Laser-Induced Breakdown Spectroscopy and Shadowgraphy of Acoustically Levitated Heptane Droplets
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis is a very interesting paper. The authors employed a series of diagnostics tools to examine the impact of droplet size and laser energy on droplet fragmentation. The paper can be published with minor modifications. I have some suggestions/comments here:
1. A zero-dimensional model for laser energy, consisting of terms for scattering, absorption, etc. is needed to better understand the physics.
2. In the experiment, is ignition of the droplet observed? From your analysis, it is not clear to me that if the n-heptane droplet is ignited and goes through combustion. If not, I am curious why the droplet is not ignited.
3. How do you control the diameter of droplet size? What is the uncertainty for that?
Author Response
Response to Reviewer 1 Comments:
This is a very interesting paper. The authors employed a series of diagnostics tools to examine the impact of droplet size and laser energy on droplet fragmentation. The paper can be published with minor modifications. I have some suggestions/comments here:
Comment1: A zero-dimensional model for laser energy, consisting of terms for scattering, absorption, etc. is needed to better understand the physics.
Response1: While a gas-phase zero-dimensional model may be feasible1, the two-phase nature of the droplet problem introduces additional complexity, involving numerous variables that present significant challenges. A closely related study2 performed an energy balance analysis of optical breakdown in a cuvette filled with water, although it did not specifically address droplets. While this approach is intriguing, incorporating such an extensive analysis into our study would exceed the scope of this work. Our study is the first to emphasize the impact of energy coupling on droplet fragmentation, and future research will focus more closely on quantifying interactions, including scattering and absorption.
Comment 2: In the experiment, is ignition of the droplet observed? From your analysis, it is not clear to me that if the n-heptane droplet is ignited and goes through combustion. If not, I am curious why the droplet is not ignited. –
Response 2: We did not observe ignition of the droplet, as indicated by the absence of OH emission (~310 nm) in the spectra (checked at various time delays after the laser pulse). The hydroxy radical (OH) is a well-established marker of heat release in combustion processes3,4, so its absence suggests that ignition did not occur. Additionally, no visible chemiluminescence from combustion was detected from the laser-droplet interaction. The laser energy absorbed by the droplet primarily causes rapid heating, vaporization, and fragmentation, rather than the gradual heating required for ignition. The deposited energy may lead to explosive phase transitions (e.g., rapid vaporization) instead of supporting a stable flame. This vaporization and fragmentation occur on a much shorter timescale than that needed for combustion reactions to initiate. Consequently, while vaporization leads to plasma formation via a different reaction pathway, the distinct timescales of fragmentation and combustion prevent sustained combustion from occurring.
We clarify this in the revised manuscript at line 120 as follows: “For all the test conditions, we did not observe any visible chemiluminescence (through combustion reactions) from the laser interaction with the droplet.”
Comment 3: How do you control the diameter of droplet size? What is the uncertainty for that?
Response 3: This reviewer comment is very helpful and we have added more detail on this issue in the (revised) supplementary files and also revised the manuscript accordingly at line 114: “We used a 3 mL syringe (BH Supplies) to inject a droplet with an initial diameter of approximately 2 mm, allowing it to evaporate until it reached the desired diameter. To estimate the time required to reach this diameter, we applied the D² law of droplet evaporation.”
Please see attachment for more information on this issue.
We also added the following text on uncertainty of droplet diameter in the revised manuscript at line 115: “We determined the droplet diameter from the image captured right before the laser hits the droplet. We used MATLAB to process the image. The pixel size was 3.3 μm which gives a pixel resolution uncertainty of 1.65 μm for the droplet diameter in each test case.”
Author Response File: 
Author Response.docx
Reviewer 2 Report
Comments and Suggestions for AuthorsThis manuscript described a study on the process of laser irradiation of heptane droplet. The important results are the two regimes under different energy densities, which could be applied to design the laser ignition system of droplet. The work is innovative, and is suitable for the publication in this journal. Some suggestions are as below.
1. Is the test of laser ablation of liquid droplets carried out in atmospheric environment? Then, when the laser irradiated on the heptane droplet, the nearby air may be ablated too. Then how to exclude the influence of air emission spectrum when collecting the heptane droplet spectrum?
2. It is suggested to list the droplet size corresponding to different laser pulse energies in a table for readers’ better understanding the manuscript.
Author Response
The authors thank the journal staff and reviewers for their time and effort in evaluating our manuscript. The reviewers' feedback has been invaluable in strengthening the paper. Our responses to each comment are provided below in red.
--------------
Response to Reviewer 2 Comments:
This manuscript described a study on the process of laser irradiation of heptane droplet. The important results are the two regimes under different energy densities, which could be applied to design the laser ignition system of droplet. The work is innovative, and is suitable for the publication in this journal. Some suggestions are as below.
COMMENT 1: Is the test of laser ablation of liquid droplets carried out in atmospheric environment? Then, when the laser irradiated on the heptane droplet, the nearby air may be ablated too. Then how to exclude the influence of air emission spectrum when collecting the heptane droplet spectrum?
RESPONSE 1: Yes, the tests are conducted in an atmospheric environment. The current study serves as a foundational investigation of plasma-droplet interactions, forming the basis for our subsequent research on laser-ignited spray combustion. Since we aim to study combustion, we are specifically interested in emissions from a combustible mixture of air and fuel and are therefore not trying to avoid emissions resulting from air ablation.
COMMENT 2: It is suggested to list the droplet size corresponding to different laser pulse energies in a table for readers’ better understanding of the manuscript.
RESPONSE 2: This is a useful suggestion; we have added Table 1 to the manuscript as shown in the attachment. However, we chose not to include the complete set of diameters at each energy (with at least 15 diameters per range) to avoid excessive use of space.
Author Response File: Author Response.docx
Reviewer 3 Report
Comments and Suggestions for AuthorsThe article under review is written on an interesting topic - the interaction of laser radiation with liquid droplets. There are a few questions and comments on the article:
- the purity of the heptane used is not indicated in the experimental methodology section;
-where are KE, SP, and LP located in Fig. 1a?
- references 3, 4, 24, 32, 34, 39-44, 48, 54, 58, 62, 63, 66, 68, and 70 lack output data;
The article under review analyzes the laser breakdown of a levitating heptane droplet depending on various experimental parameters.
Traditionally, LIBS is used to analyze the composition of the medium. In this case, the main attention is paid to the emergence of new emission lines. In this work, the threshold value of the pump energy at which the optical breakdown mode of the droplet changes is identified. In the literature, there are works on the observation of stimulated light scattering processes in droplets (SRS, SMBS, etc.), for which the threshold is determined by the substance under study and is quite simple to interpret.
The use of the obtained results for experimental modeling of ignition processes seems interesting since traditional methods are more dangerous.
Author Response
The authors thank the journal staff and reviewers for their time and effort in evaluating our manuscript. The reviewers' feedback has been invaluable in strengthening the paper. Our responses to each comment are provided below in red.
-----------
Response to Reviewer 3 Comments:
The article under review is written on an interesting topic - the interaction of laser radiation with liquid droplets. There are a few questions and comments on the article:
COMMENT1: the purity of the heptane used is not indicated in the experimental methodology section;
RESPONSE 1: We have added this information in revised manuscript in the methodology section line 107 as follows: “To create a controlled environment for the study of laser breakdown on droplets, we utilized a single-axis acoustic levitator (TinyLev, Makerfabs) to suspend a single heptane (Thermo Fisher Scientific H3504, purity96%) droplet in ambient air.”
COMMENT 2: where are KE, SP, and LP located in Fig. 1a?
RESPONSE 2: Thank you for pointing this out. We revised the optical setup to clarify the positions of the short-pass (SP) and long-pass (LP) filters, as shown below for reference. Initially, we used a Schlieren setup with a knife edge (KE); however, we transitioned to a shadowgraphy setup and subsequently removed the knife edge. The updated figure reflects this change and no longer includes a KE in the legend. Please see the attachment for the revised figure.
COMMENT 3: references 3, 4, 24, 32, 34, 39-44, 48, 54, 58, 62, 63, 66, 68, and 70 lack output data
RESPONSE 3: Thank you for pointing this out. The references have been updated to include all relevant details.
The article under review analyzes the laser breakdown of a levitating heptane droplet depending on various experimental parameters.
Traditionally, LIBS is used to analyze the composition of the medium. In this case, the main attention is paid to the emergence of new emission lines. In this work, the threshold value of the pump energy at which the optical breakdown mode of the droplet changes is identified. In the literature, there are works on the observation of stimulated light scattering processes in droplets (SRS, SMBS, etc.), for which the threshold is determined by the substance under study and is quite simple to interpret.
The use of the obtained results for experimental modeling of ignition processes seems interesting since traditional methods are more dangerous.
Author Response File: Author Response.docx
