Detachment of bacteria-laden droplet from dusty surface by centrifugal method

Abstract

Coughing and sneezing can generate thousands of infectious disease droplets. The small droplets suspend in air for long time while the large droplets settle on desk or floor surfaces quickly. Some droplets may deposit in ventilation duct. These surfaces may not be clean but with dusts on it. Ventilation duct was dusty after long period of operation. The present of the dust may affect the droplet detachment process and the survivability of the bacteria. The objective of this study is to investigate the detachment of pure droplet and bacteria-laden droplet from dusty surface. Clean and dusty surface of stainless steel were used as substrate in the experiment. Clean surface was cleaned by water and alcohol to remove dust and oil. Dusty surface was prepared by suspending Arizona Test Dust in a chamber and then deposited on the substrate by gravity. The dust loadings on the substrate was 0.5g/m2. Then the dust-loaded substrate was sterilized at 160℃ for 2 hours. The droplet was a mixture of glycerol and water to simulate the nucleus of saliva. Micrometer-sized droplets with rod-shaped bacteria, Escherichia coli (E.coli), were generated using a vibrating orifice aerosol generator. The droplet was deposit on the substrate and the contact diameter of the droplet was captured by a microscope. Tryptone soya agar was pour at the cap of a substrate holder. Detached bacteria-laden droplet was deposit on the agar. The E.coli on the agar and the dusty substrate were cultured for 12 hours and the colony forming unit were compared. After rotation in centrifuge, the pure droplet was partially detached. When the rotation speed was increased, the contact diameter of the remained portions became uniform independent of their initial contact diameter. The remained droplet was larger on dusty surface than on the clean surface. For the bacteria-laden droplet, the droplet was partially detached and E.coli can be found in both the detached portion and the remained portion. The partially detached bacteria-laden droplet can be a source of bioaerosol. The survivability of the bacteria after the detach process shall be studied.