key: cord-0198048-mf10j5si authors: Kumar, S. Santosh; Shao, Siyao; Li, Jiaqi; He, Zilong; Hong, Jiarong title: Droplet evaporation residue indicating SARS-COV-2 survivability on surfaces date: 2020-05-25 journal: nan DOI: nan sha: 23f9fe250b471c5053fc97ee80f6051b7e3c209a doc_id: 198048 cord_uid: mf10j5si SARS-CoV-2 survives and remains viable on surfaces for several days under different environments as reported in recent studies. However, it is unclear how the viruses survive for such a long time and why their survivability varies across different surfaces. To address these questions, we conduct systematic experiments investigating the evaporation of droplets produced by a nebulizer and human-exhaled gas on surfaces. We found that these droplets do not disappear with evaporation, but instead shrink to a size of a few micrometers (referred to as residues), persist for more than 24 hours, and are highly durable against changes of environmental conditions. The characteristics of these residues change significantly across surface types. Specifically, surfaces with high thermal conductivity like copper do not leave any resolvable residues, while stainless steel, plastic, and glass surfaces form residues from a varying fraction of all deposited droplets at 40% relative humidity. Lowering humidity level suppresses the formation of residues while increasing humidity level enhances it. Our results suggest that these microscale residues can potentially insulate the virus against environmental changes, allowing them to survive inhospitable environments and remain infectious for prolonged durations after deposition. Our findings can also be extended to other viruses transmitted through respiratory droplets (e.g., SARS-CoV, flu viruses, etc.), and can thus lead to practical guidelines for disinfecting surfaces and other prevention measures (e.g., humidity control) for limiting viral transmission. The ongoing COVID-19 pandemic has infected more than five million people as of writing, causing major disruption to the global economy and social order. It has been well accepted that the virus causing the disease, severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), can be transmitted through contact of virus-laden respiratory droplets on surfaces WHO 2020) . Particularly, studies have found much higher concentration of SARS-CoV-2 RNA deposited on surfaces in hospitals rather than as aerosols (Liu et al. 2020; Guo et al. 2020) , pointing to the importance of investigating the virus survivability on surfaces. As reported by two recent experiments (Chin et al. 2020; van Doremalen et al. 2020) , SARS-CoV-2 has a long survival time on different surfaces and can remain viable under different temperature and humidity levels. Specifically, Chin et al. (2020) investigated the stability of SARS-CoV-2 deposited as droplets on ten surfaces at 60% relative humidity (RH) with variation in temperatures, and found the virus to be more stable on smooth surfaces (e.g. glass and plastic), remaining viable for up to two and four days, respectively with survival time decreasing at higher temperatures. van Doremalen et al. (2020) measured virus survival time on four surfaces, at 40% RH, to vary from approximately seven hours on copper to more than three days on plastic (polypropylene). However, no study so far has provided any physical mechanisms that can explain the long survival times, the large variation between the different surface materials tested, as well as the impact of environmental changes on surface transmission. Such mechanism, related to droplet evaporation process, can be critical for understanding the carriage and transmission of SARS-CoV-2 as summarized in a recent review paper (Gralton et al. 2011 ). Here we hypothesize the evaporation characteristics of respiratory droplets may indicate SARS-CoV-2 survivability on different surfaces and under different humidity and temperature conditions. In the literature, the studies of droplet evaporation on surfaces typically involve seeded particles and focus on particle pattern formation for various applications such as inkjet/3D printing, manufacturing self-assembled structures, etc. (Partsa et al. 2018) . Only one study investigated the evaporation of ultrapure water droplets on hydrophobic substrates that generates submicron residues (He & Darhuber 2019) . There is no systematic study of such water droplet evaporation on different surfaces of interest, nor works that make connection between virus transmission with droplet evaporation. In this study, we conduct a systematic experiment to assess the evaporation process of water droplets on surfaces with deposited size ranging from 5 to 100 μm, within the range of respiratory droplets generated by human breathing and speaking (Tang 2009 ). The test surfaces are selected to match those used in Chin et al. (2020) and van Doremalen et al. (2020) . A detailed description of the experiment is provided in the Materials and Method Section at the end. Figure 1 . Example first and last frames showing the formation of (a) single residue and (b) multiple residues from single droplet evaporation on a coated glass substrate. (c) Schematic illustration of the evaporation curve which demonstrates how the droplet size changes over time. D p (0) represents the initial droplet diameter measured at the start of evaporation. T E represents the evaporation time at which the droplet shrinks to residue size D R . (d) The normalized evaporation curve calculated by averaging 100 individual droplets evaporating on the coated glass surface at a temperature of 22 ºC and humidity of 40% RH. The size is normalized by D p (0) and the time by T E . The measured time varying size from the images are used as sample points to generate a continuous evaporation curve at discrete time steps through piecewise Hermite polynomial interpolation. The standard deviation indicating the differences between the sampled droplets is presented as the shading around each data point. We found that during evaporation, droplets on the tested surfaces shrink in size and height to form a thin liquid film, leaving behind a single residue on the order of micrometers (Figure 1a and Video S1). Sometimes, the film can break up into multiple residues (Figure 1b and Video S2). Those residues then could persist for hours with no visible change in their sizes. We observed such residues appear in different forms ( Figure S1 and Video S3-S6) on all surfaces except copper at 40% RH. On the copper surface, only a faint signature of a residue can be appreciated, suggesting a film with a thickness below our resolution limit (~300 nm), much smaller than those for other surfaces. To quantify the droplet evaporation process, we measure the diameter (Dp) as a function of time (t) for the different surfaces ( Figure 1c ). We define Dp as the area equivalent diameter of the droplet to enable comparisons between non-spherical and spherical shapes observed. The initial droplet size Dp(0) is measured at the start of evaporation when the droplet begins to change in size or height. The evaporation time TE is defined as the time at which the droplet shrinks to residue size DR, i.e., Dp(TE)=DR. In cases where the droplet disappears completely, Dp(TE)=0. In cases with multiple residues, we measure DR for the individual residues separately, and define the size as the root sum of squares of the DR for all residues. To characterize the general evaporation trend of droplets of different sizes, the evaporation curves are normalized using the Dp(0) and TE corresponding to each droplet. The initial droplet diameter Dp(0) and evaporation time TE yield approximately a linear relationship under our experimental conditions for all surfaces except copper for which TE shows little dependence on Dp(0) ( Figure S2 ). For coated glass surface ( Figure 1d ), the evaporation curve exhibits an initial slow rate of change in size over a duration of ~0.8TF followed by a rapid descent to form the final residue, of about 18% of Dp(0). Compared with coated glass surface (Figure 1d ), the evaporation curves for the other surfaces show a similar trend in general ( Figure 2 ). However, the evaporation rate and residue size vary among different surfaces, depending on the surface properties including wettability, roughness and thermal conductivity. Specifically, coated glass that has strong hydrophobicity and smoothness presents the highest initial evaporation rate. The metal surfaces (i.e., copper, and stainless steel) with higher thermal conductivity exhibit steeper change in size near the end of evaporation, compared to plastic and both glass surfaces with low thermal conductivity. The copper substrate does not yield any resolvable residue at 40% RH, while the residues for the other surfaces fall within in the range of 9-22% of Dp(0). The rougher surfaces like plastic and stainless steel show larger variation in residue size compared to the smoother glass surfaces. The resolvable residues exhibit a stability in number and size for a period of 24 hours as shown in Figure 3 . Specifically, the percentage of residues that remain, referred to as residue fraction, decays gradually with time for all surfaces except for stainless steel which displays a sharp decline at the beginning, reaching a plateau at ~15% potentially due to the relatively higher thermal conductivity and a larger contact area associated with surface roughness. The uncoated glass retains the highest residue fraction (~95%), while the coated glass and plastic both yield a lower fraction of ~80% after 24 hours. The drop in residue fraction can be attributed to the evaporation of smaller residues present on these surfaces as indicated by the larger variability in residue size seen in Figure 2 . The average residue size (Figure 3b ) for all surfaces show a relative larger decrease within the first few hours, followed by an almost linear decay with a very shallow slope (-0.01 to -0.03 μm/hour) at longer durations, indicating their survival time could extend well beyond 24 hours. Average area equivalent diameter D R of residues sampled over the same duration with shaded region representing the standard error, and the dashed lines indicate linear least square fit conducted over a range of t near the end of each data set where a linear trend can be clearly observed, from above ~5 hours for coated glass to data above ~8 hours for the remaining. Once formed, these residues show strong durability even under fluctuations of ambient temperature and humidity. They can stay on plastic and glass surfaces even after the surfaces are treated with a heat gun for 60 s at a temperature of ~60 °C (measured at the surface), while the same treatment removes more than ~90% of residues on stainless steel, possibly due to its higher thermal conductivity. In comparison, we found that wiping is more effective for residue removal across all surfaces (applying Kimtech wipes for 10 s can remove >95% of the residues). We found that the residue formation process is strongly influenced by the ambient humidity. As the humidity increases from 40% RH to 60% RH, the fraction of droplets that form residues increases by ~5% on the coated glass surface, ~15% on the plastic surface with no significant change observed on the plastic and stainless steel surfaces. More importantly, at 60% RH we also observe the formation of residues on the copper surface, although for a much lower fraction of droplets. In contrast, as the humidity is reduced, the fraction of droplets that form residues decreases on all surfaces, with a maximum of 10% on coated glass at ~20% RH, with other surfaces indicating much smaller values. With a further drop in humidity to ~10% RH, none of the surfaces can form residues. The size of residues also indicates a strong dependence on the initial droplet size at each humidity investigated. For the coated glass substrate (Figure 4) , the residue size scales linearly with the initial droplet size at both humidity values with very similar slopes. Specifically, the minimum droplet size that can form a residue decreases with humidity, from ~25 μm at 40% RH to ~5 μm at 60% RH. We observe similar trends between the two humidity values for the other surfaces ( Figure S3) . Interestingly, the steel surface at 60% RH shows two specific clusters corresponding to a larger and smaller residue types, each scaling differently with initial droplet size. In addition, all surfaces show a lower scatter in residue size at the higher humidity, possibly due to a reduction in formation of multiple residues, since surface tension makes it less likely for thicker films to breakup into pieces. The formation of the microscale residues from pure water evaporation has been reported in He & Darhuber (2019) . The authors suggest that this phenomenon is caused by deliquescence by ionic compounds in the photoresist substrate, in the presence of humidity. However, such a mechanism cannot explain the observations from the current experiment using clean substrates without similar ionic compounds. Instead, the phenomena observed in our experiments can be generally attributed to an equilibrium state achieved during droplet evaporation, in which a delicate balance among substrate surface energy, gas-liquid interfacial energy, the internal energy of the evaporating droplet and its surrounding air is established. As the droplet evaporates, it increases the local humidity, which in turn decreases the evaporation rate and increases the probability for vapor to condense back onto the droplet. A residue is formed when the evaporation and condensation rates become equal before the droplet dries out completely. Based on this mechanism, larger droplets with a longer evaporation time have a higher probability of forming residues at any humidity level. As we increase humidity of the environment, the rate of condensation goes up proportionally, thereby improving the odds of smaller droplets to form residues before evaporation dries them out, which is consistent with our observation in the previous section. Likewise, a decrease in humidity will accelerate the evaporation process leading to a droplet dry out before a residue can be formed, except for the large droplets that manage to survive for a longer duration. However, differences in the substrate properties (e.g., thermal conductivity or wettability) can influence the rate of evaporation and condensation, thus leading to the observed variability in residues across the tested surfaces e.g., copper forming residues only from large droplets even at the higher humidity level. We repeated our experiments with droplets condensed from human breath instead of a nebulizer. The results also show the formation of similar stable residues persisting for a long term, with the same qualitative trends across the different surfaces. Such results point to the strong relevance of our experiment to disease transmission through respiratory exhalation, although they are not presented here in a quantitative fashion, considering the large variability of the chemical contents in human breath. Overall, our findings provide a physical mechanism contributing to the long survival time and stability of viruses under practical settings. Specifically, we suggest that the residues with size 1-2 orders larger than that of SARS-CoV-2 found in our experiments can serve as a shield, insulating the virus against extreme environmental changes (Tang 2009 ). Accordingly, the probability of forming residues and their stability can indicate the virus survivability on different surfaces. For instance, the residues are found to be most difficult to form on copper, which shows the shortest survival time of SARS-CoV-2 in van Doremalen et al. (2020) . Compared with plastic, the stainless steel has lower probability of sustaining the formed residue for long term at 40% RH, mirroring the survivability results for plastic and stainless steel reported in van Doremalen et al. (2020) . The physical insights gained from our work can be extended to other viruses that are transmitted through respiratory droplets (e.g., SARS/MERS viruses, flu viruses, etc.), particularly, to SARS-CoV-1 which has a survivability trend very similar to those of SARS-CoV-2 on different surfaces (van Doremalen et al. 2020) . Our findings suggest that high temperature (through enhancing evaporation rate) and low humidity can inhibit the formation of residues, lowering the survivability of viruses on surfaces. For temperature effect, such inference is consistent with reduced survivability of virus with increasing temperature reported in multiple studies (Casanova et al. 2010; Chan et al. 2011; Chin et al. 2020) . However, despite a number of studies investigating the humidity effect on virus survivability on surfaces (Casanova et al. 2010; Chan et al. 2011) , their experiments were conducted using virus-laden droplets of ~mm size, which forms residues at all humidity conditions tested according to our study. Therefore, the probability of residue formation cannot be used to explain the variation of virus survivability with humidity in their studies, which are likely caused by other mechanisms. The adverse effect of humidity on virus infectivity reported in the literature (Shaman & Kohn 2009; Lowen et al. 2007 ) points largely to airborne transmission, which can be explained by increased aerosol settling at higher humidity through condensation, and is not relevant to the mechanism discussed in our study. Our findings have direct practical implications on prevention measures for reducing the risks of virus infection. Specifically, our results indicate that ventilation with hot air can be effective to disinfect metal surfaces, but its effectiveness reduces drastically for surfaces with low thermal conductivity (e.g., plastic and glass) which can only be treated with wiping. Our tests show that vigorous wiping with regular water-absorbent tissue paper can remove more than 95% of the residues on surfaces if disinfecting wipes are not available. Particularly, our results derived from the experiments using droplets with size matching those generated during human breathing and speaking has specific implications for COVID-19, which displays an exceedingly high rate of spread than earlier viruses, associated with high viral loads in the upper respiratory tract and potential transmission by asymptomatic/presymptomatic individuals (Bai et al. 2020; Furukawa et al. 2020; Gandhi et al. 2020 ). Our results suggest that even tiny droplets (<20 µm) can leave residues under moderately high humidity (>40%), causing significant spread of virus through surface contamination. Therefore, our study highlights the importance in wearing masks under such conditions towards minimizing the spread of virus through normal respiratory activities e.g., breathing and speaking (Leung et al. 2020 ). In addition, lowering the indoor humidity when possible can substantially suppress the formation of such residues (e.g., below 10% RH for all surfaces), and limit the spread of viral infection through contact from such small respiratory droplets, as we continue to reopen our economy and workplaces in the future. In the end, we would also like to caution the readers from generalizing the quantitative results (e.g., evaporation rate, residue fraction, etc.) present in our experiments, since they are dependent on specific surface and environmental conditions. Accordingly, it would be of practical significance to investigate the evaporation residues over a broader range of surface substrates and under different environmental factors (e.g., humidity, temperature, etc.), which can lead to actionable prevention measures to reduce the virus transmission through contaminated surfaces. Our work will surely inspire a host of future research using more advanced diagnostic, analytical and simulation tools to elucidate the formation and characteristics of residues and their connection with virus transmission. The water droplets are generated using distilled water with TSI 9302 nebulizer operated at an input pressure of 138 kPa which produces a 5.7 L/min output rate of droplets (mean diameter ~6.4 μm) which coagulate on the surface to produce a wide range of droplet sizes. Five different surface samples, including Fisher Scientific microscope glass slide, glass slide coated with RainX hydrophobic coating, plastic (3M polypropylene tape), copper (Hillman copper sheet) and 304 stainless steel samples, are selected for testing under an ambient temperature of 22 °C and humidity varying between 10% to 60% RH. The samples are placed with the test side facing up on an inverted microscope, connected with a Flare CMOS camera (2048 pixel × 1024 pixel sensor size) sampling at 30 frames/second. We used the nebulizer to generate droplets on the substrate (deposited size range 5 to 100 μm) and imaged them simultaneously under 10x magnification (1.21 mm × 0.64 mm field of view at 0.59 µm/pixel resolution) to capture the evaporation of liquid droplets and formation of the residues. The size of evaporating droplets at each time step and the corresponding residues are extracted from the 10x microscopic images manually using ImageJ, where the size is defined as the area-equivalent diameter. We conduct residue removability tests for each substrate through heating as well as wiping. For the former, we treat each surface with a heat gun (temperature of 60 °C at the surface) for 60 seconds and observe, both qualitatively and quantitatively, the change in the residue concentration. As for the latter, we wipe the surfaces with a Kimtech wipe for approximately 10 seconds, with minimal pressure. Finally, we test the longterm stability and durability of the residues on all surfaces (except copper) by capturing images at 10x magnification for 24 hours, at 1 hour increments, in an environment with relatively stable temperature (22 °C) and humidity (40% RH). Presumed asymptomatic carrier transmission of COVID-19 Effects of air temperature and relative humidity on coronavirus survival on surfaces The effects of temperature and relative humidity on the viability of the SARS coronavirus & Poon, L. L. Stability of SARS-CoV-2 in different environmental conditions Evidence Supporting Transmission of Severe Acute Respiratory Syndrome Coronavirus 2 While Presymptomatic or Asymptomatic Asymptomatic transmission, the Achilles' heel of current strategies to control COVID-19 The role of particle size in aerosolised pathogen transmission: a review Aerosol and surface distribution of severe acute respiratory syndrome coronavirus 2 in hospital wards Evaporation of water droplets on photoresist surfaces -An experimental study of contact line pinning and evaporation residues Respiratory virus shedding in exhaled breath and efficacy of face masks Early transmission dynamics in Wuhan, China, of novel coronavirus-infected pneumonia Aerodynamic analysis of SARS-CoV-2 in two Wuhan hospitals Influenza virus transmission is dependent on relative humidity and temperature Mechanisms of pattern formation from dried sessile drops Absolute humidity modulates influenza survival, transmission, and seasonality The effect of environmental parameters on the survival of airborne infectious agents Aerosol and surface stability of SARS-CoV-2 as compared 9 with SARS-CoV-1 Geneva: World Health Organization SARS-CoV-2 viral load in upper respiratory specimens of infected patients We acknowledge the support from the University of Minnesota for this research. We would also like to thank Dr. David Pui for the equipment support, Dr. Suo Yang and Dr. Lei Feng for fruitful discussion of the results and Barbara Heitkamp for help editing the manuscript.