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Facemasks hinder the spreading of particles

Published: 15 December 2021

How do facemask hinder the spreading of liquid particles when we breathe and talk with and without facemasks? The corona pandemic has made the question of how well mouth protection prevents the spread of infection highly topical. Now, new research will be presented to the Public Health Agency of Sweden and the Swedish Research Council under the leadership of researchers from Luleå University of Technology.

"Facemasks hinder the spreading of of liquid droplets​. Our experiments show that the large particles are well captured by facemasks, while fewer numbers of smaller droplets leak out on the sides of facemasks. Ventilation design is therefore of highest importance​ in public environments", Staffan Lundström says, Professor of Fluid Mechanics at Luleå University of Technology and project leader.

The research started at the beginning of the covid-19 outbreak in Sweden

Since the beginning of 2020, this issue has been in focus for a group of researchers in the field of Fluid Mechanics in Sweden. Under the leadership of Luleå University of Technology in collaboration with KTH, the Royal Institute of Technology, Chalmers University of Technology and Lund University of Technology, the effectiveness of facemasks has been studied from various aspects.

Effects of face masks during airborne pandemic based on fluid mechanical aspects

The focus has been how well particles in our exhaled air, are captured by the type facemasks we use, in for example public transport and public environments in order to hinder the spreading of covid-19. By compiling what is known from previous studies and doing new experiments and simulations, the researchers have aimed to improve knowledge about facemasks and the spread of exhaled particles.

The research is based on close collaboration between the research groups in the field. Some results are scientifically published, others are not yet published.

Brief description of research results presented to the Public Health Agency of Sweden and the Swedish Research Council.

Luleå University of Technology (Staffan Lundström, professor of fluid mechanics, Mikael Sjödahl, professor of experimental mechanics). Model experiments have been carried out to quantify the number of particles transmitted with the flow, with and without face masks. Each mask is tested with and without side leakage. The tested masks include both homemade fabrics and masks that you can purchase at a pharmacy. The results show that the filtration efficiency is, in general, good for the tested masks in the fully sealed case. Unsurprisingly, the masks from the pharmacy performed better than the homemade fabrics. In the presence of leakage, the larger particles are removed from the flow due to inertia. However, as particle size decreases, the filtration efficiency rapidly decreases.

KTH, Royal Institute of Technology (Ramis Örlü, Associate Professor of Fluid Mechanics) We investigate experimentally the fluid dynamics of outward protection from face masks by analysing qualitatively and quantitatively the leakage and throughflow jets at the interface of mask and face. The investigation is performed by high-speed imaging and Schlieren shadowgraphy under pulsed conditions aiming at simulating speaking and sneezing conditions. The test liquids are water and artificial saliva. The results: Surgical masks are found to be excellent for frontal filtration in agreement with previous studies. Cotton based masks should be discouraged. Strong leakage and through-flow jets are escaping at the interface of face and mask at the top/nose and side/cheeks. Saturated masks had negligible effect on performance. Further studies are needed to assess whether masks should be (re)used for ecological/economical/environmental reasons.

"Our experiments with artificial saliva under pulsed conditions simulating speech and sneezing conditions show that face masks are – as known – excellent for frontal filtration of flow and particles. However, the leakage flow makes the usage a complex ventilation problem when considering masks in a societal context", Ramis Örlü says.

Chalmers University of Technology, (Srdjan Sasic, Professor of Fluid Mechanics) The mechanisms for filtering liquid droplets (10-50 micrometers) in fibrous microstructures of face masks have been investigated using the so-called LBM method. Dynamics, collection and coalescence of droplets of sizes comparable to the fiber and pore sizes relevant to mask materials are studied during a range of respiratory events (breathing, coughing). A non-Newtonian behavior of saliva is also taken into account. The results: A novel model is formulated for droplet penetration length and permeability in face mask microstructures, given the fiber size and porosity. Based on this, face masks can be developed so that they filter even better and become easier to breathe through.

"Our modelling and simulations provide design guidelines to producers of face masks on how to make a balance between having a high degree of mask efficiency and comfort for users (breathability)", Srdjan Sasic says.

Lund University, Faculty of Engineering LTH (Xue-Song Bai, professor of fluid mechanics). Advanced numerical simulations have been performed to study the spread of large droplets and aerosol flow using so-called large eddy simulations. The turbulent flow is described using Navier-Stokes equations and the droplet motion is simulated using a particle tracking equation.The droplets break up and evaporate during the spreading process and they are affected by gravity. The results: A face mask model for numerical simulation of transport of droplets/aerosol particles through face masks has been developed based on the experiments performed. The model has been used to predict the transport of droplets/aerosol particles in different environments. The simulations show that without face masks, the social distancing of 1 m is not safe, while a distance of 1.5 m is more justified. Face masks can not only filter out a majority of the droplets but they can significantly reduce the safe social distancing. The leak through the slit between the mask and the face is the main source of droplet discharge when coughing with face masks.

Transport of liquid droplets and aerosol particles in an elevator is studied under different air ventilation conditions. It turns out that the transport of small droplets and aerosol particles is significantly affected by the ventilation. It is noted that large droplets tend to fall to the ground within 1.5 m whereas small droplets and aerosol particles can spread throughout the elevator depending on the ventilation conditions.

"There is no doubt that face masks can significantly reduce the transmission of SARS-CoV-2 vrius. Our CFD simulations indicate that the safe social distance can be signficantly reduced to 1/3 of that without a face mask", Xue-Song Bai says.