Effect of the Hybrid Hydrophobic-Hydrophilic Nanostructured Surface on Explosive Boiling

Round 1

Reviewer 1 Report

This work focuses on the effect of the hybrid hydrophobic-hydrophilic nanostructured surface on the explosive boiling, where the hydrophobic surface is beneficial for bubble nucleation and the hydrophilic surface enhances the critical heat flux.

The authors present through simulations that the hybrid nanostructure can decrease the onset temperature compared to the pure hydrophilic surface. It is attributed to the effect of hydrophobic coating, which promotes the formation of bubbles and causes a quicker liquid film break. Furthermore, with the increasing of the hydrophobic coating thickness, the onset temperature of explosive boiling decreases. 

This work can be published after minor revisions

1. I kindly ask the authors to fix Fig. 3. It seems that the lower part is missing.

Thanks for your comment. We have fixed Fig. 3 in the revised manuscript. (Please see L184.)

2. The authors state that "..Fig. 7 (a) shows the evolution of average temperature of each layer for 3 nm liquid film. There is a little temperature difference between these layers, indicating the heat absorbed from the plate can be fast transferred from the bottom layer to the top layer.."

To be honest, I see a small difference up to 0.5ns. Please state on the SN ratio on this figure. Discuss a little more regarding the effect of the liquid films thickness.

Thanks for your comment. We have stated on the SN ratio on Fig. 7. Just as the reviewer said the difference is more clearly shown up to 0.5 ns (Fig. 7a). That is because at the early stage, the first layer which contacts with the bottom surface, elevates its temperature to a high level quickly, while the second and third layers will increase their temperature gradually. However, the duration of this stage is shorter compared to Fig. 7b, and the difference of each layer is also smaller than Fig. 7b. Therefore, a thicker liquid film has a larger temperature gradient, inducing an advantage to trigger the bubble nucleation, and a lower onset temperature. The previous text has been corrected as “It is found that at the early stage (before 0.5 ns), it shows a temperature difference for three layers. That is because the first layer which contacts with the bottom surface, elevates its temperature to a high level quickly, while the second and third layers will increase their temperature gradually. For the next stage (after 0.5 ns), there..” (Please see L259-L63.)

3. I suggest that the authors should include an error bar in Fig. 2.

Thanks for your comment. We have included an error bar in Fig. 2 in the revised manuscript. (Please see L162.)

4. Please check the manuscript for typos etc. There are some words in capitals, and some syntax errors that need to be resolved.

Thanks for your comment. We appreciate the reviewer’s positive comments. We have checked the English usage and grammar carefully. We have revised and marked it in the manuscript.

Reviewer 2 Report

In this paper, the authors simulated the explosive boiling on the hybrid hydrophobic-hydrophilic nanostructured surface using molecular dynamics. They found that the hybrid nanostructure can reduce the starting temperature compared to the purity of hydrophilic surface due to hydrophobic covering that promotes bubbles and causes a faster break-in liquid film. Moreover, with an increase in column width and liquid film thickness, the onset temperature of explosive boiling in the hybrid wetting surface decreases.

Specific comments:

1. The authors should improve and enlarge Fig. 1. The schematic diagram of the simulation system should be appropriately labeled, and the nanostructures should be visible. The authors can also change the color combination to enhance the visibility of the atoms.

Thanks for your comment. We have redrawn Fig. 1 and the previous text has been corrected as “The simulation system is schematically shown in Fig. 1a and Fig. 1b” (Please see L112.) and “As shown in Fig. 1c and Fig. 1d, ..” (Please see L112.)

2. As is visible from Fig.7, the time evolution of the three layers depends upon the thickness. Why do the authors have chosen only two thicknesses (3 and 6 nm)? I suggest that they include more thickness for the complete study.

Thanks for your comment. Just as the reviewer said the time evolution of the three layers depends upon the thickness. However, the research [R1-R3] about the relationship of the onset temperature (T_s) and liquid film thickness has given a definite conclusion, which T_s decreases with the increase of the film thickness. Although the surfaces in these papers are flat and pure hydrophobic or hydrophilic nanostructured, the physical mechanism of them is same. On the other hand, the key point in this work is to study the effect of the hybrid hydrophobic-hydrophilic nanostructured surface on the explosive boiling. Whether the influence of the liquid film on T_s is similar to the previous research is just a point we try to verify. Therefore, we have chosen only two thicknesses to demonstrate this relationship in this work. To make it more reasonable, the following descriptions have been added into the revised manuscript: “According to previous research [30, 34, 35], the onset temperature of explosive boiling de-creases with the increase of the film thickness, where the surfaces are flat or pure hydrophobic or hydrophilic nanostructured. To verify the relationship of above two factors on this hybrid hydrophobic-hydrophilic nanostructured surface, the trend of onset temperature with two film thicknesses is also discussed.” (Please see L244-L248.)

[R1]. Liao, M. J.; Duan, L. Q. “Explosive Boiling Of Liquid Argon Films On Flat And Nanostructured Surfaces. Numer. Heat Tr. A-Appl. 2020, 78, 94-105.

[R2]. Dou, Y.; Zhigilei, L. V.; Postawa, Z.; Winograd, N.; Garrison, B. J. Thickness effects of water overlayer on its explosive evaporation at heated metal surfaces. Nucl. Instrum. Meth. B., 2001, 180, 105–111.

[R3]. Wang, Y. H.; Wang, S. Y.; Lu, G.; Wang, X. D. Explosive boiling of nano-liquid argon films on high temperature platinum walls: Effects of surface wettability and film thickness. Int. J Therm. Sci., 2018, 132, 610–617.

3. I suggest that the authors should include an error bar in Fig. 2.

Thanks for your comment. Actually, we included case of 6 nm thickness, and the result is shown in Fig. R1. Since q_w has a similar trend to the situation of case of 3 nm thickness, we just show the Fig. 4 as an example to discuss the effect of hydrophobic coating thickness. To illustrate it more clearly, the following descriptions have been added into the revised manuscript “It should be noted that, if the film thickness increases to 6 nm, q_w shows a similar trend to the situation of case of 3 nm thickness, which is shown in Supporting Material (Fig. S1). This also illustrates that no matter what the film thickness is, the heat flux increases with the coating thickness.”(Please see L209-L212.)

Round 2

Reviewer 2 Report

The manuscript now can be accepted in the present form.