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Abstract
A dynamically reconfigurable liquid crystal (LC) photonic device is an important research field in modern LC photonics. We present a type of continuously tunable distributed Bragg reflector (DBR) based on LC polymer composites modulated via a novel optofluidic method. LC-templated DBR films are fabricated by photopolymerization under visible standing wave interference. The influences of the incident angle, incident light intensity, and content of ethanol as a pore-forming additive on the reflection behavior are discussed in detail. Then, the LC-templated DBR films are integrated into microfluidic channels and reversibly refilled by different organic solvents. The reconfigurable characteristics of optofluidic DBRs were demonstrated by changing the average refractive index (RI) of the mixed liquids and adjusting the flow rates, resulting in the dynamic and continuous variation of the reflection band within a specific visible light band. It is anticipated that the prototype optofluidic LC device will hopefully be applied to some specific scenarios where conventional means of regulation, such as electric, optical, and temperature fields, are unsuitable and possibly boost the development of microfluidic analysis techniques based on structural color.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In the past few decades, liquid crystal (LC)-based polymer composites have aroused the widespread interest of researchers in the field of LC photonics. According to the components and characteristics, LC composite materials can be categorized into polymer dispersed liquid crystals (PDLCs), polymer-stabilized liquid crystals (PSLCs), and liquid crystal polymer templates (LCPTs) [1–5]. Among them, LCPTs with reconfigurable features and good thermal and mechanical stabilities have been widely used in reflective structural color films [6], sensors [7], lasers [8,9], displays [10], and other LC photonic devices [11–14]. Notably, the cholesteric phase of LCs endows LCPTs with intriguing optical performance. J. B. Guo et al. used the wash-out/refill method to prepare cholesteric LCPT films that can reflect both left-handed circularly polarized light and right-handed circularly polarized light at the same time [4]. J. H. Liu et al. prepared and filled cholesteric LCPT films with traditional organic solvents and nematic liquid crystals (NLCs) with different RIs to adjust the reflection wavelength [15]. D. Luo et al. prepared a multicolor LC laser that can produce red, green, and blue colors by refilling NLCs containing different dyes into a cholesteric LCPT [16]. Compared with traditional control methods, such as applying an external electric or optical field, the use of liquid fillers to influence the overall performance of LC devices allows great flexibility. However, most of the current works mentioned above achieve the purpose of adjusting the reflection band by individually refilling solutions with different RIs, resulting in the discontinuity of reflected wavelengths of cholesteric liquid crystals (CLCs). In addition, for a device that has been fabricated, repeated manual replacements of the liquid fillers using the usual wash-out/refill process will also significantly reduce the operation efficiency.
Optofluidics is an interdisciplinary subject composed of optics and microfluidics. It has the advantages of miniaturization, integration, and reconfiguration [17–19]. Optofluidic devices are ready to be adjusted dynamically because of the selectable characteristics of the fluid. However, only a few works have been concerned about LCs in a microfluidic channel. A. Sengupta et al. used various physical and chemical methods to provide specific anchoring conditions on the inner surface of the microchannel and studied the arrangement of the NLC molecular directors [20]. A. E. Vasdekis et al. studied the different orientations of anisotropic NLC directors in a static and flowing state and realized an optical flow control phase modulator with a working frequency up to kHz [21]. Q. H. Wei et al. demonstrated the relationship between the texture of CLCs and the depth and width of microchannels [22]. D. Luo and K. L. Yang et al. showed that the orientation of NLCs affected by surface defects on the sidewalls could be used for high-sensitivity visual inspection and sensing [23]. However, most of the abovementioned results are more connected with the alignment and defect structure of LC fluids in flowing or static states. To the best of our knowledge, the interaction between LCPTs and fluids and associated applications in microchannels has yet to be further exploited [24].
Here, we propose a prototype optofluidic DBR with dynamic continuously tunable reflection based on LCPTs capable of the wash-out/refill property. LC-templated DBR films are fabricated by photopolymerization to understand the wave interference of biofriendly green light. We explored the influence of incident angle, incident light intensity, and ethanol content as a pore-forming additive on the reflection behaviors. After the integration of DBR films into the microchannel, micropumps were used to reversibly inject organic solvents with different RIs. The reconfigurable characteristics of the LCPTs were demonstrated by changing the average RI of the mixed liquids and adjusting the flow rates, leading to the dynamic and continuous variation of the reflection band of DBR within a specific visible light band.
2. Experimental section
2.1 Materials and methods
To fabricate the LC-templated DBR films, the polymer/LC composites were prepared by a mixture of monomers/initiator/LC/volatile solvent [12]. The reactive mesogens (RMs) used were RM257, RM82, RM006, RM021, RM010 (nRMs = 1.56, NJSJ) and a cross-linking monomer N-vinylpyrrolidone (NVP, nNVP = 1.51, Aladdin). The weight ratio of RMs is 30:20:20:20:10. For the LC material, an NLC (E7, Xianhua) was used. Rose bengal (RB, Aladdin) and N-phenylglycine (NPG, Aladdin) were added to the mixture as photoinitiators and coinitiators, respectively. Ethanol was used as a volatile pore former. Four samples S1-S4 with different constituent fractions were selected as listed in Table 1.
The mixtures were ultrasonicated for 20 min and stirred magnetically at 1200 rpm under dark light conditions for 16 hours to ensure homogeneity. The homogeneous mixtures were filled into LC cells via capillary action. LC cells were assembled by two parallel indium tin oxide-coated glass substrates. One glass substrate was PI-rubbed, and the other was processed by 3-mercaptopropyltriethoxysilane (MPTMS, Aladdin). MPTMS treatment ensures the integrity of the polymer network adhered to the glass substrate.
Table 1. Chemical composition of samples
2.2 Standing wave interference
The optical setup for preparing the LC-templated DBR films is shown in Fig. 1(a). The 532 nm beam from a single-longitudinal-mode (SLM) laser (Coherent Compass 315 M) was attenuated, expanded, collimated, and then incident on a regular prism (K9, n = 1.52) placed on a rotating stage with an accuracy of 2 degrees. The LC cell physically adhered to the prism with an RI matching liquid, benzyl alcohol (BA, n = 1.54), and the glass substrate treated by MPTMS was placed close to the prism. The path of incident light in the prism and LC Cell is shown in Fig. 1(b). The angle between the incident light and the prism surface normal is θ. At a certain angle of incidence θ, the incident light enters the LC cell through the prism. The standing wave formed as a result of interference between the incident beam and its own total internal reflection, as shown in Fig. 1(c). The thickness of the LC cell is 5 µm.
Fig. 1. (a) The optical setup for preparing the LC-templated DBR films. (b) Schematic diagram of the path of incident light in the prism and LC cell. (c) Schematic diagram of the standing wave interference of visible light. (d) Schematic illustration of the fabrication process of the LC-templated DBR films. (e) Diagram of the optofluidic DBR device.
The fabrication process of the LC-templated DBR films is shown in Fig. 1(d). First, the LC cell is filled with a polymer/LC composite mixture (Fig. 1(d), Stage 1). Next, the LC cell is illuminated with interference light to initiate the polymerization of RMs. LCs are driven to the low light intensity region of the interfering light field, as polymerization and cross-linking occur preferentially in the high light intensity region (Fig. 1(d), Stage 2). Upon opening the LC cell, the ethanol evaporates (Fig. 1(d), Stage 3). Then, the substrate coated with the LC-templated DBR films is placed in a sufficient amount of organic solvent, such as ethanol, and thoroughly washed out of the unpolymerized LCs (Fig. 1(d), Stages 4). Finally, after sufficient drying, an LC-templated DBR film with a periodically alternating laminar structure of polymer layers and nanoporous layers is obtained (Fig. 1(d), Stage 5).
2.3 Fabrication of optofluidic DBR devices
As illustrated in Fig. 1(e), the optofluidic DBR device consists of a two-layer structure. The upper layer is polydimethylsiloxane (PDMS) with microchannels. The microchannel structure (100 µm high) includes a Y-channel, a folded channel (11.65 cm length, 400 µm width), and a rectangular channel (2 cm length, 3 mm width). There are two inlets in the Y-channel and an outlet at the end of the rectangular channel. The lower layer is a glass substrate with a dried LC-templated DBR film, which is already reshaped to fit in the rectangular channel. The two layers are aligned and bonded together by oxygen plasma treatment. All liquids were stored in 5 mL syringes and driven by syringe pumps (LSP Syringe Pump LSP01-1B). The liquids are transported from the pumps to the microfluidic device through the silicone tubes inserted into the reserved injection holes in the upper PDMS layer.
All experiments were carried out at room temperature. The reflection micrograph of the sample was observed with a metallurgical microscope (Motic BA210MET). The reflectance spectrum was recorded with a fiber optic spectrometer (Ocean Optics USB4000).
3. Results and discussion
3.1 Influence of the fabrication parameters in standing wave interference
The sanding wave formed because of interference between the incident beam and its own total internal reflection. The value of the incident angle θ influences the period of the standing wave. Therefore, the incident angle θ is an essential factor in the standing wave interference. As depicted in Fig. 1(b), the angle between the two coherent beams is 2β. According to Snell's law, the relationship between β and θ can be expressed as
Experimentally, sample S2 was used, the incident light intensity was 10 mW/cm2, and the exposure time was 10 min. Figures 2(a) and 2(b) present the incident-angle-dependent reflectance spectra after the “washed-out/dried” process and after the “refilled with ethanol” process, respectively. After being washed out and dried, the reflection band can only be observed in the visible range with an incident angle θ of 12°-24°. After being refilled with ethanol, the reflection band falls within the visible light band with an incident angle θ of 4°-16°. As plotted in Fig. 2(c), in both cases, the center wavelength is redshifted as θ increases. When the dried LC-templated DBR film is further refilled with ethanol, an additional redshift occurs. The insets of Figs. 2(a) and 2(b) present nonpolarized reflection micrographs of the LC-templated DBR film (θ=12°) and the LC-templated DBR film (θ=20°), respectively. The reflection covering the visible range can be observed in both processes when θ equals 12°. However, in the case of θ=20°, the reflection band can only be observed after the “washed-out/dried” process because the reflection band redshifted and exceeded the detection band when being refilled with ethanol. The above theoretical analysis considering the propagation path of the incident light in the prism and the LC cell interprets these experimental results well.
Fig. 2. (a) Reflectance spectra of the LC-templated DBR films after washing out and drying at different incident angles θ. Insets are nonpolarized reflectance micrographs of the LC-templated DBR films at incidence angles of 12° (left) and 20° (right). (b) Reflection spectrum of the LC-templated DBR films at different θ after refilling with ethanol. Insets are nonpolarized reflectance micrographs of the LC-templated DBR films at incidence angles of 12° (left) and 20° (right). The scale bar is 100 µm. (c) The relationship of incident angles and the center wavelength of the reflective band for both stages. (d) Normalized reflectance spectra of the LC-templated DBR films recorded for different incident light intensities at a fixed incident angle of 12° and an exposure time of 10 min.
Remarkably, the intensity of incident light can also affect the position of the reflective band. The reflectance spectra after the “washed-out/dried” process were recorded for different incident light intensities at a fixed incident angle of 12° and an exposure time of 10 min. As shown in Fig. 2(d), the position of the reflectance band is blueshifted slightly and eventually stabilizes with increasing incident intensity. This phenomenon can be explained as follows: the intensity of incident light increases, and the initiation efficiency of the photoinitiator increases, consequently promoting a faster and complete reaction in the photopolymerization. In addition, when the photopolymerization is accelerated, much more unreactive E7 and ethanol are immobilized in the high light intensity region before diffusing into the low light intensity region. The immigration of reactive monomers from the low light intensity region to the neighboring high light intensity region is hampered. The superimposed effects decrease the average RI to some extent, eventually resulting in a blueshift in the reflective band. It is worth noting that the reflective band no longer blueshifts when the light intensity reaches a certain threshold.
3.2 Effect of ethanol as a pore forming additive
To understand the role of ethanol as a pore former, the weight ratio of each component is fixed except for ethanol. According to the above experimental results, the angle of incidence θ was set to 8°. The incident light intensity and the exposure time were controlled at 10 mW/cm2 and 10 min. For an LC-templated DBR film, the reflectivity is dependent upon the number of layers, layer thickness, and the RI difference between each layer [25]. The center wavelength of the reflective band λ can be determined by [26]
where n1 and n2 are the refractive indexes of the polymer layer and nanoporous layer, respectively. d1 and d2 are the corresponding layer thicknesses.As shown in Fig. 3, the center wavelength of the reflective band λ is blueshifted gradually as the ethanol content increases. The multilayer structures of all films should be similar because the weight ratios of the chemical components in the samples are the same except for ethanol. It can also be anticipated that the number of nanopores increases with ethanol content, resulting in a decrease in n2 after refilling with ethanol. According to Eq. (3), λ will also decrease with decreasing n2. The blueshift of the reflective band with increasing ethanol concentration was in accordance with the above theoretical analysis of the average RI in the LC-templated DBR films.
Fig. 3. (a) Reflectance spectra of samples S1- S4 after refilling with ethanol. (b) The evolution of the center wavelength of reflective band, showing a blue shift in the reflectance band with increasing ethanol content.
Apart from the Bragg peak position, the peak reflectance value was also found to vary with the mass of ethanol, showing a negative correlation. Provided that the number of layers in the LC-templated DBR films is kept constant, the center wavelength is mainly influenced by the RI difference between the polymer and nanoporous layers. The lower the RI difference between the two regions is, the lower the peak reflectance. In this case, although the addition of ethanol is necessary to provide a more flexible adjustment range under the same interference conditions, the addition of excess ethanol as a pore-forming additive reduces the overall reflection performance. We also found that excessive ethanol would also destroy the adhesion of the DBR film to the glass substrate. In samples S4 and S3, the adhesion was poor, and some areas were prone to fracture after separation of the LC cells. In samples S2 and S1, the adhesion was better, and no significant fracture occurred.
3.3 Reconfigurability of DBR in microchannel
The above experiments reveal the effects of ethanol, incident angle, and incident light intensity on the reflection bands of the LC-templated DBR films, providing a guideline for the integration into microchannels. To obtain a suitable offset range in the visible wavelength band, the LC-templated DBR films were prepared using the material ratios of sample S2. The incident angle θ was set at 8°, the incident light intensity was fixed at 10 mW/cm2 during photopolymerization, and the exposure time was 10 min. A mixture of BA (n=1.54) and MA (n=1.33) was injected into the microchannel integrated with the LC-templated DBR films after being washed-out and dried, through inlet 1 and inlet 2. In this case, the weight ratios of the mixed solutions of BA and MA were 1:2, 1:4, 1:6, 1:8, and 1:10, and the RI decreased in order.
As shown in Figs. 4(a) and 4(b), the reflection band is redshifted with increasing RI when mixed solutions with different RIs are injected into the microchannels. Meanwhile, the reflection intensity decreases gradually. If we assume that the thicknesses of the polymer and nanoporous layers induced by standing wave interference remain constant during the refilling process, then λ can be inferred to be related to the change in RI, as indicated by Eq. (3). In the process of refilling, the injected solution infiltrates the nanopores of the LC-templated DBR films, which is associated with changes in the average RI and structural colors. When the RI difference between the polymer and nanoporous layer increases, the reflection intensity increases simultaneously. Moreover, according to the general principle of solubility, we assumed that the swelling degree of LCPTs is promoted in the solution with more phenyl groups (BA: MA=1:2), characterized as a prominent redshift. Notably, the experimental results indicate that the reflective wavelength of LC-templated DBR films can be continuously tuned over a broad range of over 110 nm (∼516 nm-∼634 nm) by mixing solution in the microchannel to produce a gradual distribution of RI.
Fig. 4. (a) Reflection spectra of the LC-templated DBR films integrated in the microchannels when injected using mixed solutions with different weight ratios of BA and MA. (b) Variation in the center wavelength of the reflective band with different weight ratios of BA and MA.
As is well known, refilling LC-templated DBR films with mixed solutions of different RIs is a flowing mass transfer process that consists of both convection and diffusion. The convection-diffusion equation is [27]
where c, D, u, and t denote the concentration, diffusion, coefficient, flow velocity, and time, respectively. In addition, the process of solution refilling the LC-templated DBR flims in the microchannel can be regarded as a laminar flow where convection dominates based on the Peclet number and Reynolds number. The Peclet number and Reynolds number are where L, ρ, and µ are the feature length, density and dynamic viscosity, respectively. For the laminar flow case, convection only transfers mass tangential to the direction of flow velocity, and diffusion causes mass transfer in the direction perpendicular to the fluid flow.Therefore, to further demonstrate that the reflective band will change continuously when switching two different RI solutions and to examine the effect of flow rate on the tunability of optofluidic DBR devices, the mixed solution (BA: MA=1:4) was injected at different flow rates (20, 40, 60, 80 µL/min) into the microchannel prefilled with MA, and vice versa. As the injecting situation was the same at the beginning and the end of each flow rate state, the starting and ending wavelengths in the reflection spectra were almost the same. The magnitude of the fluid flow rate in the microchannel was thus varied by varying the injection flow rate.
As shown in Figs. 5(a) and 5(b), the center wavelength of the reflective band λ varies continuously and dynamically between 516 nm and 566 nm at a flow rate of 20 µL/min. It is evident that the tunable range can be further expanded if solutions with larger RI differences are selected. Figure 5(c) shows the injection of the mixed solution with different flow rates when the microchannel is filled with MA. Figure 5(d) shows the injection of MA with different flow rates when the microchannel is filled with the mixed solution in advance. Both processes show that the duration time for the reflection band to move from the starting point to the endpoint decreases as the injected flow rate increases. As depicted in Figs. 5(c) and 5(d), it is noteworthy that the process of injecting mixed solution takes less time than the process of injecting MA when the flow rate of the injected solution is the same. The concentration of BA in the free fluid region within the microchannel is higher when injecting a mixed solution compared to injecting MA into the microchannel. The high concentration difference causes faster diffusion of BA from the free fluid region into the LC-templated DBR films. In addition, there is a slight difference in the final center wavelength at different flow rates. The difference in the final center wavelength is attributed to the repeated washing and refilling of the LC-templated DBR films. It is expected that it will be resolved by a mature preparation process.
Fig. 5. Variation in reflectance spectra when (a) BA and MA (1:4) and (b) MA were injected into the microchannel at 20 µL/min. Shift of the center wavelength of the reflectance band at flow rates of 20, 40, 60, and 80 µL/min for (c) the injection of BA and MA (1:4) into the microchannel prefilled with MA and (d) the injection of MA into the microchannel prefilled with BA and MA (1:4). The insets show the duration time taken for the reflection spectrum from the start to finish at 20, 40, 60, and 80 µL/min.
4. Conclusions
In summary, DBRs based on LC polymer composites were prepared by photopolymerization through standing wave interference of visible light and integrated into microfluidic channels. The effects of incident angle and incident light intensity on the reflective band of the LC-templated DBR films and the influence of ethanol content as a pore-forming additive during the formation of LC polymer composites were investigated and discussed in detail. Broadband tunability of the reflection band was achieved by reversibly filling organic solvents with different RIs and varying the flow rate of the fluid. Remarkably, the combination of optofluidic technology and LC polymer templates with wash-out/refill properties provides a new method for the fabrication of dynamic and continuously tunable LC photonic devices. The optofluidic reconfigurability of the prototype DBR device proposed in the work is expected to be applied to some specific scenarios where conventional modulation means, such as electric, optical and temperature fields, are not suitable for modulating LCs and possibly boost the development of microfluidic analysis techniques based on structural color [28].
Funding
National Natural Science Foundation of China (62075186, 62175206); National Key Research and Development Program of China (No. 2019YFA0905800).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the corresponding author upon reasonable request.
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