Understanding the transmission phenomena of SARS-CoV-2 by virus-laden droplets and aerosols is of paramount importance for controlling the current COVID-19 pandemic. Detailed information about the lifetime and kinematics of airborne droplets of different size is relevant in order to evaluate hygiene measures like wearing masks but also social distancing and ventilation concepts for indoor environments. However, the evaporation process of expiratory droplets and aerosols is not fully understood. Consequently, the main objective of this study is to present evaporation characteristics of saliva droplets. An acoustic levitator is utilized in conjunction with microscopic imaging for recording the temporal evolution of the evaporation of saliva droplets under well-defined ambient conditions. Following the evaporation of the water content, a saliva droplet reaches a final size, which remains stable in the timescale of hours. By investigating numerous droplets of different size, it was found that the final droplet diameter correlates well to 20% of the initial diameter. This correlation is independent of the ambient conditions for a temperature range from 20 °C to 29 °C and a relative humidity from 6% to up to 65%. The experimentally obtained evaporation characteristics are implemented into a numerical model, which is based on one-dimensional droplet kinematics and a rapid mixing evaporation model. By taking into account the evaporation-falling curve as presented by Wells, the significance of the experimental results for predicting the lifetime of saliva droplets and aerosols is demonstrated. The numerical predictions may be used to determine the impact of the droplet size and the ambient conditions on the transmission risks of infectious diseases like COVID-19.

中文翻译：

---

深入了解唾液液滴和气溶胶的蒸发特性：悬浮实验和数值模拟

了解 SARS-CoV-2 通过载有病毒的飞沫和气溶胶的传播现象对于控制当前的 COVID-19 大流行至关重要。有关不同大小的空气中飞沫的寿命和运动学的详细信息与评估卫生措施（如戴口罩）以及室内环境的社交距离和通风概念有关。然而，呼出的飞沫和气溶胶的蒸发过程尚不完全清楚。因此，本研究的主要目的是展示唾液液滴的蒸发特性。声学悬浮器与显微成像结合使用，用于记录在明确定义的环境条件下唾液液滴蒸发的时间演变。随着水分蒸发，唾液滴达到最终大小，在数小时的时间尺度内保持稳定。通过研究大量不同尺寸的液滴，发现最终液滴直径与初始直径的 20% 密切相关。对于 20 °C 至 29 °C 的温度范围和 6% 至高达 65% 的相对湿度，这种相关性与环境条件无关。将实验获得的蒸发特性实现为基于一维液滴运动学和快速混合蒸发模型的数值模型。通过考虑 Wells 提出的蒸发下降曲线，证明了实验结果对预测唾液液滴和气溶胶寿命的重要性。