Directed Self-Assembly of Topological Defects of Liquid Crystal

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Directed Self-Assembly of Topological Defects of Liquid Crystal Min Jeong Shin, Min-Jun Gim, and Dong Ki Yoon Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04216 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 29, 2018

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Directed Self-Assembly of Topological Defects of Liquid Crystal Min Jeong Shin,† Min-Jun Gim,† and Dong Ki Yoon*,†, ‡ † Graduate School of Nanoscience and Technology and KINC, KAIST, Daejeon 34141, Korea ‡ Department of Chemistry, KAIST, Daejeon 34141, Korea KEYWORDS : Liquid crystals, microconfinement, surface treatment, rubbing, smectic

ABSTRACT : One of the alluring aspects of liquid crystals (LCs) is their readily controllable self-assembly behavior, leading to comprehension of complex topological structures and practical patterning applications. Here, we report on manipulating various kinds of topological defects by adopting an imprinted-polymer-based soft microchannel that simultaneously imposes adjustable surface anchoring, confinement, and uniaxial alignment. Distinctive molecular orientation could be achieved by varying surface anchoring conditions at the sidewall polymer and rubbing directions on the bottom layer. On this pioneering platform, a common LC material, 8CB (4’-n-octyl-4-cyano-biphenyl) was placed where various topological defect domains were generated in periodic arrangement. The experimental results showed our platform can change the packing behavior and even shape of topological defects by varying rubbing condition. We believe that this facile tool to modulate

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surface boundary conditions combined with topographic confinement can open a way to use LC materials in potential optical and patterning applications.


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1. INTRODUCTION Compared to a case where defect-free domain of liquid crystal (LC) have been spotlighted in display and the other opto-electonic devices, fabrication of periodic topological defect domains has been another interesting field of LC researches due to its potential application in lithography and template-based nanotechnology.1-9 There have been already numerous attempts to fabricate and control topological defect domains, and further studies on their structures are going on.2, 10-12 Because generating periodic arrays of defect structures cannot be naturally accomplished, it requires several artificial techniques, for example, mechanical shearing, chemical surface modification and topographic confinement.13-17 There has been a suggestion that it is possible to generate periodic arrays of topological defects when smectic LC materials are placed in a confined geometry with the specific surface treatment.1-3, 5, 11, 18 A common way to impose the confinement to LC materials is to exploit microchannel made by silicon or polymer material, where the additional surface treatment can be added to control the orientation of LC molecules. These well-designed boundary conditions finely control the generation of a highly ordered topological defect structures, called toric focal conic domains (TFCDs), which is the typical defect in smectic LC phase, as a result of competition between bulk elasticity of LCs and surface anchoring conditions.2, rubbing.8,

19-21

22-23

Another easy and well-known way to control molecular orientation is

With this method, microscopic grooves are rendered on the substrates,

inducing unidirectional alignment of LC molecules along a preferred direction. In particular, the effect of rubbing is highly remarkable when LC materials pass through the nematic (N) to smectic A (SmA) phase transition as Ok et al reported the two-dimensional orientation of TFCDs through a subsequent second rubbing process in another direction.8 This can be


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realized by strong influence of molecular directors in the N phase on spatial arrangements of TFCDs in the SmA phase. Here, we demonstrate the periodic array of smectic defect structures with aid of imprinted polymer-based soft microchannel and rubbing method, suggesting a way to generate various kinds of textures that have not been achieved by an individual method mentioned above. In earlier studies, antagonistic surface boundary condition on solid silicon (Si)-based microchannel could not be realized, and only single anchoring condition have been possibly applied to the solid topographic surfaces. Instead, closed microfluidic device made of PDMS(polydimethylsiloxane) reliefs and a glass substrate have been reported to induce different anchoring on one side of the channel. Disclination line defect which is frequently observed in nematic phase was intensively studied in this system.

24-25

Our experimental

platform, however, could establish an open microchannel system by using PDMS reliefs as a cast wherein anchoring condition of sidewall is determined by the injected material-either planar or homeotropic inducing polyimide (PI) solution. The imprinted polymer would impose confinement on liquid crystalline molecules along with surface anchoring. Another variable, rubbing condition, could be added on the bottom alignment layer in different direction with respect to the channel. As a result, the various topological defects in the LC phases, depending on the boundary conditions, could be achieved. The resultant optical textures were directly observed by polarized optical microscopy (POM) and laser scanning fluorescence confocal microscopy (LSFCM) to show the detailed molecular orientation and layer arrangement of defects in the N and SmA phase.

2. RESULT AND DISCUSSION


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In order to realize our experimental platform, a simple imprinting technique was employed. A PDMS replica, made from Si microchannel master mold (width 20µm, depth 7µm) was fabricated via a soft lithographic method. The detailed procedure was addressed in the Experimental Section. A bottom glass substrate was treated with a PI polymer solution giving a planar surface anchoring (Figure 1a), which was then rubbed along one direction (green arrows). The PDMS mold was oriented facing the molded microchannel with the PI-treated bottom substrate. By introducing a vertical alignment inducing PI (VAPI) solution to the closed geometry (Figure 1b), the channel was designed to give different surface anchoring at each surface; vertical LC alignment could be driven by air-LC interface and side walls, while planar anchoring would be induced at the bottom of the molded channel. After evaporating solvent of PI solution, the mold was removed leaving the imprinted VAPI microchannel on the planar alignment induced PI (PAPI) coated glass substrate (Figure 1c). Even though the PI sidewalls were shrunk during the solvent evaporation and polymerization, the confinement effect on LC molecules was strong enough to control the generation of desired defect structures (Figure S2 in supporting information). And then 8CB molecules were introduced in the polymer microchannel by capillary force in the isotropic state (∼45 °C), to see the generation of defect structures at each LC phase (Figure 1d). 8CB molecule shows two distinct LC phases, N and SmA phase, respectively on cooling. Generally, LC molecules in the N phase have a long range orientation order, and rubbing method becomes powerful tool to orient LC molecules along rubbing direction in this phase, whose effect is considerably maintained even in the arrangement of SmA layers.26-27 As a result, one dimensionally oriented LC molecules in the N phase, are developed to have layers in the SmA phase, thereby generate different types of textures (Figure S1). To investigate the surface anchoring and rubbing dependent molecular arrangement, pristine or rubbed


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substrates coated with both types of PI were prepared to make three different anchoring conditions, which were PA, VA, hybrid anchoring systems with cell gap of 5µm (Figure 2, Figure S1). When 8CB was subjected to the planar pristine cell (Figure S1 a,e,i), typical schlieren textures of N phase were transformed into fan-shaped textures in the SmA phase. When rubbing was introduced (Figure S1 b,f,j), the optically uniaxial texture was obtained over the N to SmA phase temperature range. In homeotropic cells (Figure S1 c,g,k), the sample exhibited the totally dark image between crossed polarizers through the whole LC phases because of homeotropic aligned LC molecules. Rubbing on homeotropic cell, however, would tilt vertically oriented molecules along one direction, so that the cell showed relatively bright image (Figure S1 d,h,l). Figure 2 shows the optical textures obtained through N to SmA in the hybrid anchoring cell with schematic illustrations of molecular orientation at each condition. This anchoring condition is known to lead smectic layers to go through elastic deformation, giving off curved layer defects like focal conic domains (FCDs) or toric focal conic domains (TFCDs).2, 7-8, 10 Under the pristine condition (Figure 2a), schlieren textures composed of point defects and dark brushes were observed in the N phase. With decreasing temperature, this sample showed tilted FCDs at the transition point from the N to SmA phase (Figure 2c),26 followed by creating TFCDs all over the substrate at far lower temperature in the SmA phase (Figure 2e). It is preferential to form TFCDs in this antagonistic condition because VAPI causes perpendicular alignment of molecules at the top while layer bending toward bottom substrate is required to fulfill planar anchoring condition as shown in the schematic image (Figure 2g). When this condition was combined with uniaxial groove (Figure 2b,d,f), array of tilted FCDs could be obtained after passing through uniform N textures. Self-assembly of LC molecules, in this case, would be affected by uniaxially aligning manner, and non-toroidal FCDs were


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formed where eccentric ellipse and hyperbola line pairs allineated along the rubbing direction (Figure 2h). As shown in Figure 2 and Figure S2, this kind of simple experimental platform combining rubbing and surface treatment has been already conducted in numerous studies.8, 10, 12, 26

With just a simple imprinting process added, we could create a new micron scale system which imposes confinement, controllable surface anchoring and uniaxial alignment on LC molecules simultaneously. The selected anchoring conditions at each different surface were as followed: planar anchoring at the bottom substrate and the homeotropic state at sidewalls and the air-LC interface. The other variables were with/without rubbing process and the rubbing direction with respect to the microchannel. Therefore, three different conditions were made, and each condition could govern the orientation of 8CB molecules in different manners. In the absence of rubbing (Figure 3a,d,g), long range ordered optical textures were observed in the N phase, prior to being transformed into defect structures in the SmA phase with brushes at the sidewall. To estimate the molecular orientation, first order retardation plate (λ= 520nm) was inserted between the sample and analyzer. If molecules or their optical axis are oriented parallel and perpendicular to the slow axis of the retardation plate, the image shows cyan blue and yellow color, respectively. Magenta color is displayed when LC molecules are aligned vertical, parallel and perpendicular to the polarizers. The right part of each image in Figure 3 corresponds to optical data obtained with the retardation plate. As indicated by color change from cyan blue to yellow, the director of LC molecules in the N phase was expected to go through gradual elastic deformation from the sidewalls toward the center of the microchannel (Figure 3a). It represented LC molecules were mostly positioned perpendicular with respect to the sidewall, but parallel to channel direction in the middle. At TN-SmA, tilted FCDs were generated from the sidewall, revealing the molecular directors were


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maintained as those formed in the N phase (Figure 3d). These tilted FCDs did not appear right adjacent to sidewalls, instead it seemed being nucleated from the side brush textures. However, at far decreased temperature in the SmA phase(Figure 3g), tilted FCDs were transformed into TFCDs, retaining side brush textures for elastic energy minimization.8, 26, 28 This change was further facilitated by the absence of uniaxial grooves on the bottom layer which allowed for molecules to readily form radial arrangement. One of the interesting features of the texture was the brush emanating from VAPI wall, which might be an array of parabolic FCDs as reported in the previous studies (Figure S3). 6, 11, 29 This was resulted from local layer undulation which subsequently tilt layers to fulfill all of anchoring conditions, maintaining equidistant layer thickness. Also, POM and LSFCM images revealed the molecules were organized in principally perpendicular direction to sidewalls with subtle undulation as illustrated by alternative arrangement of yellow and blue color (Figure S3c). This phenomenon might occur due to the strong anchoring of the PI alignment layer, thus splay deformation was suppressed in this region. When rubbing was done on the underlying PAPI layer along the direction of microchannel (Figure 3b,e,h), two distinct domains were observed in the N phase. The broad yellow color shown along the middle of the channel indicated that most of LC molecules were aligned parallel with respect to the microchannel (Figure 3b). The homeotropic anchoring given at sidewalls induced perpendicular molecular orientation relative to rubbing direction. This sample went through the N to SmA phase transition, exhibiting an array of tilted FCDs with side brush patterns (Figure 3e). The arcs of the FCDs were formed along the rubbing direction because the molecules in the N phase were originally oriented parallel to the rubbing direction.12, 30 When the temperature went below TN-SmA, these FCDs were changed rapidly into the periodic arrangement of TFCDs with sidewall brushes (Figure 3h).


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The last condition that introduced perpendicular microgrooves to the channel direction led to generation of another type of defect structures. In the N phase, it was noticeable that the molecules were aligned perpendicular to the channel direction, giving a pseudo-single domain (Figure 3c). Passing through TN-SmA, an array of FCDs which was arranged along the rubbing direction was developed (Figure 3f). Then a periodic zigzag pattern was gradually developed in the SmA phase (Figure 3i).10 The subtle difference in birefringence between each condition came from a minute difference in LC thickness, which was originated from the free surface of LC-air interface, and this variance was unavoidable. The optical textures acquired at each condition could be maintained after thermal annealing process, showing its capability to be used as a template. For example, periodic arrangement of zigzag pattern was maintained after thermal annealing process for 24 hours at least. (Figure S4) The competition of anchoring forces between free surface and walls, combined with rubbing and micron-scale confinement enabled fabrication of various periodic defect arrays. To demonstrate the versatility of our method, we extended the study by using different anchoring condition at sidewalls by using PAPI material. Giving planar anchoring condition at the sidewalls indicates that the free surface opened to the air is the only source to act as homeotropic boundary. For this, we treated the PAPI on both bottom and side walls of a microchannel, keeping rubbing condition the same as in Figure 3. The uniform vertical arrangement of LC molecules near PAPI sidewalls-air interface was formed (Figure 4). Figure 4a represents the typical irregular N texture resulting from degenerate azimuthal alignment of LC molecules. During the phase transition, FCDs were developed and the defect textures lay over the whole area (Figure 4d), which is comparable with the antagonistic case (Figure 3). TFCD arrays with semicircular domains from the sidewalls were generated at the lower SmA phase temperature (Figure 4g).1, 11 When rubbing was present (Figure 4b,e,h and


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Figure 4c,f,i), similar textures were emerged except for the direction of fan-shaped FCD arrays. Passing through TN-SmA, the fan-shaped FCDs were gradually transformed into tilted FCDs due to effect of uniaxial alignment. Compared to sidewall region where anchoring force was superior to rubbing, middle part of the channel was filled by four-lobe FCDs with slight tilt toward rubbing direction. Therefore, the dramatic structural variation of topological defects in this condition could not be observed not like in VAPI-sidewall’s case (Figure 3). How LC molecules are oriented in antagonistic boundary condition is model sketched in Figure 5. As characteristic textures-showing simultaneous effect of confinement, surface anchoring and rubbing-were well represented in case of Figure 3g, e and i, schematic models corresponding to each case are suggested. In antagonistic boundary condition, the LC molecules near VAPI sidewalls were faced to adopt three different anchoring forces coming from sidewall, air, and underlying PAPI layer. A model in Figure 5a is presented to explain the optical textures in Figure 3g.11 The above schematic diagram shows top-down view of layer arrangement of defect structure, while the bottom one is cross-section view, in which red walls represent VAPI layer. The green domain in Figure 5a indicates TFCD domains surrounded by parabolic FCD array, noted by two yellow bold lines. 11 The layers must bend at the sidewalls as illustrated in bottom diagram in Figure 5a to meet the constraints coming from air and VAPI layer. The surface profile on this specific condition also supported this phenomenon in which curved surface topology at microchannel-LC interface and ~100nm depression at the middle were observed. (Figure S5) Analysis on the surface morphology on the other texture, the zigzag pattern, was already reported in ref 10. Figure 5b depicts a situation at TN-SmA. Due to the presence of uniaxial grooves, the molecular directors in the middle tend to align along the channel direction while the orthogonal arrangement is preferred at sidewalls. Therefore, it is highly probable that molecules go


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through gradual orientational change from the sidewall to center. This behavior in the N phase is maintained even at TN-SmA, where layer mismatch (violet region) is unavoidable. Even though the uniaxial alignment effect is present at the bottom PAPI layer, the mismatch between the smectic layers, which were forming the tilted FCDs and located near the domain boundary, would vanish at lower temperatures, giving zero eccentricity. Hence, the relatively stable defect structures, TFCDs, would be spontaneously formed. Thus, the same texture as Figure 5a emerged in SmA phase. On the other hand, coincidence in molecular directors induced from VAPI sidewall and PAPI rubbed substrate results in periodic arrangement of zigzag optical patterns representing alternative FCDs (Figure 5c).10, 31 Therefore, confinement and uniform molecular arrangement due to the sidewall and rubbing were effective to develop periodic arrangement of FCDs, which has not been realized on the flat substrate.

3. CONCLUSION In summary, we proved that different types of anchoring forces coming from each surface of the topographical patterns and rubbing effect can be achieved in a single system. Not like the conventional treatment used in the solid substrate, a variety of surface anchoring conditions could generate various types of topological defect arrays of LC phase. Direct observation of the optical textures in altered rubbing conditions indicated the uniqueness of our LC control platform as a promising lithographic tool. Other than smectic phase, more complex LC materials would be controlled by adjusting anchoring conditions and rubbing according to the needs.

4. EXPERIMENTAL METHODS


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Materials and sample preparation: A Si microchannel with width of 20µm and depth of 7µm was chemically cleaned with acetone, ethanol, and DI water respectively. Prior to treating the Si wafer with F-OTS (tridecafluoro-1,1,2,2-tetrahydrooctyltrichloro-silane, Gelest, Inc.), this Si wafer was put under O2 plasma for 5 min. 100 µL of as-received F-OTS stored in a dried vial and the cleaned Si microchannel were placed in a dessicator together and put under vacuum for 1 hr. After an hour, this microchannel was heated at 120 ℃ . The un-cross-linked PDMS (polydimethylsiloxane, Sylgard 184A, Sewang Hitech) was prepared by mixing PDMS precursors and curing agent(Sylgard 184B, Sewang Hitech) in a ratio of 10:1, and this mixture was poured onto a master Si-microchannel. After removing air bubbles by placing the sample in the dessicator under vacuum for 1 hr, it was cured for 4 hours in an oven at 65 ℃. The flexible PDMS mold was then fabricated by peeling it off from the Si wafer. Glass slides chemically rinsed via the same cleaning method above were spin-coated with PI material (PAPI: PIA-5550-02A, JNC) to induce planar anchoring on liquid crystalline materials. The surface treated substrates were soft-baked at 90℃ for 100s and then cured at 200℃ for 2 hr. Depending on the desired defect types, the glass substrates were rubbed or not rubbed using a rubbing machine (RMS-50-M, Namil). The PDMS mold made in the previous step was placed on the glass slides, forming closed micro-channels. In this step, three different conditions could be made; one was the PDMS mold put on a non-rubbed glass slide, another was PDMS mold put on the rubbed substrate with channel direction parallel to rubbing direction, and the other was the mold put on the slide with channel direction perpendicular to rubbing direction. The vertical alignment inducing PI (VAPI: AL60702, JSR) material was then injected in the associated PDMS mold-glass substrate with capillary


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force. This mold-glass slide system was heated at 90℃ for 15 min to dry off the solvent. The mold was removed along the long axis of the microchannel in order not to distort the shape of microchannel. After removing the PDMS film from the substrate, the microchannel made of VAPI at sidewalls with PAPI treated bottom was cured at 200℃for 2hrs. Finally, the microchannel imposing hybrid anchoring (vertical at sidewalls and planar on underlying part) was fabricated. In order to study the versatility of our platform, microchannel imposing planar alignment of molecules at both the sidewalls and bottom was also produced with minute modification to the method above, which was injecting PAPI, instead of VAPI, to the PDMS mold-glass slide combination. The LC material, 8CB (4’-n-octyl-4-cyano-biphenyl, Synthon Chemicals), was injected in the microchannel via capillary force at its isotropic phase (T>40.5℃). The sample was cooled at a rate of 0.1℃/min. For observation with LSFCM, 8CB was mixed with 0.01 wt% of a fluorescent dye molecule, N,N’-bis(2,5-di-tert-butylphenyl)-3,4,9,10-perylenedicarboximide(BTBP)

(dissolved

in

Chloroform at 0.01 wt% concentration) which was excited at 488nm and emitted in the range of 510-550nm, at 1:1 ratio and heated at 60℃ to remove any solvent.

Characterization: The optical textures of the LC material in various conditions were investigated by DTLM (LV100POL, Nikon), and further information on molecular orientations at the SmA phase could be obtained from LSFCM (C2 plus, Nikon) that was embedded with a cooled chargecoupled device (CCD) colour camera (DS-Ri1, Nikon). The surface morphology and height


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profile of the optical pattern were analyzed via AFM(Bruker, Multimode-8) in the tapping mode. The temperature was controlled with the heating stage (LINKAM LTS420) and a temperature controller (LINKAM TMS94).


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Figure 1. An experimental scheme to generate antagonistic boundary system with confinement and rubbing. (a) Planar PI coating on glass substrate, followed by rubbing. (b) PDMS put on the underlying substrate and injection of PI solution. (c) Removal of PDMS mold and thermal treatment to produce a novel LC control system. (d) Injection of LC material. The green arrow indicates the rubbing direction.


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Figure 2. POM images of sandwich cells that have antagonistic surface anchoring condition with schematic illustration of layer arrangement. (a,c,e) Pristine condition cell. (b,d,f) Rubbed cell with rubbing direction indicated by green arrow. (g) Schematic illustration of layer orientation in case of pristine cell. (h) In case of rubbed cell. (Scale bar, 40 µm).


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Figure 3. Smectic 8CB liquid crystal confined in antagonistic boundary microchannel on cooling. (a,d,g) In absence of rubbing. (b,e,h) Rubbing direction parallel to the microchannel. (c,f,i) Rubbing direction perpendicular. Right figure of each image was obtained with retardation plate (λ = 530 nm) inserted to show molecular orientation. (Scale bar, 20 µm) At the bottom of the figure, magnified images of white-dashed box were provided. (Scale bar, 10 µm)


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Figure 4. Smectic 8CB liquid crystal confined in homogeneous condition microchannel on cooling. (a,d,g) In absence of rubbing. (b,e,h) Rubbing direction parallel to the microchannel. (c,f,i) Rubbing direction perpendicular. Right figure of each image was obtained with retardation plate (λ = 530 nm) inserted to show molecular orientation. (Scale bar, 20 µm) At the bottom of the figure, magnified images of white-dashed box were provided. (Scale bar, 10 µm)


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Figure 5. Schematic modeling of defect structures in antagonistic boundary microchannel and cross-sections. (a) TFCD (green domain) surrounded by parabolic FCDs (yellow bold lines) and its cross-section (X-X’). The yellow bold lines correspond to two parabolas of parabolic FCDs passing through each other at the focal point with normal orientation. (b) FCD along channel direction with parabolic FCDs (yellow bold lines) at side. Red domain indicates molecules going through gradual polar angle change from front to back of the page. (c) FCD arrays in perpendicular manner, forming periodic zigzag pattern. The red dot and red line represent hyperbola line of FCDs. A nail “T” means director change of which its end going inward through the page.


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SUPPORTING INFORMATION The following files are available free of charge. Figures showing POM images of LC, sandwiched between homogeneously planar and homeotropic anchoring conditions. SEM image of the PDMS mold and Polymer-imprinted microchannel (antagonistic and homogeneous boundary anchoring conditions, respectively). Figures of POM and LSFCM image of LC texture acquired in antagonistic boundary condition. Figures of optical pattern (zigzag pattern) maintained after 24 hours of thermal annealing process at 33.5℃. AFM surface profile on the optical pattern obtained in the antagonistic boundary conditions and its height profile across the microchannel. (PDF)

AUTHOR INFORMATION * E-mail: [email protected] ACKNOWLEDGMENT This study was supported by a grant from the National Research Foundation (NRF) and funded by the Korean Government (2017R1E1A1A01072798, 2015R1A1A1A05000986 and 2017M3C1A3013923).


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Langmuir

level. Physical Review E 2010, 82 (4), 041705.


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Table of Contents (TOC) Graphic of this manuscript


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Directed Self-Assembly of Topological Defects of Liquid Crystal

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