Topological Defect-Guided Regular Stacking of Focal Conic Domains in Hybrid-Aligned Smectic Liquid Crystal Shells

Round 1

Reviewer 1 Report

The manuscript describes observations obtained on hybrid aligned LC shells in the vicinity of the N-S_A transition. The relative density of the LC and those of the inner and outer liquids plays a crucial role in the formation of the structure in the shell, since the system is in the gravity field. Several density configurations have been studied and some interesting phenomena were found. The work is fully empirical, it is entirely based on observation. Some handwaving explanations are given, though they are plausible but without any theoretical calculation and/or computational simulation they remain speculations. 

The paper is clearly written, some critics can be formulated about the figures. The snapshots are not always demonstrative. Effects, which should be “easy to recognise”  according to the text, are sometimes not visible at all.

Questions and comments:

1. How do the systems described in chapters 3.1 and 3.2 behave when observed perpendicular to gravity?

2. It would be useful to indicate the temperature in the snapshots of figures 2, 4, 5 and 6. Is the temperature in Fig.5a and 5h the same? After heating back to the N phase, will the shells be randomly distributed and oriented like in 5a and 5b?

3. In several places of the text T_NI is indicated, instead of T_NS, see e.g lines 267, 292, 311, 313, 320.

As already said, the work describes only microscopic observations, nevertheless contains interesting findings. I suggest publication in Crystals.

Author Response

We thank the reviewer for the careful reading of our manuscript and the valuable feedback. Our response to the critical points is as follows:

The work is fully empirical, it is entirely based on observation. Some handwaving explanations are given, though they are plausible but without any theoretical calculation and/or computational simulation they remain speculations.

Indeed, the study is fully empirical, a fact that we have not in any way tried to hide. However, we have analyzed our experimental results very carefully, leading to conclusions that are clear and uncontroversial regarding the structures that give rise to the textures. The speculative part is restricted to the discussion of why the structures and phenomena develop. While we fully agree that theory and computer simulations are needed to support our conjectures (as we wrote multiple times already in the first version), we do anchor our conjectures in previous work in the literature, also including theory and computer simulations, which consider other cases of frustrating confinement that nevertheless have sufficient similarities to be relevant for the phenomena we observe. Therefore, there is a sound foundation for our conjectures, which we hope is sufficient at this stage. It should be emphasized that the experimental challenges to produce the results presented in the paper are not insignificant, and we hope that the many surprising observations resulting from them will inspire further theoretical and computer simulation studies.

The snapshots are not always demonstrative. Effects, which should be “easy to recognise” according to the text, are sometimes not visible at all.

We apologize for the lack of clarity. In the revision, we have tried to be more specific in our references to features in figures, and when there are variations between shells, we have acknowledged this rather than referring simply to what happened in the majority of the shells, as we sometimes did in the first version.

How do the systems described in chapters 3.1 and 3.2 behave when observed perpendicular to gravity?

We did not study this. It should be noted that the experiments with tilted microscope are not easy to carry out, as components easily move during the reorientation. Moreover, the heavy microscope must be held by the operator throughout the experiment, as the instrument is not made for operation perpendicular to gravity; hence the experiment is quite demanding for the operator. For these reasons, we could not do all experiments with the non-standard microscope orientation.

It would be useful to indicate the temperature in the snapshots of figures 2, 4, 5 and 6. Is the temperature in Fig.5a and 5h the same?

We fully agree that this would be very useful, and it is truly frustrating that we cannot provide the exact information. The movies were recorded in a way that was supposed to record the temperature continuously. Unfortunately, this failed and there was either no or only incomprehensible temperature information throughout the movies. We did not discover the problem until after the experiments were concluded. It is even difficult to estimate the temperature since the 0.01~K/min cooling/heating rate was only applied in the near vicinity of the nematic-smectic transition; since further away no dramatic changes with temperature take place, faster cooling/heating rates were applied. We therefore prefer to indicate only if a photo is obtained above or below the nematic-smectic transition temperature, which is indicated with numerical precision. Also the clearing point is given quantitatively; hence readers can understand the approximate temperatures.

After heating back to the N phase, will the shells be randomly distributed and oriented like in 5a and 5b?

No, they are not. This is seen in Fig. 6 and discussed in the corresponding text.

In several places of the text T_NI is indicated, instead of T_NS, see e.g lines 267, 292, 311, 313, 320.

We apologize for this repeated error and thank the reviewer for making us aware of it. We have corrected the text and hope that instances of the error are now removed.

Reviewer 2 Report

Noh and Lagerwall present an interesting and elegant study of defect transformations in shells during a nematic-to smectic A phase transition. The authors observe new phenomena, such as the realignment of the symmetry axis from being along the gravity field to being perpendicular to it. Another interesting observation is that the thicker part of the shell shows lower retardance than the thin part. All observations are carefully substantiated and explained qualitatively. The paper is richly illustrated and properly referenced. I have only two minor comments.
In the Abstract, the authors might wish to replace "vertical" and "horizontal" with "along the gravity direction" and "perpendicular to it" to improve clarity.
It is not clear what was the purpose of adding the reactive monomer RM257. Was it polymerized? Was there any photoinitiator? Was there any filter to prevent spontaneous polymerization? The abstract adds to the confusion by mentioning "fully polymerizable" liquid crystals, but the main text does not provide much information on the possible photopolymerization.

Author Response

We thank the reviewer for their encouraging feedback and helpful suggestions for further improving the manuscript. Our response to the critical points is as follows:

In the Abstract, the authors might wish to replace "vertical" and "horizontal" with "along the gravity direction" and "perpendicular to it" to improve clarity.

We thank the reviewer for this suggestion, which we have implemented.

It is not clear what was the purpose of adding the reactive monomer RM257. Was it polymerized? Was there any photoinitiator? Was there any filter to prevent spontaneous polymerization?

The main purpose was to bring down the temperature of the nematic-smectic transition closer to room temperature, and to facilitate density matching in the vicinity of the transition. We have added this information to the revised manuscript.

The abstract adds to the confusion by mentioning "fully polymerizable" liquid crystals, but the main text does not provide much information on the possible photopolymerization.

We have rephrased this part to avoid the confusion.