A Novel Approach for the Creation of Electrically Controlled LC:PDMS Microstructures
, Piotr Sobotka 1, Monika Grom 1, Szymon Baczyński 1, Marcin Juchniewicz 2, Kasper Marchlewicz 3 and Artur Dybko 3
Round 1
Reviewer 1 Report
The paper of K. Rutkowska et al is devoted to the application of electrically controlled liquid crystals (LC) in PDMC based organic microfluidic channels. The authors proposed the methods how to control the channels as well as what will be the orientation of LC in them. The results can be useful for the sensors in particular used for biology.
The paper can be published after a certain revision due to the comments below.
- It can be useful, if the authors can characterize LC orientation on the surface more precisely (anchoring energy, thermo and UV stability etc). They should also mention the photolalignment as a very useful method for LC alignment in the surface with the high resolution and stability (see e.g. http://as.wiley.com/WileyCDA/WileyTitle/productCd-0470065397.html)
- The authors used some simulation results (Fig.2), but there is no comparison between the simulations and experiment. It should be also very useful to understand, what is optimal LC parameters for the microfluidic (elastic, viscous constants, dielectric and optical parameters).
- The authors used the electrolyte, but it will increase the amount of ions in LC volume and certain electrohydrodynamic instabilities may become possible (see e.g. http://www.amazon.com/gp/product/product-description/0387947086/ref=dp_proddesc_0/102-0026271-1929709?ie=UTF8&n=283155&s=books)
Author Response
We thank the Referee for her/his comments on our manuscript crystals-1711293 entitled A novel approach for the creation of electrically-controlled LC:PDMS microstructures by K.A. Rutkowska, P. Sobotka, M. Grom, S. Baczyński, M. Juchniewicz, K. Marchlewicz, A. Dybko. We have carefully considered the Reviewer's comments. Please find below our point-by-point answers. Modifications added accordingly in reply to the Referee's suggestions have been marked in blue in the file sensors-1711293_round1.docx containing a new version of the manuscript.
Response to comments of Reviewer #1:
The paper of K. Rutkowska et al. is devoted to the application of electrically controlled liquid crystals (LC) in PDMC based organic microfluidic channels. The authors proposed the methods how to control the channels as well as what will be the orientation of LC in them. The results can be useful for the sensors in particular used for biology. The paper can be published after a certain revision due to the comments below.
Remark 1: It can be useful, if the authors can characterize LC orientation on the surface more precisely (anchoring energy, thermo and UV stability etc). They should also mention the photoalignment as a very useful method for LC alignment in the surface with the high resolution and stability (see e.g. http://as.wiley.com/WileyCDA/WileyTitle/productCd-0470065397.html)
Answer: We agree with the Reviewer's opinion that the more detailed characterization of the LC molecular orientation on the PDMS surface and the information about the photoalignment (as one of the valuable methods for LC ordering) should be included in the manuscript. In fact, it should be emphasized that the essential condition for receiving and controlling the anisotropic properties of LC material is to obtain a specific orientation of its molecules within the sample volume. Typically, the surface ordering methods are employed by applying suitable substances, e.g., rubbed polymer films (generally polyimides) for planar and surfactants (such as lecithin or chromolane) for homeotropic (vertical) orientation, respectively. Alternative technologies for LC alignment include obliquely evaporated thin films (oxides) deposition, micro-grooving, nanopatterning, as well as a non-contact method of photoalignment. Whatever technique is applied, special equipment and/or suitable sample preparations are required. Therefore, it is expected that structures made of PDMS attract considerable interest as this material allows, in principle, for the intrinsic vertical orientation of liquid crystal molecules (with no either orienting layers or additional processes required).
The introductory part of the paper has been changed to include additional information (as suggested by the Reviewer #1). In particular, the text that has been inserted is marked in blue:
{line 68: Equally important is the fact that besides the possibility of complex microstructures fabrications, PDMS may be used to design and develop a substrate with desirable anchoring energy for LC molecules. It allows adjusting LC pretilt angle from 0 to 90 degrees as a function of PDMS content in poly(vinyl cinnamate) with azimuthal anchoring energy of ~6 × 10–6 J/m2 at intermediate pretilt angles [20]. PDMS thin films have also been used as alignment layers for LCs in display applications with a possible linkage between PDMS curing conditions (i.e., time and temperature) and final surface free energy value (in the range of ~20–30 × 10–3 J/m2) [21,22]. With such a pretty low surface free energy (lower than that for commercial polyimide SE-4811 used in the display industry) within a wide temperature range (up to 80 °C), the PDMS films are capable of promoting vertical alignment (VA) of liquid crystal molecules [22]. Eventually, untreated PDMS is characterized by high hydrophobicity and low surface anchoring energy, allowing for the spontaneous homeotropic (i.e., vertical) orientation of LC molecules on the elastomer surface [19]. It has to be underlined that such considerations related to the ordering of liquid crystal molecules inside photonic microstructures are not pointless in the present case. It should be remembered that the functionality of LC:PDMS structures, especially when it comes to optical applications, highly depend on the LC molecular arrangement. The essential condition for receiving and controlling the anisotropic properties of LC material is to obtain a specific orientation of its molecules within the sample volume. Typically, the surface ordering methods are employed by applying suitable substances, e.g., rubbed polymer films (generally polyimides) for planar and surfactants (such as lecithin or chromolane) for homeotropic (vertical) orientation, respectively [23-25]. Alternative technologies for LC alignment [26] include obliquely evaporated thin films (oxides) deposition, micro-grooving, nanopatterning, as well as a non-contact method of the photoalignment [27]. Whatever technique is applied, the strong anchoring is obtained for the azimuthal anchoring energy of more than 10‒5 J/m2. The latter may be achieved in the rubbed polyimide layer (~10‒4 J/m2) [24,25] or in the photoaligning materials (up to 7.2 × 10‒5 J/m2 for sulfuric azo-dye SD1) [27,28] but with an assistance of the special equipment and after suitable sample preparations. Therefore, it is not surprising that structures made of PDMS attract considerable interest as this material allows, in principle, for the intrinsic vertical orientation of liquid crystal molecules (with no either orienting layers or additional processes required) [19,21,29] with the azimuthal anchoring energy [20] of about one order of magnitude lower than for lecithin [30].}
About UV stability of PDMS – it was already mentioned in lines 46-47 {Specifically, it is characterized by high transmittance in a wide spectral range (from 240 to 1100 nm), including UV, which is a rare feature for polymers.}. Thermal stability of the surface free energy in the wide range of temperatures were studied in [22], as cited in the text.
Remark 2: The authors used some simulation results (Fig.2), but there is no comparison between the simulations and experiment. It should be also very useful to understand, what is optimal LC parameters for the microfluidic (elastic, viscous constants, dielectric and optical parameters).
Answer: We agree that the lack of direct comparison between simulation and experimental results may be awkward for some readers. We want to underline that the numerical simulations presented in this paper were performed only to get a general idea and the first set of geometrical parameters to be checked in the experimental conditions. For this moment, quantitative comparison between numerical and experimental data is not easy to be realized due to the significant approximations made when performing the calculations. More complex studies are planned to be performed to combine the various aspects, which are the calculation of molecular reorientation (as shown in subsection 2.3) but for periodical electrodes and with the assumption of dielectric constant spatial distribution resulting from the molecular arrangement (such numerical calculations will be made in the cycle – taking into account elastic, dielectric and optical parameters of specific liquid crystalline material). Moreover, we plan to calculate the modes in so-formed waveguide channels, as well as to consider the LC:PDMS structures to be possibly applied as the long-period gratings (LPG). For this purpose, additional calculations must be performed by implementing one of the suitable models (e.g., the coupled-mode theory). Due to the limited volume of the manuscript, detailed results on this subject will be reported in a future publication.
The paper's conclusions have been modified as described below. In particular, the text that has been inserted is marked in blue.
{line 823: Moreover, we consider the LC:PDMS structures to be possibly applied as the long-period gratings (LPG). For this purpose, complex numerical studies have to be performed, including more accurate determination of the LC molecular reorientation under periodic electric field, as well as the implementation of a suitable model for LPG description (e.g., the coupled-mode theory) [80-82]. Due to the limited volume of the manuscript, detailed results on this subject will be reported in a future publication.}
line 848: Presented studies give a general idea and the first set of geometrical parameters to be checked in the experimental conditions with no quantitative comparison between numerical and experimental data to be easily realized due to the significant approximations made when performing the calculations.}
Remark 3: The authors used the electrolyte, but it will increase the amount of ions in LC volume and certain electrohydrodynamic instabilities may become possible (see e.g. http://www.amazon.com/gp/product/product-description/0387947086/ref=dp_proddesc_0/102-0026271-1929709?ie=UTF8&n=283155&s=books)
Answer: We agree with the Reviewer's opinion that applying the electric voltage to the LC sample may increase the number of ions in LC volume, resulting potentially in electrohydrodynamic instabilities, as described in the suggested literature. An additional sentence has been added to the introduction to inform the readers of this possibility. The text is marked in blue in the new version of the manuscript.
{line 384: Referring to the topic of electrical conductivity, it is worth noting that despite careful purification of liquid crystalline material, there may remain a residual concentration of ionic impurities giving a considerable ionic electrical conductivity, σ ≈ 10‒7‒10‒12 Ω‒1 cm‒1 (with the ions forming near the electrodes) [25]. An alternating electric field has been applied to avoid possible electrohydrodynamic instabilities in the nematic (related, e.g., to the nonuniform distribution of the space charge concentrated near one of the electrodes). In addition, special efforts to avoid the electrolyte entering the central channel with the liquid crystal material have been made during the filling of the structure.}
Reviewer 2 Report
The paper discusses the design, fabrication, and testing methods for electrically tuned LC:PDMS structures. It is well written and organized. Here I summarized my concerns about an electro-optical sensor device as follows, and I hope they will be useful to authors.
- How about the thickness of the device? PDMS is one of the famous flexible materials and has been applied in a lot of devices. How about the flexibility of fabricated micro-fluid devices?
- Is there any specific design for the central LCs channel? Why is the circle-pore-channel preferred?
- As shown in structure 2A and structure 2B, the increase of effective length of electrode is effective to decrease the accelerating voltage. How about to deduce the height of each sub-electrode array? I believe when connecting electrodes that are far from the central LCs is made to become closer, the switch of LCs could be drawn under a much lower external voltage.
- I am quite curious about the relationships between the input and output signals. As it is an LCs based electro-optical device, and herein LCs switching is accelerated by external voltage on patterned electrodes, so is there any numerical formula that could be established to discuss the output signal like brightness change or length of electrically aligned LCs in central channel with the inputted voltage and the parameters of electrodes?
- How about the life time and the cyclical stability of a device? If this device is used for sensors, is it possible to maintain its electro-optical performance and accuracy after many times acceleration?
Author Response
We thank the Referee for her/his comments on our manuscript crystals-1711293 entitled A novel approach for the creation of electrically-controlled LC:PDMS microstructures by K.A. Rutkowska, P. Sobotka, M. Grom, S. Baczyński, M. Juchniewicz, K. Marchlewicz, A. Dybko. We have carefully considered the Reviewer's comments. Please find below our point-by-point answers. Modifications added accordingly in reply to the Referee's suggestions have been marked in blue in the file sensors-1711293_round1.docx containing a new version of the manuscript.
Response to comments of Reviewer #2:
The paper discusses the design, fabrication, and testing methods for electrically tuned LC:PDMS structures. It is well written and organized. Here I summarized my concerns about an electro-optical sensor device as follows, and I hope they will be useful to authors.
We would like to thank the Reviewer for appreciating our work and finding it interesting and valuable. We also believe that the results we obtained may be helpful for researchers working on liquid crystal optofluidics, with a particular interest in exploiting PDMS microchannels to create liquid crystalline electro-optical sensors.
Remark 1: How about the thickness of the device? PDMS is one of the famous flexible materials and has been applied in a lot of devices. How about the flexibility of fabricated micro-fluid devices?
Answer: The thickness of the device itself is of about single centimeters (which is mainly the thickness of the PDMS layer bonded to the glass substrate with a thickness of about 1mm). The channels' height is about 30μm (depending on the specific mold used for fabrication). It is a truth that PDMS elastomer is considered flexible material, but the channels are well reproduced (when fabricated using the cast-and-mold technique) without any destortions in their size and shape related to the PDMS flexibility. The photos of the channels' cross-section are presented in our recent paper (which was added to the bibliography list and is mentioned in the text). Of course, it has to be underlined that all tests related to the analysis of the LC molecular (re)orientation were performed in stationary conditions – i.e., with no additional forces (like stretch or pressure) applied to the sample. Additional text has been added to make it clear:
{line 302: Despite the flexibility of the PDMS material, the assumed rectangular cross-sectional shape of the central and side channels is perfectly reproduced in the elastomer volume. High-resolution photos of the channels (performed with SEM) are presented in [73].
line 319: It has to be underlined that all tests related to the analysis of the LC molecular (re)orientation were performed in stationary conditions – i.e., with no additional forces (like stretch or pressure) applied to the sample.}
Remark 2: Is there any specific design for the central LCs channel? Why is the circle-pore-channel preferred?
Answer: The shapes of all channels in the microstructure are enforced by the mold geometry. In all analyzed cases, the channels are characterized by rectangular cross-sections. The circle-pore-channels (mentioned by the Reviewer #2) are those used in the infiltration process (their shapes and sizes are irrelevant and related only to the technique used for cutting or drilling them - using the biopsy punch or the micro-drill in the liquid-nitrogen-cooled sample, respectively). Additional information about channels' geometry has been added to avoid possible confusion. The changes are indicated in blue in the new document.
{line 295: Each fabricated sample contained a set of holes allowing for the liquid crystal and the electrode material to be introduced into the relevant microchannels. These vertical inlets must be drilled on both ends of microchannels to allow an air to escape during filling the structures. Their shapes and sizes are irrelevant and related to the technique of drilling them.}
Remark 3: As shown in structure 2A and structure 2B, the increase of effective length of electrode is effective to decrease the accelerating voltage. How about to deduce the height of each sub-electrode array? I believe when connecting electrodes that are far from the central LCs is made to become closer, the switch of LCs could be drawn under a much lower external voltage.
Answer: It is correct that the driving voltage value can be effectively reduced when bringing the electrodes closer to the central channel. Indeed it is the main direction in further redesigning the sample to optimize its performance. Keeping in mind that minimizing the periodicity of the meander electrodes (which are required to get the optical structure in the type of the Bragg grating or long-period grating, LPG) is essential in the subsequent samples, we are considering elevating the height of the electrode channels (what could additionally simplify their infiltration with the metal eutectic). The additional explanations have been added to the text.
{line 813: It is worth noting that the main direction in further redesigning the sample to optimize its performance is bringing the electrodes closer to the central channel allowing for an effective reduction of the driving voltage value. Keeping in mind that minimizing the periodicity of the meander electrodes (which are required to get the optical structure in the Bragg grating type) is essential in the subsequent samples, we are considering elevating the height of the electrode channels (which could additionally simplify their infiltration with the metal eutectic).
line 823: Moreover, we consider the LC:PDMS structures to be possibly applied as the long-period gratings (LPG). For this purpose, complex numerical studies have to be performed, including more accurate determination of the LC molecular reorientation under periodic electric field, as well as the implementation of a suitable model for LPG description (e.g., the coupled-mode theory) [81-83]. Due to the limited volume of the manuscript, detailed results on this subject will be reported in a future publication.}
Remark 4: I am quite curious about the relationships between the input and output signals. As it is an LCs based electro-optical device, and herein LCs switching is accelerated by external voltage on patterned electrodes, so is there any numerical formula that could be established to discuss the output signal like brightness change or length of electrically aligned LCs in central channel with the inputted voltage and the parameters of electrodes?
Answer: As already mentioned, we plan to adapt our structures as liquid crystalline waveguides whose propagational properties are changed using an external electric field. Firstly by changing the molecular orientation, the effective refractive index for particular polarization changes as described e.g., by the equation in the caption of Fig. 2. When changing the spatial distribution of the effective refractive index within the liquid crystalline core, the modal characteristic for the waveguide channel is obviously changing (what can be calculated using any mode solver). Moreover, another option to be considered is applying the proposed LC:PDMS structure as the long-period grating (LPG) – the optical characteristic of such a structure may be calculated using different methods such, e.g., the coupled-mode theory and others. In fact, our further considerations are related to decreasing the microstructure's geometrical size and combining different calculation models – i.e., the spatial distribution of the electric field from the periodic electrodes and reorientation of the LC molecules under its influence, modal characteristics for so-formed liquid crystalline waveguides - to design optimal LPG to be electrically driven in the proposed configuration. Keeping in mind the complexity of the problem, it is definitely hard to propose a simple numerical formula that could describe the output signal (like e.g. light intensity) change with applied voltage. When it comes to the formula describing the length of electrically aligned LCs in the central channel with the applied voltage and the parameters of electrodes, it should be much easier to be elaborated (of course, depending on the additional parameters – such as physical properties of LC material and other geometrical parameters of the sample). However, due to the limited volume of the manuscript, detailed results on this subject will be reported in a future publication. We have added this information to the conclusions of the paper.
{line 823: Moreover, we consider the LC:PDMS structures to be possibly applied as the long-period gratings (LPG). For this purpose, complex numerical studies have to be performed, including more accurate determination of the LC molecular reorientation under periodic electric field, as well as the implementation of a suitable model for LPG description (e.g., the coupled-mode theory) [81-83]. Due to the limited volume of the manuscript, detailed results on this subject will be reported in a future publication.
line 848: Presented studies give a general idea and the first set of geometrical parameters to be checked in the experimental conditions with no quantitative comparison between numerical and experimental data to be easily realized due to the significant approximations made when performing the calculations.}
Remark 5: How about the life time and the cyclical stability of a device? If this device is used for sensors, is it possible to maintain its electro-optical performance and accuracy after many times acceleration?
Answer: We have monitored the stability of our LC:PDMS devices for a few month-time-period, and we have not observed any deterioration of its switching properties and changes in its electro-optical performance. Importantly, after switching the voltage off, the molecules get back to their initial arrangement (forced by the PDMS surfaces) in milliseconds (typical for the LC reorientation process). The main aspect to be considered when studying the cyclical stability of the presented device is the stability of the LC molecular alignment obtained in the LC:PDMS microstructures, and it was proven to not vary with time (as mentioned in our previous paper [19]). The stability of the LC:PDMS device against multiple usages has not been studied in detail yet, and will be the subject of our investigations in the very next future. Anyhow, it should be underlined that the studies of the other research groups have not indicated any instabilities when it comes to repeatable steering with the electric field, with no issues related to the electro-optical performance maintenance [22]. We have added additional information to the manuscript:
{line 830: The stability of LC:PDMS devices has been monitored for a few month-time-period with no deterioration of their switching properties nor changes in their electro-optical performance observed. The most crucial aspect to be considered when studying the cyclical stability of the presented device is the stability of the LC molecular alignment obtained in the LC:PDMS microstructures, and it was proven to not vary with time (as mentioned in our previous paper [19]). Moreover, it is worth underlining that the studies of the other research groups have not indicated any instabilities when it comes to repeatable driving PDMS-based VA LC cells with the electric field, with no issues related to the electro-optical performance maintenance [22].}
We hope that the modifications introduced into the manuscript according to the Referees' suggestions have made it more straightforward and more suitable for publication in its revised form.
Yours sincerely,
Katarzyna Rutkowska
