key: cord-0957198-dfjccrfr
authors: Kwak, Dong-Bin; Kim, Seong Chan; Kuehn, Thomas H.; Pui, David Y.H.
title: Quantitative analysis of droplet deposition produced by an electrostatic sprayer on a classroom table by using fluorescent tracer
date: 2021-08-12
journal: Build Environ
DOI: 10.1016/j.buildenv.2021.108254
sha: 654f53c37fa96e0b45cb3ebe89457d7987b3e59c
doc_id: 957198
cord_uid: dfjccrfr

Due to the ongoing COVID-19 pandemic situation, measures to mitigate the risk of transmission of the SARS-CoV-2 virus in an indoor setting are urgently needed. Among the various types of disinfectant methods, electrostatic spraying is often applied to decontamination in public places. For quantitatively characterizing electrostatic spraying, we developed the novel evaluation method by using a fluorescent tracer. By applying this method, we performed three different experiment cases (static test on a table, static test on a cylinder, and dynamic test on a table) to figure out its unique characteristics (Coulombic fission and wraparound effect) and measure its performance in various aspects. To be specific, bimodal distribution with peak sizes of ∼10 and ∼100 μm was found due to Coulombic fission. Otherwise, a unimodal distribution with a peak size of ∼100 μm occurred for the uncharged droplets. As a result, the effective contact area increased by 40–80 % due to small progeny droplets. The wraparound effect was examined on two different cylinders: copper (Cu) and polyvinyl chloride (PVC) pipe. When the target surface was not charged (Cu 0 kV and PVC 0 kV), the average normalized concentrations on the backside of the cylinder (θ = 180°) increased by around 67 % for charged droplets. Meanwhile, when the target surface was highly charged (PVC –19 kV), the average normalized concentrations at θ = 180° were increased more than two times for charged droplets.

Under the current COVID-19 pandemic situation, it is urgently required to establish general 34 measures to mitigate the risks of SARS-CoV-2 virus transmission in indoor environments. While 35 there is a controversial discussion regarding the dominant transmission path, it is well known that 36 fomite transmissions are considered to be one of the sources of SARS-CoV-2 virus transmission 37 [1] [2] [3] [4] . It means that expiratory droplets generated from infected persons settle on shared surfaces 38 such as desks, furniture, door handles, and tools, and those can be transmitted to the next individual 39 who touched the surfaces afterward. In addition to the risk of fomite transmission, the virus within 40 human saliva can survive up to 3 days on plastic, and stainless steel, which makes surface 41 decontamination to be the most practical way to mitigate virus spread [3, 5] . For those reasons, As main surface decontamination methods like surface wiping, trigger sprayers, pressure 47 sprayers, and electrostatic sprayers are used with disinfectant solutions for various places from 48 households to public places such as schools, public transportations, and hospitals [18, 19] . Among 49 the several types of sprayers, electrostatic sprayers are mainly applied for public place 50 decontaminations due to their time and labor effectiveness over other sprayers. Their 51 characteristics to deliver the droplets to hidden surfaces of the objects, which is called the 52 wraparound effect, result in full even coverage on hardly reaching surfaces [20] [21] [22] . The 53 electrostatic nozzle application has been developed in numerous industrial applications including 54 particle synthesis [23] , micro-patterning [24] , thin-film deposition [25, 26] , measurement device 55 [27] [28] [29] , oil burner [30] , and agriculture [31-34] due to its unique characteristics. Although the 56 characterization of droplet deposition pattern and droplet size distribution is essential for 57 optimizing the disinfectant sprayer, to our best knowledge, there is no research conducted through 58 quantitatively characterizing the electrostatic sprayer performance. 59

For tracking a large number of droplets, we employed the fluorescent tracer method. Due 60 to technological advances in various industries, aerosol transport tracking studies have gained great 61 J o u r n a l P r e -p r o o f (GFP/FITC filter) with an excitation wavelength range of 465-495 nm and an emission wavelength 120 range of 515-555 nm was used in this study. The raw images were captured through the 121 fluorescence microscope under 10× magnification, 4×4 binning working mode, and 1 s exposure 122 time (640×540 pixels and binned pixel size equal to 2.6 µm). 123

Recognizing images and retrieving image information was performed by using commercial 124 software, MATLAB Release 2020a. To extract the fluorescent droplet information and remove the 125 background noise, the color threshold segmentation which is one of the most commonly applied 126 segmentation methods [43, 44] was applied to eliminate parts of the image that fall outside a 127 specified color range. Based on 256-color values (0-255), the RGB color threshold range was set 128 from 0 to 255 for red, 75 to 255 for green, and 0 to 50 for blue. The filtered images were analyzed 129 after changing them to binary images. and SC refer to narrow band, and sharp-cut, respectively. (For interpretation of the references to 149 color in this figure legend, the reader is referred to the web version of this article.) 150 151

In order to quantitatively characterize the electrostatic sprayer performance with or without 153 an electrostatic charge, three different experiment cases were performed, as shown in Fig 2. For 154 quantitatively determining how far, wide, and evenly the sprayed solution can be delivered, a static 155 test on a table (482.6(W)×1524(L) mm) was performed, as shown in Fig. 2a the electrostatic sprayer performance in a more practical aspect. The electrostatic sprayer with or 171 without electrostatic charge was continuously and horizontally operated by walking along the front 172 for 10 seconds (Front) or along the sides for 5 seconds (Side). In each case, the spray nozzle tip 173 was maintained at a height of 1 m and 0.2 m from the edge of the desk, as shown in Fig. 2e . 

where dp is the aerosol state particle (droplet) diameter, Q is the liquid flow rate, and f is the 193 vibrating frequency. The operating condition and of FMAG and calculated droplet diameter were 194 summarized in Table 1 ).

(2) 209

Where dp,contact is contact diameter measured by fluorescence microscope and is the contact angle. 

A static test on a table was conducted as shown in Fig. 2a . One of the biggest features in the difference between the presence or absence of an electrostatic 297 charge on droplets is that the number of peak points (modes) in the distribution of the contact 298 diameter is measured by a fluorescence microscope. If there is no electrostatic charge on the 299 droplets, a unimodal distribution with a peak size of ~100 µm occurred, but if there is an 300 electrostatic charge on the droplets, a bimodal distribution with peak sizes of ~10 and ~100 µm 301 was shown. Small contact diameters (< 10 µm) were not found in the case without an electrostatic 302 charge but in the case with an electrostatic charge. However, in both cases, most of the droplets 303 were deposited on the cover glasses with a distance of 0.27L, and as the distance from the spray 304 One of the most striking features of charged droplets generated by the electrostatic sprayers 318 is Coulombic fission. In the case of droplets without electrostatic charge, as shown in Fig. 7(a) , 319 the effect of simply reducing the size of the droplets occurred. However, in the case of charged 320 droplets, as shown in Fig. 7(b) , not only evaporation but also parent droplet breakup occurred 321 caused by the greater electrostatic repulsion force caused by surface charge compared to surface 322 tension, resulting in creating progeny droplets [48] . Aerosol state droplet diameters in Fig. 7 were 323 calculated by using contact diameters in Fig. 6 and the ratio (B = 0.407) between the contact 324 diameter and the aerosol state droplet diameter in eq. (4). Fig. 7(c) illustrates the variation of peak 325 droplet diameter according to the distance from the sprayer nozzle. The peak droplet diameters 326 gradually decreased regardless of the presence of an electrostatic charge on the droplet. It is 327 worthwhile to mention that smaller droplets may include droplets generated by a breakup in highly 328 charged liquid jets in addition to droplets generated by Coulombic fission. In order to quantitatively describe the benefits of progeny droplets which were originated 337 from the breakup of charged droplets in terms of electrostatic sprayer performance, the increment 338 of effective contact area based on the same volume of droplets was introduced. Here, we assumed 339 that the volume of sprayed solution was the same on both sprayer modes. The relative number of 340 droplets between progeny droplets and parent droplets with an electrostatic charge for the 'single' 341 droplet volume without an electrostatic charge at the peak droplet diameter can be obtained as where Voff is the droplet volume without an electrostatic charge at the peak droplet diameter. Von,1st, 345

and Von,2nd are the droplet volume with an electrostatic charge at the first (progeny) and second 346 (parent) peak droplet diameter, respectively. The subscript of i indicates the different distance from 347 the sprayer nozzle, e.g., i = 0.27L. 1st and 2nd are the number distribution ratio of the first and 348 second peak droplet diameter, which can be obtained from Fig. 7b . β is the number correction 349 factor for adjusting the value of the left and right sides term. Here, β indicates the relative number 350 of droplets compared to the single droplet volume without an electrostatic charge at the peak 351 droplet diameter. Through using eq. (7) and the number distribution ratio and droplet diameters 352 which can be obtained from Fig. 7 (a) and (b) , the relative number of droplets based on the single 353 volume of sprayed droplet without an electrostatic charge was calculated as shown in Table 2 

where Aeff is effective contact area, (dp/2B) 2 . The calculated increment of effective contact area 358 by using the electrostatic sprayer were 0.27L = 42.1%, 0.50L = 76.6%, and 0.73L = 61.4%. From 359 this analysis, we can conclude that spraying the charged droplets can drastically improve the 360 contact area between the solution and surface due to small progeny droplets. 361 362 Table 2 . The relative number of charged droplets compared to the single volume of sprayed 363 droplets without an electrostatic charge. The peak droplet diameter and number distribution ratio 364 between the first and second peak droplet probability density were obtained from Fig. 7 . The 365 number correction factor was calculated from eq. (7). The increment of effective contact area based 366 on the same volume of droplets without an electrostatic charge ( i) was calculated from eq. (8 

Although it is a well-known fact that an electrostatic sprayer has a wraparound effect on a 371 

For evaluating the electrostatic sprayer in a real application situation, the electrostatic 428 sprayer was continuously applied by walking along the front of the table for 10 seconds (Front) or 429 along the side of the table for 5 seconds (Side). Fig. 10 shows the normalized deposition pattern 430 results of the dynamic test on a table. For the front moving direction case, the overall averaged 431 normalized concentration is approximately 0.475 for uncharged droplets, and 0.660 for charged 432 droplets. In terms of covered areas based on the criteria value of the normalized fluorescent tracer 433 concentration of 0.6, the percentage of covered areas for the front moving direction cases showed 434 a significant difference between uncharged droplets (11.5%) and charged droplets (50.9%). As 435 shown in the previous static tests on a table or cylinder, it continuously showed better performance 436 in the charged droplets, but a very interesting result came out in the side moving direction case of 437 the dynamic test experiment. The overall averaged normalized concentrations were measured as 438 0.384 for uncharged droplets, and 0.310 for charged droplets. In the side moving direction case, 439 the covered areas of uncharged (14.8%) and charged droplets (18.5%) did not show a significant 440 difference compared with the front moving direction case. These results might be due to the fact 441 that the deposition pattern for uncharged droplets was formed in an elongated elliptical shape along 442 the spraying direction, however, the deposition pattern for charged droplets was created in a 443 triangular shape which gradually narrowing the sprayed area from the sprayed place, which is 444 illustrated in Fig. 5 . In the overall dynamic test, it was confirmed throughout all the results that the 445 hotspot was biased to one side. This is probably because when the spray is first sprayed, it is not 446 sprayed consistently, and when the spray is activated, a large amount is produced at the beginning. 447

It is worthwhile to mention that since we quantitatively investigated these features, these 448 differences depending on the spraying method can be found. Here, from a macro perspective, 449 electrostatic sprayers are recommended to cover large areas with short spray distances. In addition, 450

it is expected that more detailed and precise user guidelines can be presented if additional 451

investigations are carried out. 

In the present study, we reported the quantitative analysis and its method of electrostatic 466 sprayer performance by using fluorescent tracer. For estimating the sprayer droplet size in the 467 aerosol state by using the fluorescence microscope, we generated monodisperse droplets by using 468 FMAG and obtained the ratio between the contact diameter and the aerosol state droplet diameter 469 of 0.407, with a corresponding contact angle of 20° between droplets and micro cover glasses. In 470 addition, we calibrated the fluorometer for quantitatively estimating the amount of deposited 471 fluorescent droplets. We also compared the normalized areas obtained by the fluorescence 472 microscope with the signal values measured by the fluorometer and as a result, the relationship 473 between the two data was linear. Therefore, we believe that the introduced fluorescent tracer 474 method in this study can be widely employed to quantitatively characterize the deposited droplets 475 even in complex conditions and geometries. In addition, the fluorescent tracer method might be 476 very useful in various applications such as analyzing semiconductor yield degradation due to 477 particle contamination. 478

For the static test on a table, we presented the deposition pattern by changing the sprayer 479 mode without or with an electrostatic charge and found out that the deposition pattern was formed 480 as an elongated elliptical shape along the spraying direction for the sprayer mode in absence of an 481 electrostatic charge, however, the triangular shaped-deposition pattern was created for the 482 electrostatic charge mode. In a fluorescence microscope investigation, we found that small droplets 483 were created when there is an electrostatic charge on the droplets due to Coulombic fission of the 484 parent droplet. The increment of effective contact area due to Coulombic fission was determined generating monodisperse fluorescent liquid droplets and calculated droplet diameter by using 747 eq. (1). 748 Table 2 . The relative number of charged droplets compared to the single volume of sprayed 749 droplets without an electrostatic charge. The peak droplet diameter and number distribution 750 ratio between the first and second peak droplet probability density were obtained from Fig. 7 . 751

The number correction factor was calculated from eq. (7). The increment of effective contact 752 area based on the same volume of droplets without an electrostatic charge ( i) was calculated 753 from eq. (8). 

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