Characteristics of Respiratory Microdroplet Nuclei on Common Substrates*

To evaluate the role of surface in the transmission of respiratory virus, in particularly SARS-CoV-2, microdroplets of artificial saliva (approximate diameter 10 μm) were generated using an advanced inkjet printing technology to replicate the aerosol droplets, and were subsequently deposited to five different types of substrates (glass, stainless steel, polytetrafluoroethylene (PTFE), stainless steel, acrylonitrile butadiene styrene (ABS), and melamine) that are commonly in contact with human. The droplets were found to evaporate within a short timeframe (less than 3 seconds), which is consistent with previous reports. Both fluorescence microscopy and atomic force microscopy were used to characterise the droplet nuclei formed. We found that droplets of 10 μm diameter (or less) would form a solid nuclei on a surface, independent of the surface characteristics, which clarifies the antiviral surface strategies towards small droplets. It is worth noting that the interfacial energy has a significant influence on the characteristics of the resulting nuclei: droplets of artificial saliva would spread on substrates of high surface free energy (SFE) before forming a nuclei, but result in a nuclei of minimal dimension on substrates of low SFE. Nanomechanical measurements confirm that the nuclei possess similar surface adhesion (~20 nN) and Young's modulus (~4 MPa), supporting a core-shell structure of the droplet nuclei, which could have a critical impact on the surface viability of viruses. We would like to highlight that the interfacial details of proteinaceous droplet is much more complex than that of pure solvent, which is affected by the characteristics of the solid substrates.