Physico-chemical characteristics of evaporating respiratory fluid droplets

Abstract

The detailed physico-chemical characteristics of respiratory droplets in ambient air, where they are subject to evaporation, are poorly understood. Changes in the concentration and phase of major components in a droplet-salt (NaCl), protein (mucin) and surfactant (dipalmitoylphosphatidylcholine)-may affect the viability of any pathogens contained within it and thus may affect the efficiency of transmission of infectious disease by droplets and aerosols. The objective of this study is to investigate the effect of relative humidity (RH) on the physico-chemical characteristics of evaporating droplets of model respiratory fluids. We labelled these components in model respiratory fluids and observed evaporating droplets suspended on a superhydrophobic surface using optical and fluorescence microscopy. When exposed to continuously decreasing RH, droplets of different model respiratory fluids assumed different morphologies. Loss of water induced phase separation as well as indication of a decrease in pH. The presence of surfactant inhibited the rapid rehydration of the non-volatile components. An enveloped virus, ϕ6, that has been proposed as a surrogate for influenza virus appeared to be homogeneously distributed throughout the dried droplet. We hypothesize that the increasing acidity and salinity in evaporating respiratory droplets may affect the structure of the virus, although at low enough RH, crystallization of the droplet components may eliminate their harmful effects.

Keywords: aerosol transmission; crystallization; influenza; mucus; phase separation; relative humidity.

Figure 1.

Representative bright-field optical images of evaporating droplets containing different components exposed at constant RH of 60%. Scale bar represents 20 µm. (Online version in colour.)

Figure 2.

Evaporation rate of droplets at different RHs as a function of dimensionless droplet diameter (D/D_o). (a), (b) and (c) are plots obtained from droplets aerosolized from 3C, 4C and HS solutions, respectively. At each RH, the minimum D/D_o value corresponds to the equilibrium diameter (D_eq) calculated according to the Kohler equation or to the diameter of the solids in the case of RH lower than the efflorescence RH (ERH) of NaCl (approx. 44%). The legend for each RH is indicated in c. RH was held constant at the value indicated for that curve. Evaporation rates were obtained for droplets in contact with a superhydrophobic surface and without ventilation. Hence, results can only be interpreted qualitatively in applying them to airborne droplets under real environmental conditions.

Figure 3.

Optical confocal image of 3C droplets exposed initially at 95% RH, ramped down to 80% at 1%/min. The droplets became transparent at 80% RH. (Online version in colour.)

Figure 4.

Fluorescence images of droplets exposed to decreasing RH. Droplets were aerosolized from a solution containing NaCl and mucin in water. The red colour indicates mucin. Droplets were initially exposed at 100% RH, and then the RH was ramped down at 5%/min. Droplets exposed at near-100% RH appeared similar to those at 95%. Scale bar represents 20 µm.

Figure 5.

Fluorescence images of droplets exposed to decreasing RH. Droplets were aerosolized from a solution containing NaCl, mucin and DPPC in water. The red and green colours indicate mucin and DPPC, respectively. Droplets were initially exposed at 100% RH, and then the RH was ramped down at 5%/min. Droplets exposed at near-100% RH appeared similar to those at 95%. Scale bar represents 20 µm.

Figure 6.

Composite fluorescent image of a 4C droplet containing ϕ6 virus exposed to 29% RH. The bright green dots approximately 1 µm in size may indicate the location of the virus. All scale bars are 20 µm.