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Abstract：
COVID-19 spreads through patients’ exhalation droplets， hence a comprehensive understanding of muco-saliva fragmentation provides crucial insight. This paper scrutinizes a study of the breakup of mimic of contagious viscoelastic exhalation fluid， conducted by MIT researchers： “Visualization of sneeze ejecta： steps of fluid fragmentation leading to respiratory droplets”， in which the authors profoundly contributed to the understanding of hazardous sneeze ejecta caused by sneezing. In this article， some of their nicely incorporated experimental methods and break through will be described and analyzed with the integration of fluid dynamics.
1.Introduction
1.1 What the research is about
MIT researchers’ observation indicates disintegration of muco-saliva occurs not only in trachea， but outside human airway after ejection， which explains the high infectivity of airborne influenza strains like influenza and SARS （1）. However， uncertainties on the range of droplet distribution determined by droplet size and the detail of fluid fragmentation persist within the field （1）. They have visualized the four stages of the mucus decomposition through high-speed videography. The researchers also measured physical quantities like relaxation time， Reynold number， and Deborah number.
1.2 Why this research is important
This work over ejecta disintegration is concerned with the production of droplets of muco-salivary liquids and the viscoelasticity of this material controls the fragmentation of this liquid as it is ejected by sneezing and coughing events. Thus， providing insights into the formation of the droplets is the reason why this research is contributive.
To better understand the innovation of this research， it is helpful to consider previous work and how it could be expanded. First of all， the fluid dynamicsof violent expirations remains poorly understood relative to industrial flows.Second， coughs and sneezes， neurological reflex actions triggered by irritation of the nose or trachea， are two typical ways of spreading contagious droplets （2）. Albeit the mechanism of sneezing is analogous to that of coughing， little is known about the dynamics after pharynx constriction in a sneeze （3，4）.
Hence this research is innovative for these aspects. First， the MIT researchers unveiled the fluid dynamics of sneeze and the features of each stages. Second， through the calculation of the relaxation time， important dimensionless numbers relevant to the problem of sneeze ejecta， the Deborah number and the Reynolds number were estimated， along with many other fluid dynamics features. The researchers discovered the crucial role viscoelasticity plays in mucus fragmentation. Moreover， B. E. Scharfman et al.， the conductors of this research focused on sneezing， an uncommon subject， and compared the dynamics with that of coughing （5，6，7）， which provides tremendous and insight into the formation of sneeze-induced droplets. In the work of B. E. Scharfman et al.， the total duration of the sneeze was 134.5 ms with an estimated Reynolds number for the gas cloud of ReG = 105， indicating that the sneeze ejection is drastic and fully turbulent.
By juxtaposing the droplets’ propagation of cough and sneeze， as shown in Fig 2 and 3， the difference between a sneeze and a cough is evident. While sneeze ejecta is still relatively condensed and pendant in the air after 0.34 seconds 70 centimeters away， droplets of cough ejecta can barely be detected by bare eye after 0.15 seconds.
As shown in Fig4， when the mucus first burst out of human airway， it is flattened into sheet; then as the muco-salivary liquid stretches further， the bag structure bursts and mucus on the ligament configurates like beads-on-string due to the spontaneous Rayleigh-Plateau instability. Finally， beads merge and droplets form.
姓名：宋若水
城市：北京市
年級：12年级
目标专业：生物医学工程专业
在撰写本篇研究报告前，我阅读了有MIT研究员B. E. Scharfman等人的研究论文《Visualization of sneeze ejecta： steps of fluid fragmentation leading to respiratory droplets》。从中我了解了流体动力学及软质材料的相关内容，如雷诺数、松弛时间、Deborah数等无纲常量的物理意义、定义式及计算式。此外我了解了如何从文章摘要中抓取一篇论文的重要信息及如何撰写评论性论 文。
COVID-19 spreads through patients’ exhalation droplets， hence a comprehensive understanding of muco-saliva fragmentation provides crucial insight. This paper scrutinizes a study of the breakup of mimic of contagious viscoelastic exhalation fluid， conducted by MIT researchers： “Visualization of sneeze ejecta： steps of fluid fragmentation leading to respiratory droplets”， in which the authors profoundly contributed to the understanding of hazardous sneeze ejecta caused by sneezing. In this article， some of their nicely incorporated experimental methods and break through will be described and analyzed with the integration of fluid dynamics.
1.Introduction
1.1 What the research is about
MIT researchers’ observation indicates disintegration of muco-saliva occurs not only in trachea， but outside human airway after ejection， which explains the high infectivity of airborne influenza strains like influenza and SARS （1）. However， uncertainties on the range of droplet distribution determined by droplet size and the detail of fluid fragmentation persist within the field （1）. They have visualized the four stages of the mucus decomposition through high-speed videography. The researchers also measured physical quantities like relaxation time， Reynold number， and Deborah number.
1.2 Why this research is important
This work over ejecta disintegration is concerned with the production of droplets of muco-salivary liquids and the viscoelasticity of this material controls the fragmentation of this liquid as it is ejected by sneezing and coughing events. Thus， providing insights into the formation of the droplets is the reason why this research is contributive.
To better understand the innovation of this research， it is helpful to consider previous work and how it could be expanded. First of all， the fluid dynamicsof violent expirations remains poorly understood relative to industrial flows.Second， coughs and sneezes， neurological reflex actions triggered by irritation of the nose or trachea， are two typical ways of spreading contagious droplets （2）. Albeit the mechanism of sneezing is analogous to that of coughing， little is known about the dynamics after pharynx constriction in a sneeze （3，4）.
Hence this research is innovative for these aspects. First， the MIT researchers unveiled the fluid dynamics of sneeze and the features of each stages. Second， through the calculation of the relaxation time， important dimensionless numbers relevant to the problem of sneeze ejecta， the Deborah number and the Reynolds number were estimated， along with many other fluid dynamics features. The researchers discovered the crucial role viscoelasticity plays in mucus fragmentation. Moreover， B. E. Scharfman et al.， the conductors of this research focused on sneezing， an uncommon subject， and compared the dynamics with that of coughing （5，6，7）， which provides tremendous and insight into the formation of sneeze-induced droplets. In the work of B. E. Scharfman et al.， the total duration of the sneeze was 134.5 ms with an estimated Reynolds number for the gas cloud of ReG = 105， indicating that the sneeze ejection is drastic and fully turbulent.
By juxtaposing the droplets’ propagation of cough and sneeze， as shown in Fig 2 and 3， the difference between a sneeze and a cough is evident. While sneeze ejecta is still relatively condensed and pendant in the air after 0.34 seconds 70 centimeters away， droplets of cough ejecta can barely be detected by bare eye after 0.15 seconds.
As shown in Fig4， when the mucus first burst out of human airway， it is flattened into sheet; then as the muco-salivary liquid stretches further， the bag structure bursts and mucus on the ligament configurates like beads-on-string due to the spontaneous Rayleigh-Plateau instability. Finally， beads merge and droplets form.
姓名：宋若水
城市：北京市
年級：12年级
目标专业：生物医学工程专业
在撰写本篇研究报告前，我阅读了有MIT研究员B. E. Scharfman等人的研究论文《Visualization of sneeze ejecta： steps of fluid fragmentation leading to respiratory droplets》。从中我了解了流体动力学及软质材料的相关内容，如雷诺数、松弛时间、Deborah数等无纲常量的物理意义、定义式及计算式。此外我了解了如何从文章摘要中抓取一篇论文的重要信息及如何撰写评论性论 文。