- Email: [email protected]
The opto-thermal effect on encapsulated cholesteric liquid crystals
Solid State Electronics xxx (xxxx) xxx–xxx
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Solid State Electronics journal homepage: www.elsevier.com/locate/sse
The opto-thermal effect on encapsulated cholesteric liquid crystals ⁎
Yu-Sung Liua, , Hui-Chi Linb, Kin-Min Yangb a b
Department of Electronic Engineering, National Formosa University, No. 64, Wunhua Rd., Huwei Township, Yunlin County 63201, Taiwan, ROC Department of Electro-Optical Engineering, National Formosa University, No. 64, Wunhua Rd., Huwei Township, Yunlin County 63201, Taiwan, ROC
A R T I C L E I N F O
A B S T R A C T
Keywords: Micro-encapsulated cholesteric liquid crystals Opto-thermal Heat conduction equation
In this study, we implemented a micro-encapsulated CLC electronic paper that is optically addressed and electrically erasable. The mechanism that forms spot diameters on the CLC films is discussed and verified through various experimental parameters, including the thickness of CLCs and Poly(2,3-dihydrothieno-1,4-dioxin)-poly(styrenesulfonate) (PEDOT:PSS), pump intensity, and pumping time. The opto-thermal effect, brought on by the PEDOT:PSS absorbing layer, causes the spot diameters on the cholesteric liquid crystal thin films to vary. According to our results, the spot diameter is larger for a sample with a thinner cholesteric liquid crystal layer with the same excitation conditions and same thickness of the PEDOT layer. The spot diameter is also larger for a sample with a thicker PEDOT under the same excitation conditions and same thickness of the cholesteric liquid crystal layer. We proposed a simple heat-conducting model to explain the experimental results, which qualitatively agree with this theoretical model.
1. Introduction Cholesteric liquid crystals (CLCs) can be encapsulated in droplet form and retain its bistability as long as the droplet size is significantly larger than the pitch of the CLCs [1–4]. In this study, the CLCs are encapsulated using the emulsification method [5–9], in which CLCs, water, and a water-dissolvable polymer are all mixed together. The water dissolves the polymer to form a viscous solution that does not dissolve the CLCs. This mixture is combined with a high-speed propeller blade to form an emulsion that contains a number of micron-size CLC capsules. The emulsion is then coated on a plastic-conducting substrate while the water is allowed to evaporate, leaving the micro-encapsulated CLCs suspended in the polymer matrices. Once the water evaporates, a second layer of conducting polymer is coated on top of CLC emulsion to form a device. Due to the high viscosity of the CLC emulsion, the device can be made using a roll-to-roll process [6,7]. The encapsulated CLCs mainly undergo four states in this article. First, most of the directors of the CLC molecules are perpendicularly aligned to the film surface with a sufficiently strong electric field across the film. Referred to as the homeotropic state, this state is unstable and immediately transfers to a planar state once the strong electric field is removed. In the planar state, in which the helical axis of the CLC directors is perpendicular to the micro-cell surface, the Bragg reflection [10–13] of the CLCs results in the reflection of incident light, thus making the film appear bright. On the other hand, the film in the planar state, which is stable, can be switched to the focal conic state, another ⁎
stable state; therefore it exhibits bistability with either the help of weak electric fields across the film or appropriate heating methods. The CLCs preserve the helical structure in the focal conic state, but the helical axes are irregularly distributed in a short-range order and form numerous small discontinuities in the reflective index, which result in the highly random scattering of incident light on the film surface; therefore the film appears dark. The transition from the planar to focal conic state is reversible if a sufficiently strong electric field is applied again. The processes described above can also be circulated. Many methods have been suggested to alter CLC behavior. Among them, the photo-chemical reaction was explored by doping azo dyes into the CLCs. The main mechanism is the trans-cis photoisomerization effect of the azo molecules excited by the optical electric field [14,15]. However, some researchers proposed that the opto-thermal effect that causes the phase changes in the CLCs may have a better reaction rate than the photo-chemical effect [16]. In this study, we exploited the opto-thermal effect and applied a layer of micro-encapsulated CLC emulsion on the single flexible APET/ITO (Amorphous Polyethylene Terephthalate/ Indium Tin Oxide) substrate. Due to the low absorption of the CLC layer, another layer of PEDOT:PSS (Poly(2,3-dihydrothieno1,4-dioxin)-poly(styrenesulfonate)) was used as the optical absorption and electrically conducting layer. Consequently, the sample film can be patterned with the opto-thermal effect, and the pattern created on the film can be erased by an external electric field and returned to the originally blank film due to the bistability of the CLCs. The film structure is demonstrated in Fig. 1, in which a layer of the micro-
Corresponding author. E-mail address: [email protected] (Y.-S. Liu).
http://dx.doi.org/10.1016/j.sse.2017.09.005
0038-1101/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Liu, Y.-S., Solid State Electronics (2017), http://dx.doi.org/10.1016/j.sse.2017.09.005
Solid State Electronics xxx (xxxx) xxx–xxx
Y.-S. Liu et al.
Temperature u (ÛC) Fig. 1. The sample structure.
encapsulated CLCs was first coated on the APET/ITO through rod coating. Then, the PEDOT:PSS was applied on top of the CLC layer. Unwanted solvent was evaporated by placing the sample into an oven. Before we get into the experimental details, we present a theoretical simulation in Section 2 in order to qualitatively explain the phenomena that we encountered in Section 3.
0
5
Time t (s) 10
2. Theoretical simulation
0
Depth x (ȝm)
Fig. 2. The overall simulation result.
2.1. Theoretical considerations Our motivation for creating a good resolution film is the optothermal effect produced by a tightly focused laser. Said focused laser can deliver heat to form a tiny dark spot, a localized distribution of focal conic CLCs, on the film as long as the laser pumping time, which corresponds to the amount of heat transferred to the film at a constant laser power, as well as the same laser focusing conditions, carefully controlled. To understand the temperature distribution over time within the sample film, we simplify the temperature distribution u(t,x) as a function of time t and sample depth x. The heat-conducting equation when using a laser as the heat source is [17–19]
∂ ⎛ ∂u ⎞ ∂u κ + q ̇ = ρC , ∂x ⎝ ∂x ⎠ ∂t
(1)
where u, t, and x represent the temperature, time, and depth within the sample film, respectively. q is the heat in Joules, and q̇ is the time derivative of q. ρ is the material density (g/cm3). C and κ are the heat capacity (J/(g·K)) and the thermal conductivity (W/(cm·K) ), respectively. The laser heating rate is described as follows:
q ̇ = I0 (1−R) μe−μx ,
(2)
where I0 is the incident laser intensity, μ is the absorption coefficient (1/ m ), and R is the sample surface reflection. When simplified, R is negligible, resulting in the following equation:
q ̇ = I0 μe−μx .
(3)
Applying proper boundary conditions, we obtain the following simulation results. 2.2. Simulation results Fig. 3. The simulation considering experimental conditions. The red dash-lines indicate the boundary between the CLCs and PEDOT:PSS. (a) Curves A, B, and C have 1-, 2-, and 4unit thickness of CLCs, respectively, and all of them have 1-unit thickness of PEDOT:PSS. (b) Curves A, B, and C have 4-unit thickness of CLCs and have 1-, 2-, and 3-unit thickness of PEDOT:PSS, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The overall simulation is shown in Fig. 2, where the u, t, and x axes indicate the sample temperature, passing time, and sample depth, respectively. The laser pump is switched on at t = 0 and off at t = 5. The low heat-absorption CLC layer extends from x = 0 to x = 4. Then, the high heat-absorption PEDOT:PSS layer goes from x = 4 to x = 5. Looking from the left, we can plot a temperature versus time graph. The overall sample temperature gradually increases once the laser turns on and quickly drops once the laser is turned off. Likewise, we can plot a temperature versus depth graph when looking from the front. In this graph, the sample temperature gradually increases to its maximum at the vicinity between the CLCs and PEDOT:PSS and then decreases.
shows the simulated temperature distributions of the samples with the same of PEDOT:PSS thickness but varying thicknesses of the CLC layer. In Fig. 3a, curves A, B, and C have one-, two-, and four-unit thickness of CLCs, respectively, while all have one unit thickness of PEDOT:PSS. As shown in the figure, the thinner the CLC layer is, the greater the temperature increase is (as in curve A). Since CLCs can slightly absorb heat, the laser intensity decreases somewhat when traveling through the thin CLC layer; upon reaching the high absorption PEDOT:PSS layer, a high temperature rise takes place at the boundary between the CLCs and PEDOT:PSS. Therefore, the sample features an overall large temperature. As a result of that high temperature, a large spot may form on the
2.3. Calculation results considering experimental conditions To compare the simulation with the experimental results, we select to change the thickness of only the CLC or PEDOT:PSS layer at each time, while maintaining the same thickness for the other layer. Fig. 3a 2
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film as a result of the high temperature. When the laser travels through the thickest CLC sample (curve C), its intensity dissipates at the front of the CLCs and finally generate temperature when it reaches the PEDOT:PSS. In this case the temperature rise is limited. In contrast, Fig. 3b maintains the thickness of all the CLC layers the same while changing the thickness of the PEDOT:PSS layer. In Fig. 3b, curves A, B, and C have four-unit thickness of the CLC layers and have one-, two-, and three-unit thicknesses of the PEDOT:PSS layer, respectively. The laser travels the same distance over the CLC part in those three curves, so their laser intensity dissipation is the same until it reaches the high absorption PEDOT:PSS layer. The thicker PEDOT:PSS can absorb more energy, thus resulting in a higher temperature in the sample, as indicated in curve C, which also results in a larger spot being developed on the film. Since the CLCs and PEDOT:PSS have different heat capacities, thermal conductivities, and absorption coefficients, the simulation results show that the location of the maximum heat accumulated within the sample, as well as that of the maximum temperature, is not exactly where the CLC and PEDOT:PSS layers meet.
(a)
(b)
dark spot
Fig. 4. The sample before (a) and after (b) pump excitation.
3. Experimental results 3.1. Appearance of the pumped sample Before we move on to further discuss the results of our analysis, we will first present the most initial phenomenon we observed. Fig. 4a shows the original sample film before any laser excitation. Looking at the figure, the layers that appear, in order, would be the transparent APET/ITO, the greenish CLC emulsion, and the blackish PEDOT:PSS. The planar state was achieved using a strong electric field applied between the conducting ITO and PEDOT:PSS. A black circle was used to mark the area on the film to be pumped by the laser beam. In Fig. 4b, a diode-pumped solid-state laser with a wavelength of 532 nm served as the pump beam. The pump laser would enter the sample from the transparent APET/ITO side. The sample with several laser pumping times is shown within the marked circle. The dark spots, caused by the low reflection on the surface, are attributed to the result of high scattering in the focal conic state. 3.2. Influence of the polarization angles between the probe and pump on probe transmission To verify whether the high scattering of CLCs in the focal conic state could be attributed to the electric field in the polarized pump laser or simply caused by heating of the pump laser, another 632 nm He-Ne laser was added as a probe beam, while a silicon detector was placed behind the sample to measure the transmitted probe intensity. The pump polarization was fixed constantly at zero degrees, while the probe polarization was rotated by a half-wave plate to create different angles with regard to pump polarization. If the CLC molecules were aligned by the electric field of the polarized pump and created a specific distribution with regard to pump polarization, the probe transmittance would then change with this distribution. The dependence of probe transmittance on the angles between the probe and pump polarizations at pump intensities 0.21 and 0.61 W/cm2 is shown in Fig. 5a and b, respectively. The result of the probe transmittance of the sample pumped for 10, 20, and 40 s is displayed in black, red,1 and blue, respectively. At the low pump intensity of 0.21 W/cm2, shown in Fig. 5a, the probe transmittances at 10, 20, and 40 s are generally circles with different radii and no favorite oriented distribution. At the high pump intensity of 0.61 W/cm2, shown in Fig. 5b, the probe transmittance distributions at 20 and 40 s nearly overlap due to reaching saturation,
Fig. 5. The dependence of probe transmittance on the angles between the probe and pump polarizations at pump intensities (a) 0.21 and (b) 0.61 W/cm2.
while that at 10 s is unsaturated and has a smaller radius. We also note that the unsaturated curve (black curve, 10 s) in Fig. 5a features a smaller radius than that (black curve, 10 s) in Fig. 5b, due to the lower laser pump intensity in the former experiment. As seen in this figure, probe transmittance is generally independent of the angles extended between the probe and pump polarizations. This finding suggests that the transmittance change in CLCs is not related to pump field alignment, and it can thus be inferred therefore that the transmittance change may primarily be related to the heat generated from the pump beam or the opto-thermal effect, but not the photo-chemical reaction. 3.3. Spot sizes Figs. 6 and 7 show the experimental spot sizes measured at different pump intensities (displayed in different colors) for several pumping
1 For interpretation of color in Fig. 5, the reader is referred to the web version of this article.
3
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Fig. 7. The spot diameters measured at various pump times and pump intensities, where the cholesteric liquid crystal layer is the same but the thicknesses of the PEDOT:PSS layer in (a) and (b) are 5.5 μm and 2.5 μm, respectively. Fig. 6. The spot diameters measured at various pump times and pump intensities, where the PEDOT:PSS layer is the same but the thicknesses of the cholesteric liquid crystal layer in (a) and (b) are 25 μm and 12 μm, respectively.
experimental trend is also consistent with the theoretical prediction in Fig. 3b, in which the simulation curve C has the thicker PEDOT:PSS and thus the higher overall temperature within the sample, which is related to the thicker PEDOT:PSS layer in the sample of Fig. 7a.
times. Fig. 6 shows a comparison of two samples having CLC thicknesses of 25 and 12 μm, respectively, with the same PEDOT:PSS layer. In contrast, two samples with the PEDOT:PSS thicknesses of 5.5 and 2.5 μm, respectively, and the same thickness CLC layer are compared in Fig. 7. The y-axis corresponds to the spot diameters measured at specific pumping times shown on the x-axis. In both figures, a stronger pump intensity results in larger spot diameters for the same pumping time. Furthermore, at the same pump intensity, the spot diameter becomes saturated at a certain pumping time, which suggests that the pump intensity can adjust the spot diameters after saturation, while prior to saturation, the spot diameter can be controlled by carefully adjusting the pumping time. In Fig. 6b, the sample with a thinner CLC layer exhibits a larger spot diameter compared with that in Fig. 6a. These two samples have the same highly absorptive PEDOT:PSS layer, but the sample with the thinner CLC layer behaves like a smaller heat reservoir in the CLC part and dissipates less energy, resulting in a higher temperature distribution within the sample, as well as larger spots. This trend qualitatively agrees with the theoretical prediction in Fig. 3a. In Fig. 3a, the simulation curve A has the thinner CLCs and thus the higher overall temperature within the sample, which corresponds to the thinner CLC sample of Fig. 6b. Likewise, Fig. 7a, which has a thicker PEDOT:PSS layer, reveals a larger spot diameter than Fig. 7b. The two samples in Fig. 7 have the same CLC layer and thus the same heat reservoir. Therefore, the sample with the thicker PEDOT:PSS layer, which absorbs heat well and works as a temperature generator, would exhibit a higher temperature distribution and form larger spot diameters. This
3.4. Gray-Scale photo Fig. 8a shows a normal gray-scale photo. We tried to reproduce that same photo on the CLC sample film by extracting the gray-scale value (0–254) of each pixel in the two-dimensional photo. The two-dimensional gray-scale matrix was then mapped to an appropriate pumping time matrix. According to this pumping time matrix, a diode-pumped solid state laser with a wavelength of 532 nm would function as the
Fig. 8. The photos of (a) a normal picture and (b) a transferred pattern on the sample film.
4
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reflective display. SID Symp Dig Tech Pap 2003;34:959. [2] Yang DK, Wu ST. Fundamentals of liquid crystal devices. 1st ed. England: Wiley; 2006. [3] Li Y, Suen JJY, Prince E, Larin EM, Klinkova A, Thérien-Aubin H, et al. Colloidal cholesteric liquid crystal in spherical confinement. Nat Commun 2016;7:12520. [4] Kobashi J, Yoshida H, Ozaki M. Planar optics with patterned chiral liquid crystals. Nat Photon 2016;10:389. [5] Shiyanovskaya I, Green S, Magyar G, Doane JW. Single substrate encapsulated cholesteric LCDs: coatable, drapable and foldable. SID Symp Dig Tech Pap 2005;36:1556. [6] Stephenson SW, Johnson DM, Kilburn JI, Mi XD, Rankin CM, Capurso RG. Development of a flexible electronic display using photographic technology. SID Symp Dig Tech Pap 2004;35:774. [7] McCollough GT, Johnson CM, Weiner ML. Roll-to-roll manufacturing considerations for flexible, cholesteric liquid crystal (ChLC) display media. SID Symp Dig Tech Pap 2005;36:64. [8] Hiji N, Kakinuma T, Araki M, Hikichi Y. Cholesteric liquid crystal micro-capsules with perpendicular alignment shell for photo-addressable electronic paper. SID Symp Dig Tech Pap 2005;36:1560. [9] Bai PF, Hayes RA, Jin ML, Shui LL, Yi ZC, Wang L, et al. Review of paper-like display technologies. Progress in electromagnetics research 2014;147:95. [10] Yeh P, Gu C. Optics of liquid crystal displays. 2nd ed. New Jersey: Wiley; 2010. [11] de Gennes PG, Prost J. The physics of liquid crystals. 2nd ed., Oxford, New York, 1995. [12] Khoo IC. Liquid crystals. 2nd ed. New Jersey: Wiley; 2007. [13] Khoo IC, Wu ST. Optics and nonlinear optics of liquid crystals. 1st ed. Singapore: World Scientific; 1993. [14] Geng Jun, et al. Electrically addressed and thermally erased cholesteric cells. Appl Phys Lett 2006;89:081130. [15] Hrozhyk Uladzimir A et al., Optically switchable, rapidly relaxing cholesteric liquid crystal reflectors. Opt Exp, 18(9) 9561 (2010). [16] Tzeng SYT, Chen CN, Tzeng Y. Thermal tuning band gap in cholesteric liquid crystals. Liq Cryst, 37(9) 1221 (2010). [17] Zubair SM, Chaudhry MA. Heat conduction in a semi-infinite solid due to timedependent laser source. Int J Heat Mass Trans, 139(14) 3067 (1995). [18] Yilbas BS, Sami M, Al-Farayedhi A. Closed-form and numerical solutions to the laser heating process. Proceedings of the institution of mechanical engineers, 212 part3 (1997). [19] Cengel YA. Heat transfer: a practical approach. 2nd ed. New York: McGraw-Hill; 2002.
laser heating source. The pumping laser passes through an electronic shutter and enters the CLC sample, which is fixed on a two-dimensional translation stage. The open times of this electronic shutter and the moving of the translation stage are controlled by a custom computer program. Fig. 8b displays the pattern transferred from Fig. 8a by the laser’s direct-writing on the CLC thin films. The resolution of this transferred pattern relied not only on the gray-scale value that we extracted, but also the distance between each pixel and the focusing ability of the pumping laser beam. Instead of using the heat transfer machine, which transfers heat to the CLC sample by directly contacting the sample film, we applied a laser direct-writing method and achieved a photograph with good resolution on the sample film. 4. Conclusion In this study, we implement a micro-encapsulated CLC electronic paper that is optically addressed and electrically erasable. The mechanism of spot diameters formed on the CLC films is discussed and verified through a number of experimental parameters, including the thickness of the CLC and PEDOT:PSS layers, pump intensity, and pumping time. Under the same excitation condition, the thicker the CLC layer is, the smaller the spot size we obtain. Furthermore, the large spots form because of a thick and high-absorption PEDOT:PSS layer. The optical alignment due to the electrical field of the pump beam is experimentally excluded in this study. Furthermore, a simple optothermal and heat-conducting model was proposed to explain our experimental results, which qualitatively agree with this theoretical model. References [1] Yang DK, Lu ZJ, Chien LC, Doane JW. Bistable polymer dispersed cholesteric
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Contents lists available at ScienceDirect
Solid State Electronics journal homepage: www.elsevier.com/locate/sse
The opto-thermal effect on encapsulated cholesteric liquid crystals ⁎
Yu-Sung Liua, , Hui-Chi Linb, Kin-Min Yangb a b
Department of Electronic Engineering, National Formosa University, No. 64, Wunhua Rd., Huwei Township, Yunlin County 63201, Taiwan, ROC Department of Electro-Optical Engineering, National Formosa University, No. 64, Wunhua Rd., Huwei Township, Yunlin County 63201, Taiwan, ROC
A R T I C L E I N F O
A B S T R A C T
Keywords: Micro-encapsulated cholesteric liquid crystals Opto-thermal Heat conduction equation
In this study, we implemented a micro-encapsulated CLC electronic paper that is optically addressed and electrically erasable. The mechanism that forms spot diameters on the CLC films is discussed and verified through various experimental parameters, including the thickness of CLCs and Poly(2,3-dihydrothieno-1,4-dioxin)-poly(styrenesulfonate) (PEDOT:PSS), pump intensity, and pumping time. The opto-thermal effect, brought on by the PEDOT:PSS absorbing layer, causes the spot diameters on the cholesteric liquid crystal thin films to vary. According to our results, the spot diameter is larger for a sample with a thinner cholesteric liquid crystal layer with the same excitation conditions and same thickness of the PEDOT layer. The spot diameter is also larger for a sample with a thicker PEDOT under the same excitation conditions and same thickness of the cholesteric liquid crystal layer. We proposed a simple heat-conducting model to explain the experimental results, which qualitatively agree with this theoretical model.
1. Introduction Cholesteric liquid crystals (CLCs) can be encapsulated in droplet form and retain its bistability as long as the droplet size is significantly larger than the pitch of the CLCs [1–4]. In this study, the CLCs are encapsulated using the emulsification method [5–9], in which CLCs, water, and a water-dissolvable polymer are all mixed together. The water dissolves the polymer to form a viscous solution that does not dissolve the CLCs. This mixture is combined with a high-speed propeller blade to form an emulsion that contains a number of micron-size CLC capsules. The emulsion is then coated on a plastic-conducting substrate while the water is allowed to evaporate, leaving the micro-encapsulated CLCs suspended in the polymer matrices. Once the water evaporates, a second layer of conducting polymer is coated on top of CLC emulsion to form a device. Due to the high viscosity of the CLC emulsion, the device can be made using a roll-to-roll process [6,7]. The encapsulated CLCs mainly undergo four states in this article. First, most of the directors of the CLC molecules are perpendicularly aligned to the film surface with a sufficiently strong electric field across the film. Referred to as the homeotropic state, this state is unstable and immediately transfers to a planar state once the strong electric field is removed. In the planar state, in which the helical axis of the CLC directors is perpendicular to the micro-cell surface, the Bragg reflection [10–13] of the CLCs results in the reflection of incident light, thus making the film appear bright. On the other hand, the film in the planar state, which is stable, can be switched to the focal conic state, another ⁎
stable state; therefore it exhibits bistability with either the help of weak electric fields across the film or appropriate heating methods. The CLCs preserve the helical structure in the focal conic state, but the helical axes are irregularly distributed in a short-range order and form numerous small discontinuities in the reflective index, which result in the highly random scattering of incident light on the film surface; therefore the film appears dark. The transition from the planar to focal conic state is reversible if a sufficiently strong electric field is applied again. The processes described above can also be circulated. Many methods have been suggested to alter CLC behavior. Among them, the photo-chemical reaction was explored by doping azo dyes into the CLCs. The main mechanism is the trans-cis photoisomerization effect of the azo molecules excited by the optical electric field [14,15]. However, some researchers proposed that the opto-thermal effect that causes the phase changes in the CLCs may have a better reaction rate than the photo-chemical effect [16]. In this study, we exploited the opto-thermal effect and applied a layer of micro-encapsulated CLC emulsion on the single flexible APET/ITO (Amorphous Polyethylene Terephthalate/ Indium Tin Oxide) substrate. Due to the low absorption of the CLC layer, another layer of PEDOT:PSS (Poly(2,3-dihydrothieno1,4-dioxin)-poly(styrenesulfonate)) was used as the optical absorption and electrically conducting layer. Consequently, the sample film can be patterned with the opto-thermal effect, and the pattern created on the film can be erased by an external electric field and returned to the originally blank film due to the bistability of the CLCs. The film structure is demonstrated in Fig. 1, in which a layer of the micro-
Corresponding author. E-mail address: [email protected] (Y.-S. Liu).
http://dx.doi.org/10.1016/j.sse.2017.09.005
0038-1101/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Liu, Y.-S., Solid State Electronics (2017), http://dx.doi.org/10.1016/j.sse.2017.09.005
Solid State Electronics xxx (xxxx) xxx–xxx
Y.-S. Liu et al.
Temperature u (ÛC) Fig. 1. The sample structure.
encapsulated CLCs was first coated on the APET/ITO through rod coating. Then, the PEDOT:PSS was applied on top of the CLC layer. Unwanted solvent was evaporated by placing the sample into an oven. Before we get into the experimental details, we present a theoretical simulation in Section 2 in order to qualitatively explain the phenomena that we encountered in Section 3.
0
5
Time t (s) 10
2. Theoretical simulation
0
Depth x (ȝm)
Fig. 2. The overall simulation result.
2.1. Theoretical considerations Our motivation for creating a good resolution film is the optothermal effect produced by a tightly focused laser. Said focused laser can deliver heat to form a tiny dark spot, a localized distribution of focal conic CLCs, on the film as long as the laser pumping time, which corresponds to the amount of heat transferred to the film at a constant laser power, as well as the same laser focusing conditions, carefully controlled. To understand the temperature distribution over time within the sample film, we simplify the temperature distribution u(t,x) as a function of time t and sample depth x. The heat-conducting equation when using a laser as the heat source is [17–19]
∂ ⎛ ∂u ⎞ ∂u κ + q ̇ = ρC , ∂x ⎝ ∂x ⎠ ∂t
(1)
where u, t, and x represent the temperature, time, and depth within the sample film, respectively. q is the heat in Joules, and q̇ is the time derivative of q. ρ is the material density (g/cm3). C and κ are the heat capacity (J/(g·K)) and the thermal conductivity (W/(cm·K) ), respectively. The laser heating rate is described as follows:
q ̇ = I0 (1−R) μe−μx ,
(2)
where I0 is the incident laser intensity, μ is the absorption coefficient (1/ m ), and R is the sample surface reflection. When simplified, R is negligible, resulting in the following equation:
q ̇ = I0 μe−μx .
(3)
Applying proper boundary conditions, we obtain the following simulation results. 2.2. Simulation results Fig. 3. The simulation considering experimental conditions. The red dash-lines indicate the boundary between the CLCs and PEDOT:PSS. (a) Curves A, B, and C have 1-, 2-, and 4unit thickness of CLCs, respectively, and all of them have 1-unit thickness of PEDOT:PSS. (b) Curves A, B, and C have 4-unit thickness of CLCs and have 1-, 2-, and 3-unit thickness of PEDOT:PSS, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The overall simulation is shown in Fig. 2, where the u, t, and x axes indicate the sample temperature, passing time, and sample depth, respectively. The laser pump is switched on at t = 0 and off at t = 5. The low heat-absorption CLC layer extends from x = 0 to x = 4. Then, the high heat-absorption PEDOT:PSS layer goes from x = 4 to x = 5. Looking from the left, we can plot a temperature versus time graph. The overall sample temperature gradually increases once the laser turns on and quickly drops once the laser is turned off. Likewise, we can plot a temperature versus depth graph when looking from the front. In this graph, the sample temperature gradually increases to its maximum at the vicinity between the CLCs and PEDOT:PSS and then decreases.
shows the simulated temperature distributions of the samples with the same of PEDOT:PSS thickness but varying thicknesses of the CLC layer. In Fig. 3a, curves A, B, and C have one-, two-, and four-unit thickness of CLCs, respectively, while all have one unit thickness of PEDOT:PSS. As shown in the figure, the thinner the CLC layer is, the greater the temperature increase is (as in curve A). Since CLCs can slightly absorb heat, the laser intensity decreases somewhat when traveling through the thin CLC layer; upon reaching the high absorption PEDOT:PSS layer, a high temperature rise takes place at the boundary between the CLCs and PEDOT:PSS. Therefore, the sample features an overall large temperature. As a result of that high temperature, a large spot may form on the
2.3. Calculation results considering experimental conditions To compare the simulation with the experimental results, we select to change the thickness of only the CLC or PEDOT:PSS layer at each time, while maintaining the same thickness for the other layer. Fig. 3a 2
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film as a result of the high temperature. When the laser travels through the thickest CLC sample (curve C), its intensity dissipates at the front of the CLCs and finally generate temperature when it reaches the PEDOT:PSS. In this case the temperature rise is limited. In contrast, Fig. 3b maintains the thickness of all the CLC layers the same while changing the thickness of the PEDOT:PSS layer. In Fig. 3b, curves A, B, and C have four-unit thickness of the CLC layers and have one-, two-, and three-unit thicknesses of the PEDOT:PSS layer, respectively. The laser travels the same distance over the CLC part in those three curves, so their laser intensity dissipation is the same until it reaches the high absorption PEDOT:PSS layer. The thicker PEDOT:PSS can absorb more energy, thus resulting in a higher temperature in the sample, as indicated in curve C, which also results in a larger spot being developed on the film. Since the CLCs and PEDOT:PSS have different heat capacities, thermal conductivities, and absorption coefficients, the simulation results show that the location of the maximum heat accumulated within the sample, as well as that of the maximum temperature, is not exactly where the CLC and PEDOT:PSS layers meet.
(a)
(b)
dark spot
Fig. 4. The sample before (a) and after (b) pump excitation.
3. Experimental results 3.1. Appearance of the pumped sample Before we move on to further discuss the results of our analysis, we will first present the most initial phenomenon we observed. Fig. 4a shows the original sample film before any laser excitation. Looking at the figure, the layers that appear, in order, would be the transparent APET/ITO, the greenish CLC emulsion, and the blackish PEDOT:PSS. The planar state was achieved using a strong electric field applied between the conducting ITO and PEDOT:PSS. A black circle was used to mark the area on the film to be pumped by the laser beam. In Fig. 4b, a diode-pumped solid-state laser with a wavelength of 532 nm served as the pump beam. The pump laser would enter the sample from the transparent APET/ITO side. The sample with several laser pumping times is shown within the marked circle. The dark spots, caused by the low reflection on the surface, are attributed to the result of high scattering in the focal conic state. 3.2. Influence of the polarization angles between the probe and pump on probe transmission To verify whether the high scattering of CLCs in the focal conic state could be attributed to the electric field in the polarized pump laser or simply caused by heating of the pump laser, another 632 nm He-Ne laser was added as a probe beam, while a silicon detector was placed behind the sample to measure the transmitted probe intensity. The pump polarization was fixed constantly at zero degrees, while the probe polarization was rotated by a half-wave plate to create different angles with regard to pump polarization. If the CLC molecules were aligned by the electric field of the polarized pump and created a specific distribution with regard to pump polarization, the probe transmittance would then change with this distribution. The dependence of probe transmittance on the angles between the probe and pump polarizations at pump intensities 0.21 and 0.61 W/cm2 is shown in Fig. 5a and b, respectively. The result of the probe transmittance of the sample pumped for 10, 20, and 40 s is displayed in black, red,1 and blue, respectively. At the low pump intensity of 0.21 W/cm2, shown in Fig. 5a, the probe transmittances at 10, 20, and 40 s are generally circles with different radii and no favorite oriented distribution. At the high pump intensity of 0.61 W/cm2, shown in Fig. 5b, the probe transmittance distributions at 20 and 40 s nearly overlap due to reaching saturation,
Fig. 5. The dependence of probe transmittance on the angles between the probe and pump polarizations at pump intensities (a) 0.21 and (b) 0.61 W/cm2.
while that at 10 s is unsaturated and has a smaller radius. We also note that the unsaturated curve (black curve, 10 s) in Fig. 5a features a smaller radius than that (black curve, 10 s) in Fig. 5b, due to the lower laser pump intensity in the former experiment. As seen in this figure, probe transmittance is generally independent of the angles extended between the probe and pump polarizations. This finding suggests that the transmittance change in CLCs is not related to pump field alignment, and it can thus be inferred therefore that the transmittance change may primarily be related to the heat generated from the pump beam or the opto-thermal effect, but not the photo-chemical reaction. 3.3. Spot sizes Figs. 6 and 7 show the experimental spot sizes measured at different pump intensities (displayed in different colors) for several pumping
1 For interpretation of color in Fig. 5, the reader is referred to the web version of this article.
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Fig. 7. The spot diameters measured at various pump times and pump intensities, where the cholesteric liquid crystal layer is the same but the thicknesses of the PEDOT:PSS layer in (a) and (b) are 5.5 μm and 2.5 μm, respectively. Fig. 6. The spot diameters measured at various pump times and pump intensities, where the PEDOT:PSS layer is the same but the thicknesses of the cholesteric liquid crystal layer in (a) and (b) are 25 μm and 12 μm, respectively.
experimental trend is also consistent with the theoretical prediction in Fig. 3b, in which the simulation curve C has the thicker PEDOT:PSS and thus the higher overall temperature within the sample, which is related to the thicker PEDOT:PSS layer in the sample of Fig. 7a.
times. Fig. 6 shows a comparison of two samples having CLC thicknesses of 25 and 12 μm, respectively, with the same PEDOT:PSS layer. In contrast, two samples with the PEDOT:PSS thicknesses of 5.5 and 2.5 μm, respectively, and the same thickness CLC layer are compared in Fig. 7. The y-axis corresponds to the spot diameters measured at specific pumping times shown on the x-axis. In both figures, a stronger pump intensity results in larger spot diameters for the same pumping time. Furthermore, at the same pump intensity, the spot diameter becomes saturated at a certain pumping time, which suggests that the pump intensity can adjust the spot diameters after saturation, while prior to saturation, the spot diameter can be controlled by carefully adjusting the pumping time. In Fig. 6b, the sample with a thinner CLC layer exhibits a larger spot diameter compared with that in Fig. 6a. These two samples have the same highly absorptive PEDOT:PSS layer, but the sample with the thinner CLC layer behaves like a smaller heat reservoir in the CLC part and dissipates less energy, resulting in a higher temperature distribution within the sample, as well as larger spots. This trend qualitatively agrees with the theoretical prediction in Fig. 3a. In Fig. 3a, the simulation curve A has the thinner CLCs and thus the higher overall temperature within the sample, which corresponds to the thinner CLC sample of Fig. 6b. Likewise, Fig. 7a, which has a thicker PEDOT:PSS layer, reveals a larger spot diameter than Fig. 7b. The two samples in Fig. 7 have the same CLC layer and thus the same heat reservoir. Therefore, the sample with the thicker PEDOT:PSS layer, which absorbs heat well and works as a temperature generator, would exhibit a higher temperature distribution and form larger spot diameters. This
3.4. Gray-Scale photo Fig. 8a shows a normal gray-scale photo. We tried to reproduce that same photo on the CLC sample film by extracting the gray-scale value (0–254) of each pixel in the two-dimensional photo. The two-dimensional gray-scale matrix was then mapped to an appropriate pumping time matrix. According to this pumping time matrix, a diode-pumped solid state laser with a wavelength of 532 nm would function as the
Fig. 8. The photos of (a) a normal picture and (b) a transferred pattern on the sample film.
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reflective display. SID Symp Dig Tech Pap 2003;34:959. [2] Yang DK, Wu ST. Fundamentals of liquid crystal devices. 1st ed. England: Wiley; 2006. [3] Li Y, Suen JJY, Prince E, Larin EM, Klinkova A, Thérien-Aubin H, et al. Colloidal cholesteric liquid crystal in spherical confinement. Nat Commun 2016;7:12520. [4] Kobashi J, Yoshida H, Ozaki M. Planar optics with patterned chiral liquid crystals. Nat Photon 2016;10:389. [5] Shiyanovskaya I, Green S, Magyar G, Doane JW. Single substrate encapsulated cholesteric LCDs: coatable, drapable and foldable. SID Symp Dig Tech Pap 2005;36:1556. [6] Stephenson SW, Johnson DM, Kilburn JI, Mi XD, Rankin CM, Capurso RG. Development of a flexible electronic display using photographic technology. SID Symp Dig Tech Pap 2004;35:774. [7] McCollough GT, Johnson CM, Weiner ML. Roll-to-roll manufacturing considerations for flexible, cholesteric liquid crystal (ChLC) display media. SID Symp Dig Tech Pap 2005;36:64. [8] Hiji N, Kakinuma T, Araki M, Hikichi Y. Cholesteric liquid crystal micro-capsules with perpendicular alignment shell for photo-addressable electronic paper. SID Symp Dig Tech Pap 2005;36:1560. [9] Bai PF, Hayes RA, Jin ML, Shui LL, Yi ZC, Wang L, et al. Review of paper-like display technologies. Progress in electromagnetics research 2014;147:95. [10] Yeh P, Gu C. Optics of liquid crystal displays. 2nd ed. New Jersey: Wiley; 2010. [11] de Gennes PG, Prost J. The physics of liquid crystals. 2nd ed., Oxford, New York, 1995. [12] Khoo IC. Liquid crystals. 2nd ed. New Jersey: Wiley; 2007. [13] Khoo IC, Wu ST. Optics and nonlinear optics of liquid crystals. 1st ed. Singapore: World Scientific; 1993. [14] Geng Jun, et al. Electrically addressed and thermally erased cholesteric cells. Appl Phys Lett 2006;89:081130. [15] Hrozhyk Uladzimir A et al., Optically switchable, rapidly relaxing cholesteric liquid crystal reflectors. Opt Exp, 18(9) 9561 (2010). [16] Tzeng SYT, Chen CN, Tzeng Y. Thermal tuning band gap in cholesteric liquid crystals. Liq Cryst, 37(9) 1221 (2010). [17] Zubair SM, Chaudhry MA. Heat conduction in a semi-infinite solid due to timedependent laser source. Int J Heat Mass Trans, 139(14) 3067 (1995). [18] Yilbas BS, Sami M, Al-Farayedhi A. Closed-form and numerical solutions to the laser heating process. Proceedings of the institution of mechanical engineers, 212 part3 (1997). [19] Cengel YA. Heat transfer: a practical approach. 2nd ed. New York: McGraw-Hill; 2002.
laser heating source. The pumping laser passes through an electronic shutter and enters the CLC sample, which is fixed on a two-dimensional translation stage. The open times of this electronic shutter and the moving of the translation stage are controlled by a custom computer program. Fig. 8b displays the pattern transferred from Fig. 8a by the laser’s direct-writing on the CLC thin films. The resolution of this transferred pattern relied not only on the gray-scale value that we extracted, but also the distance between each pixel and the focusing ability of the pumping laser beam. Instead of using the heat transfer machine, which transfers heat to the CLC sample by directly contacting the sample film, we applied a laser direct-writing method and achieved a photograph with good resolution on the sample film. 4. Conclusion In this study, we implement a micro-encapsulated CLC electronic paper that is optically addressed and electrically erasable. The mechanism of spot diameters formed on the CLC films is discussed and verified through a number of experimental parameters, including the thickness of the CLC and PEDOT:PSS layers, pump intensity, and pumping time. Under the same excitation condition, the thicker the CLC layer is, the smaller the spot size we obtain. Furthermore, the large spots form because of a thick and high-absorption PEDOT:PSS layer. The optical alignment due to the electrical field of the pump beam is experimentally excluded in this study. Furthermore, a simple optothermal and heat-conducting model was proposed to explain our experimental results, which qualitatively agree with this theoretical model. References [1] Yang DK, Lu ZJ, Chien LC, Doane JW. Bistable polymer dispersed cholesteric
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