New research details how the Bristol-led research conducted confocal microscopy to take three dimensional images of woven fabric, the kind of material typically used to make cloth masks. Collaborators at Surrey then used the image to perform Lattice Boltzmann simulations of the air flow through fabric, a common technique used by physicists to analyse fluid dynamics. 

The findings from the simulations enabled the team to calculate the filtration efficiency for particles a micrometre (one thousandth of a millimetre) and larger in diameter. For particles with a diameter of 1.5 micrometres – the typical size of Covid-bearing particles – the team estimated woven fabric is only 2.5 to 10% efficient, because most of the air flow is channelled through relatively large gaps between the fabric’s yarns. Multiple layers of fabric improve efficiency in a roughly linear fashion, meaning triple-layer cloth masks are up to 30% efficient, but this is still poor in comparison with the material used for FFP2 masks, which is typically more than 90% efficient. 

Read the full University of Bristol press release

Paper: Modeling the filtration efficiency of a woven fabric: The role of multiple lengthscales by R Sear et al. in Physics of Fluids.