"Face masks hinder the spreading of liquid droplets. Our experiments show that the large particles are well captured by face masks, while fewer numbers of smaller droplets leak out on the sides of face masks. Ventilation design is therefore of highest importance in public environments", says Staffan Lundström, 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 Chalmers University of Technology, KTH, the Royal Institute of Technology, and Lund University of Technology, the effectiveness of face masks has been studied from various aspects within fluid dynamics.
The focus has been how well particles in our exhaled air are captured by the type face masks we use, in for example public transport and public environments, in order to prevent the spreading of covid-19. By compiling what is known from previous studies and carrying out new experiments and simulations, the researchers have aimed to improve knowledge about face masks and the spread of exhaled particles.
Paving the way for more efficient face masks
The new research results also enable the development of future face masks that have improved filtering qualities and that, at the same time, are 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, i.e. breathability", says Srdjan Sasic, professor of Fluid Dynamics at Mechanics and Maritime Sciences at Chalmers University of Technology.
Significantly reducing the social distancing
As part of the study, simulations have been carried out that show that without face masks, the social distancing of 1 meter is not safe, while a distance of 1.5 meter is more justified. Face masks can not only filter out a majority of the droplets, but they can significantly reduce the safe social distancing.
"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 significantly reduced to one third of that without a face mask", says Xue-Song Bai, professor in fluid mechanics at LTH, Lund University, Faculty of Engineering.
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:
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.
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: 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", says Ramis Örlü.
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.
Text: Katarina Karlsson, Luleå University of Technology and Lovisa Håkansson, Chalmers