energetics is limited as are characterization methods. So, we have been investigating the nucleation and stability of reverse twist disclinations in nematic liquid crystals. We present a method for the controlled generation of disclination loops and our observations and measurements of their nucleation. In a typical experiment, we use two rubbed polyimide planar aligned sub- strates. Our method uses a mechanical rotation of the alignment substrate to induce a twist in the liquid crystal bulk. The substrates are initially positioned with their anchoring directions aligned, and then we rotate the bottom substrate +100˚ (counter-clockwise) relative to the top substrate. At twist angles greater than 90˚, the liquid crystal is in a su- per-twisted state. With a +100˚ twist, the symmetry of the nematic director allows We have designed and fabricated a liquid crystal observation system that can dynamically change the twist angle (θ), thickness (d) and temperature (T) of a liquid crystal sample. The twist angle is varied through a mechanical rotation of the cell’s bottom substrate. A stepper motor, piezo actuator and capacitance sensor provide precise closed loop At left, a defect generated by our technique is shown. A +1/2 twist disclination defect loop separates regions of opposite handed twist. Above, a time lapse sequence of the nucleation process around a piece of debris is shown. The photos were taken one minute apart. The minimum cell thickness is 20μm, and the twist angle is 155˚. Bulk defects are characterized by a uniform round shape and a visible black defect line. Above, the nucleation angle of the twist disclination structure is shown as a function of minimum cell thickness. We observe transitions at twist angles much less than the 180˚ predicted by Leslie’s theory. Our observations suggest that this transition cannot be attributed to a slipping at the surface as has been previously proposed. Slipping at the surface is characterize by an irregular border and an apparent solid friction interaction between the structure and the surface coating. We utilize a convex lens (typically f=1300mm) for the bottom substrate. This has the advantage of localizing the region of minimum thickness to the center of the cell even in the case of non uniform substrate alignment (fig a). However, it is critical that the bottom substrate be precisely perpendicular to the axis of rotation. This calibration (fig b) is completed with just the bottom substrate installed in the machine. A laser is reflected off the constantly rotating substrate onto a distant screen. The tilt micrometer screws are adjusted until the laser spot on the screen remains stationary through the entire rotation. This is the same laser lever technique used in an atomic force microscope. We seek to understand that mechanism in which twist disclinations form in the bulk. Contrary to previously proposed theories, our work suggests that the formation of reverse twist domains is not due to a simple slipping of the surface alignment, but involve a tilt mitigated transition in the bulk even at twist angles less than 180˚. The formation of defect loops in the bulk are often facilitated by the presence of debris. Initial observations also suggest that flow effects play a crucial role in the transition. Our future work strives to quantify the role of flow in the formation of defects as well as determine the precise mechanism by which the transition occurs. We also seek to map the full energetic landscape of the disclination, including it’s line energy and the stability and existence of metastable states. We have also used our experimental apparatus to optically measure the cell thickness, and have used our dynamic manipulation of the cell thickness and twist angle as an extra degree of freedom in constructing a method to characterize other standard material parameters as well. positioning of the upper substrate relative to the bottom substrate. The temperature is regulated by circulating water through a custom thermal chamber which encloses the liquid crystal sample (fig c). Mechanical stability was a primary design consideration. The system allows for micrometer tilt adjustment of the bottom substrate relative to both the axis of rotation and top substrate. An integrated polarizing microscope, illumination system and camera provide for observation. The system is operated and automated using LabView. The software can vary the thickness, twist, take time lapse photo graphs, monitor the distance sensor as well as automate these processes for extended data collection. b. Laser Lever Specification Absolute Resolution Displacement Precision Twist Angle 1 degree 0.001 degree Thickness 0.5 μm 0.01 μm Temperature 1˚C 0.1˚C Optical 7.7μm (100x magnification on 110ppi display) Rotational flatness ±1µm/180˚ rotation Z π - θ Top Bottom θ Top Bottom a. b. c. the orientation to follow either a counter- clockwise +100˚ “super-twist,” or a clock- wise −80˚ “preferred-twist.” When su- per-twist and preferred twist domains exist simultaneously in the bulk, a +1/2 defect loop divides the two regions [5]. There- fore, the growth of the preferred-twist domain is regulated by the competition of its reduced elastic twist energy and the in- creased energetic cost of the growing boundary defect, the cell thickness and the relative surface alignment. a. schematic representation of our experimental cell with a right hand super twisted region (red) and a left-hand preferred twist region (orange). Projected director confor- mations of super-twisted (b) and preferred-twisted (c) domains. Surface f s = w sin θ In order for a reverse twist domain to be nucleated the super twist must be unwound. Two mechanisms have been proposed (shown left). The bulk mechanism is a tilt to a 90˚ state with no twist, first proposed by Leslie [3]. Leslie’s equations predict a tilt transition at 180˚ twist, and allow no tilt solution for twist angles less than 180˚ (see below). Leslie also suggested that reverse twist domains observed at lower twist angles are due to slipping weak surface anchoring. At left, the solutions to Leslie’s equations are shown [4] for various ratios of elastic constants. Note, that no 90˚ tilt solution exits for twist angles less than 180˚ 0˚ 60˚ 120˚ 180˚ 240˚ Minimum cell thickness (μm) Disclination nucleation angle 0 12.5 25 37.5 50 1 mm 1 mm 1 mm 1 mm 1 mm 1 mm C M Y CM MY CY CMY K DefectGeneration copy.pdf 1 7/1/14 4:03 PM
Transcript of Controlled Generation of Nematic Twist Disclinationsand capacitance sensor provide precise closed...
Controlled Generation of Nematic Twist Disclinations
Liquid crystal configuration
Experimental setup
Experimental results Related and future work
Abstract
Christopher Culbreath Nik Glazar Hiroshi YokoyamaLiquid Crystal Institute. Kent State University. Kent, Ohio.
[1] J. Fukuda and H. Yokoyama, Phys. Rev. E 66, 012703 (2002).[2] De Gennes and Prost. The Physics of Liquid Crystals Second Edition. Oxford Sci Pub. 1993.
[1] J. Fukuda and H. Yokoyama, Phys. Rev. E 66, 012703 (2002). [2] De Gennes and Prost. The Physics of Liquid Crystals Second Edition. Oxford Sci Pub. 1993. [3] F M Leslie. Pramana Suppl. No 1. 1975 [4] T J Scheffer. J. Appl Phys. 49(12) 1978. [5] T. Nagaya, H. Orihara, and Y. Ishibashi. J. Phys. Soc. of Japan. 56 (5) 1987
Cell Thickness (d) Temperature (T) Twist Angle (θ)
Schematic overview
Bulk
Dynamic control variables
Defect generation Bulk vs surface Nucleation twist angle
System descriptionImplementation details
Transition Mechanism
Halogen Lamp
Lock-in amplifier
Rotating bottom substrate and thermal chamber
Fixed top substrateand thermal chamber
Stepper motor coarse thicknessadjustment
Piezoactuator
Temperature controlledrecirculating water bath
Objective
Analyzer
Liquid Crystal
Camera
Capacitancedistance sensor
632nm Filter
Polarizer
_+
Labview control
Mechanical rendering c . Upper thermal chamber
a. Convex bottom substrate
Disclination lines play a decisive role in determining the equilibrium structures of topologically constrained liquid crystal systems [1] including cholesteric blue phases, twist grain boundary phases and liquid crystal colloids [2]. The extra energy associated with disclinations is key to stabilizing one particular director structure over another, yet our knowledge of disclination energetics is limited as are characterization methods. So, we have been investigating the nucleation and stability of reverse twist disclinations in nematic liquid crystals. We present a method for the controlled generation of disclination loops and our observations and measurements of their nucleation.
In a typical experiment, we use two rubbed polyimide planar aligned sub-strates. Our method uses a mechanical rotation of the alignment substrate to induce a twist in the liquid crystal bulk. The substrates are initially positioned with their anchoring directions aligned, and then we rotate the bottom substrate +100˚ (counter-clockwise) relative to the top substrate. At twist angles greater than 90˚, the liquid crystal is in a su-per-twisted state. With a +100˚ twist, the symmetry of the nematic director allows
We have designed and fabricated a liquid crystal observation system that can dynamically change the twist angle (θ), thickness (d) and temperature (T) of a liquid crystal sample. The twist angle is varied through a mechanical rotation of the cell’s bottom substrate. A stepper motor, piezo actuator and capacitance sensor provide precise closed loop
At left, a defect generated by our technique is shown. A +1/2 twist disclination defect loop separates regions of opposite
handed twist. Above, a time lapse sequence of the nucleation process around a piece of debris is shown. The photos were
taken one minute apart. The minimum cell thickness is 20µm, and the twist angle is 155˚.
Bulk defects are characterized by a uniform round shape and a visible black defect line.
Above, the nucleation angle of the twist disclination structure is shown as a function of minimum cell thickness. We observe transitions at twist angles much less than the 180˚ predicted by Leslie’s theory. Our observations suggest that this transition cannot be attributed to a slipping at the surface as has been previously proposed.
Slipping at the surface is characterize by an irregular border and an apparent solid friction interaction between the structure and the surface coating.
We utilize a convex lens (typically f=1300mm) for the bottom substrate. This has the advantage of localizing the region of minimum thickness to the center of the cell even in the case of non uniform substrate alignment (fig a). However, it is critical that the bottom substrate be precisely perpendicular to the axis of rotation. This calibration (fig b) is completed with just the bottom substrate installed in the machine. A laser is reflected off the constantly rotating substrate onto a distant screen. The tilt micrometer screws are adjusted until the laser spot on the screen remains stationary through the entire rotation. This is the same laser lever technique used in an atomic force microscope.
We seek to understand that mechanism in which twist disclinations form in the bulk. Contrary to previously proposed theories, our work suggests that the formation of reverse twist domains is not due to a simple slipping of the surface alignment, but involve a tilt mitigated transition in the bulk even at twist angles less than 180˚. The formation of defect loops in the bulk are often facilitated by the presence of debris. Initial observations also suggest that flow effects play a crucial role in the transition. Our future work strives to quantify the role of flow in the formation of defects as well as determine the precise mechanism by which the transition occurs. We also seek to map the full energetic landscape of the disclination, including it’s line energy and the stability and existence of metastable states. We have also used our experimental apparatus to optically measure the cell thickness, and have used our dynamic manipulation of the cell thickness and twist angle as an extra degree of freedom in constructing a method to characterize other standard material parameters as well.
positioning of the upper substrate relative to the bottom substrate. The temperature is regulated by circulating water through a custom thermal chamber which encloses the liquid crystal sample (fig c). Mechanical stability was a primary design consideration. The system allows for micrometer tilt adjustment of the bottom substrate relative to both the axis of rotation and top substrate. An integrated polarizing microscope, illumination system and camera provide for observation. The system is operated and automated using LabView. The software can vary the thickness, twist, take time lapse photo graphs, monitor the distance sensor as well as automate these processes for extended data collection.
b. Laser Lever
Specification Absolute Resolution Displacement PrecisionTwist Angle 1 degree 0.001 degreeThickness 0.5 µm 0.01 µmTemperature 1˚C 0.1˚COptical 7.7µm (100x magnification on 110ppi display)Rotational flatness ±1µm/180˚ rotation
Z
π - θ
Top
Bottom
θ
Top
Bottom
a.
b.
c.
the orientation to follow either a counter-clockwise +100˚ “super-twist,” or a clock-wise −80˚ “preferred-twist.” When su-per-twist and preferred twist domains exist simultaneously in the bulk, a +1/2 defect loop divides the two regions [5]. There-fore, the growth of the preferred-twist domain is regulated by the competition of its reduced elastic twist energy and the in-creased energetic cost of the growing boundary defect, the cell thickness and the relative surface alignment.
a. schematic representation of our experimental cell with a right hand super twisted region (red) and a left-hand preferred twist region (orange). Projected director confor-mations of super-twisted (b) and preferred-twisted (c) domains.
Surface fs= w sin θ
2.96 µm /px
In order for a reverse twist domain to be nucleated the super twist must be unwound. Two mechanisms have been proposed (shown left). The bulk mechanism is a tilt to a 90˚ state with no twist, first proposed by Leslie [3]. Leslie’s equations predict a tilt transition at 180˚ twist, and allow no tilt solution for twist angles less than 180˚ (see below). Leslie also suggested that reverse twist domains observed at lower twist angles are due to slipping weak surface anchoring.
At left, the solutions to Leslie’s equations are shown [4] for various ratios of elastic constants. Note, that no 90˚ tilt solution exits for twist angles less than 180˚
Control of molecular orientation is critical in organic electronic devices. Liquid crystals depends on
surface-induced alignment, organic conductors require well-alignment carrier paths, and solar cells need highly
oriented active molecular layers for efficient separation of charge carriers. The maskless photoalignment is a
technique that allows non-contact, photo-induced control of molecular orientation in polymers and organic materials
directly in the profile required for specific device applications. We developed a digitally controlled
ultra-violet light micro-pattern generator with the smallest resolution down to 2 micrometer. The polarization state of
UV light pattern can be synchronized with computer generated patterns so as to reconstruct a complicated
polarization modulated patterns on the projected surface. The operation of the developed instrument has been
demonstrated for liquid crystal micro-optics and bistable displays. A straightforward extension is a 3D rapid prototyping of bulk embedded anisotropic organic
structures.
Photoalignment
UV lightsource
Mask Polarizer Photoalignmentsubstrate
Patterned alignmentsurface
NN
SO3Na
NaO3S
NN OH
HO
HOOC
COOH
PhotoalignmentAbstract
Control of molecular orientation is critical in organic electronic devices. Liquid crystals depends on surface-induced alignment, organic conductors require well-alignment carrier paths, and solar cells need highly oriented active molecular layers for efficient separation of charge carriers. The maskless photoalignment is a technique that allows non-contact, photo-induced control of molecular orientation in polymers and organic materials directly in the profile required for specific device applications. We developed a digitally controlled ultra-violet light micro-pattern generator with the smallest resolution down to 2 micrometer. The polarization state of UV light pattern can be synchronized with computer generated patterns so as to reconstruct a complicated polarization modulated patterns on the projected surface. The operation of the developed instrument has been demonstrated for liquid crystal micro-optics and bistable displays. A straightforward extension is a 3D rapid prototyping of bulk embedded anisotropic organic structures.
Pre-tilt photoalignment
Photorubbing is a process in which a cross-linking type photoalignment material is irradiated with periodically modulated polarized UV light. These polarized UV stripes are translated across the substrate to yield a stable pretilt angle in the resulting alignment surface1. Our maskless photoalignment system has been equipped with a motorized translating platform at the lower exposure stage. The speed and direction of the translation are integrated into the sytem Labview control. This functionality make the rapid fabrication of photorubbed surfaces possible, with control of the scan rate, mask perodicity and exposure time.
UV lightsource
Periodicmask
Polarizer Translatingsubstrate
0˚
60˚
120˚
180˚
240˚
Minimum cell thickness (µm)
Dis
clin
atio
n nu
clea
tion
angl
e
0 12.5 25 37.5 50
1 mm
1 mm
1 mm
1 mm
1 mm 1 mm
C
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CM
MY
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CMY
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DefectGeneration copy.pdf 1 7/1/14 4:03 PM
