Figure 1. Schematic illustration of the different transmission routes of respiratory tract viruses via large (≥ 100 μm), intermediate (~10-100 μm), and small (< 5 μm) droplets.

Figure 2. Structure of a coronavirus particle.

Figure 3. Schematic illustration of a virus-laden respiratory droplet after being released by an infected person (A) and after the loss of some water content through evaporation (B). The water part of the droplet is colored light blue in A while blue in B. Yellow “bubbles” of various sizes indicate the nonaqueous components such as salt, proteins, and surfactants. The virus particles are drawn as core-shell structures with the core colored in red.

Figure 4. Schematic illustration of the evaporation of a respiratory droplet in a turbulent airflow.

Figure 5. Schematic illustration of the evaporation of a droplet on the surface of a porous material.

Figure 6. Fluorescence images of model droplets exposed to decreasing relative humidity. A droplet contains water, NaCl, mucin (red), and DDPC (green). DDPC: 1,2-dihexadecanoyl-sn-glycero-3-phosphocholine. Reprinted with permission from Vejerano and Marr.50 Copyright (2018) The Royal Society.

Figure 7. Schematic illustration of a respiratory droplet with uniformly distributed virus particles (A) and aggregated clusters of virus particles (B). The virus particles are drawn as core-shell structures with the core colored in red.

Figure 8. The distribution of virus particles in a droplet on a surface is conjectured to be affected by the evaporation dynamics and internal flow. The virus particles can possibly aggregate at the edge of the deposited droplet (A) or the droplet-air interface (B). The virus particles are drawn as core-shell structures with the core colored in red.

Figure 9. Schematic illustration of the trajectory (black dashed lines) of an individual virus particle in a respiratory droplet (A) and a mucus layer (green chains) (B). The virus particles are drawn as core-shell structures with the core colored in red.