Energy-Efficient Liquid Crystal Smart Window with a Clear View

Round 1

Reviewer 1 Report

Report on the manuscript “Energy-efficient liquid crystal smart window with a clear-view” by Chan-Heon An et al.

The manuscript is related to design of liquid crystal smart windows.

I have read the manuscript carefully. Maybe the manuscript could be useful for people with interest in smart windows and similar devices. However the manuscript is written unclearly and it is hard to be understood both by reviewers and by journal readers. To make an appropriate revision of the manuscript, the text has to be substantially reworked.

Below a list of remarks to the manuscript is given.

1)     First, about terminology. The authors systematically mention “vertical alignment”, “horizontal direction” and “vertical direction”. It seems that “horizontal direction” and “vertical direction” imply that the windows are oriented vertically. When the authors start speaking about the optical cell, they also consider it to be oriented vertically and use the same words (“horizontal direction” and “vertical direction”). However, when experiments and calculations are made on optical cells, the orientation of molecules, helix, the incident and scattered light is indicated with respect to the plane of the cell and its normal. For example, Fig. 3 shows photographs taken under a polarizing microscope. Was the cell oriented vertically and so was the polarizing microscope oriented horizontally? This would be an unusual orientation of the microscope. To avoid confusion I suggest that the authors introduce a coordinate system (x,y,z) and show it in the figures. I suggest additionally pointing the orientation of molecules, light rays and so on also with respect to the coordinate system.

2)      It is not clear what is shown in different parts of Fig. 1. Are these images of the central and outer parts of the window? Does Fig. 1 represent a drawing by the authors or is it repoduced from some other source? Besides the images of Fig. 1 show clear blue sky. It is known that light from the sky is polarized. Do the authors account for the polarization of light in their analysis?

3)      It seems the surfaces of the cell were covered by alignment layer. What was the alignment (creating homogeneous or homeotropic orientation)? 

4)      In Fig. 2(c)) it should be shown by arrows which curve corresponds to the left axis, and which to the right axis.

5)      The manuscript states that the helix pitch is 8.75 mm. Is this value of the pitch conserved in cells with thickness 12.5 mm and 5.5 mm, or the pitch is changed, or even the helix is untwisted?

6)      In page 3 of the manuscript it is stated that absorption coefficients of X12 α⊥ and α∥ were 0.024 μm^-1 and 0.205 μm^-1. It seems that α⊥ and α∥ mean directions perpendicular and parallel to the molecular long axis (the authors do not explain it). However in page 2 it is written that “Dichroic dyes … strongly absorb light when irradiated along their long axis and weakly absorb light when irradiated along their short axis”. This contradicts what is written in page 3 about α⊥ and α∥.

7)      The authors write about the absorption coefficient of X12 molecule. However the dye molecules are oriented in liquid crystal host and in calculations one should account for the degree of orientational ordering.

8)      The authors state they used the mixture E7. It is a nematic liquid crystal. Why does the cholesteric liquid crystal appear with helix pitch 8.75 mm? Or is it related only to theory and there were no experiments? In this case what photographs are given in Fig. 3(b)? Are these photographs of empty cells fabricated at KNU, as can be read in the center of photographs?

9)      Section 3 of the manuscript is titled “3. Experimental result of the fabricated device”. However this section reports calculations “using the commercial software tool”. Do the authors call their calculations “experimental result”?

10)  In Fig. 4 it is not stated whether it is experiment or calculation. In Fig. 4(a) it is stated that there are two values of transmittance, given by red and black curves. However in the figure there is only one red curve, and there is no black curve.

 In summary, for the manuscript to be reviewed in essence, the text of the manuscript has to be substantially improved.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

The paper describes the optical properties of the guest-host liquid crystal cell with a uniform lying helix structure and makes a comparison with the properties of the vertically-aligned cell. The ULH cell shows the same transmittance in the normal and horizontal directions, while in the vertical direction the transmittance is smaller, which at the same time provides good view through the window and decreases the amount of sunlight entering a house. Meanwhile, the UV cell enables good view only in a normal direction, thus the ULH cell has better properties for application in smart glass in windows.

The work described in this paper is interesting and important for future applications. However, the description of the studies is in some parts unclear: the investigated mixture is not fully characterized, the experimental technique and equipment for the transmittance measurement are not mentioned, and the uncertainties of the presented transmittance values are not given. These details must be completed before acceptance.

lines 36-37 “Specifically, the transmittances in the vertical and horizontal directions were 44.2% and 29.5%, respectively.” – I believe it was meant to be “horizontal and vertical”, like in the abstract.

lines 54-68 I am a bit confused by the description in this part. According to Figure 2, the VA cell absorbs weakly when the direction of light is along the z-axis – the long axis – and the vector of electric field is in the xy-plane. However, in line 55 there’s written “strongly absorb light when irradiated along their long axis”. I would like to ask the Authors to clarify what they mean by the direction of irradiation – is that the direction of light (then there’s a contradiction), or the direction of the E vector (then it is correct)?

line 91 – What is the temperature range of the nematic phase of the E7? It is important from the application point of view.

line 92 – What is the molecular structure of the X12 dye? It should be added to Figure 3 or a link should be given.

line 103 – The black line in Figure 4(a) could be made thicker, to be visible beneath the red line. Also, how was the transmittance measured (the method and experimental set-up are not described) and what are the uncertainties of the presented transmittances?

line 36 decreasing -> it decreases

line 93 – The parallel and perpendicular symbols should be in subscripts and −1 should be in superscripts.

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Report on revised manuscript “Energy-efficient liquid crystal smart window with a clear-view” by Chan-Heon An et al.

 Dear editors and authors of the manuscript,

 In their responses to reviewer’s comments the authors write: “We appreciate it very much for your valuable comments… Following the reviewer’s comments…” and so on, many times. But really the authors as a rule did not follow the comments of the reviewer.

For instance, about point (4) of the reviewer’s report.

The authors write: “Following the reviewer’s comments, we added arrows in Figure 2d of the revised manuscript”. However Figure 2d in the new version (old Figure 2c) is the same as Figure 2c in the first version without any added arrows.

About the photo in Figure 3b. It remains completely unclear why the authors provide photographs (Figure 3b) with label “KNU”.

In all papers on spectroscopy of liquid crystals the absorption coefficient is given for light polarization parallel and perpendicular to the long molecular axes (or parallel and perpendicular to liquid crystal director). The authors in their calculations do not mention light polarization. For what light polarization were measured the two curves in Figure 3a and in what liquid crystal, nematic or cholesteric, is also not indicated.

In conclusion section, the authors write: “our experiments have demonstrated the remarkable optical…” The experiment in the manuscript is “POM and cell images” in Figure 3b. Only here the authors remember about polarization of light. But these photos in the microscope do not have relation to calculations and the text of the manuscript.   

And so on.

 I realize that in order to prepare the text in good form, it is necessary to have a lot of successive reports and then responses to every report. So my proposal is as follows. Since the article can be useful to researchers working on smart windows, maybe publish the manuscript in current form, since for design of smart windows the listed shortcomings of the manuscript are not essential.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

The Authors addressed all my queries. I have only one issue: the temperature of the nematic/isotropic liquid transition, added by the Authors to the text, is taken from "Dispersion of Diamond Nanoparticles in Nematic Liquid Crystal/Polymer Materials". This publication is mentioned in the answers to the referee, but it is not cited in the paper. It should be cited (thus only minor revision is necessary).

Author Response

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Author Response File: Author Response.docx