Pore-Scale Transport and Two-Phase Fluid Structures in Fibrous Porous Layers: Application to Fuel Cells and Beyond

Abstract

We present pore-scale simulations of two-phase flows in a reconstructed fibrous porous layer. The three-dimensional microstructure of the material, a fuel cell gas diffusion layer, is acquired via X-ray computed tomography and used as input for lattice Boltzmann simulations. We perform a quantitative analysis of the multiphase pore-scale dynamics, and we identify the dominant fluid structures governing mass transport. The results show the existence of three different regimes of transport: a fast inertial dynamics at short times, characterised by a compact uniform front, a viscous-capillary regime at intermediate times, where liquid is transported along a gradually increasing number of preferential flow paths of the size of one–two pores, and a third regime at longer times, where liquid, after having reached the outlet, is exclusively flowing along such flow paths and the two-phase fluid structures are stabilised. We observe that the fibrous layer presents significant variations in its microscopic morphology, which have an important effect on the pore invasion dynamics, and counteract the stabilising viscous force. Liquid transport is indeed affected by the presence of microstructure-induced capillary pressures acting adversely to the flow, leading to capillary fingering transport mechanism and unstable front displacement, even in the absence of hydrophobic treatments of the porous material. We propose a macroscopic model based on an effective contact angle that mimics the effects of the such a dynamic capillary pressure. Finally, we underline the significance of the results for the optimal design of face masks in an effort to mitigate the current COVID-19 pandemic.

Respiratory droplets interception in fibrous porous media

“…Given the efficiency of the numerical algorithm, larger porous domains could also be simulated, possibly representing hydrophobic/hydrophilic multilayer structures such as the one composing 3ply disposable face masks, which induce porosity and permeability gradients along the flow directions that strongly affect two-phase flow behaviors. 43,66 Nevertheless, to simulate droplets smaller than 1 μ m, where diffusion is the main filtration mechanisms, hybrid lattice-Boltzmann–Lagrangian-based methods could be preferred in order to solve scales smaller than the carrying flow resolution for the droplets transport and limit the computational cost. 67 To reconstruct the microstructure via non-destructive measurement techniques, such as x-ray computed tomography, is another interesting approach to access geometrical information of more realistic geometries, possibly identifying the effects of degradation of the materials.…”

“…We make use of the lattice Boltzmann methodology (LBM) for simulating droplet transport through a set of artificially generated fibrous layers, to mimic and represent mask fibrous layers composed of variously oriented fibers. The LBM is well suited for simulations of two-phase flows in complex geometries, such as in fibrous layers, 43 given its intrinsic computational efficiency that allows to reach pore-scale resolution at a limited computational cost. 44 We use the open-source code lbdm; 45 a series of validation test cases of the two-phase flow algorithm are found in our previous works.…”

Liquid Repellence of Phobic Fiber Networks

“…Fibrous networks constitute a core class of porous media and include materials such as non-woven fabrics, electrospun mats, and papers. Their highly desired mechanical strength, flexibility, permeable structure, and scalable manufacturing have led to applications in batteries, apparel, functional fabrics, liquid–liquid separations, and fluid barriers. − Understanding the wetting behavior of these materials, especially their wetting barrier properties, is key to successfully applying design principles and has been a key thrust of research. − …”

Asymmetric invasion in anisotropic porous media

“…However, such a stabilising mechanism, which mainly depends on the fluid-fluid dynamic viscosity ratio, can be opposed by other relevant factors, such as the instability generated at the pore scale with high values of the advancing contact angle of the invading fluid [15], inertial forces [16,17], or geometrical configurations that provides a positive permeability gradient along the fluid invasion direction [14]. The latter mechanism of instability, which promotes the kinetic roughening of the invading front, has been observed both in porous materials reconstructed via X-ray computed tomography [18] and in polydimethylsiloxane (PDMS) microfluidic devices manufactured with controlled pore size gradients [19].…”