Time-resolved X-ray radiography of through-thickness liquid transport in partly saturated needle-punched nonwovens

TL;DR

This study uses advanced X-ray imaging to analyze how liquid moves through nonwoven fiber networks, revealing how needling affects flow pathways and transport efficiency, which is crucial for optimizing filtration and insulation materials.

Contribution

It introduces a combined micro-CT and time-resolved X-ray radiography method to quantify dynamic liquid transport in partly saturated nonwovens, highlighting the impact of needling on flow pathways.

Findings

Liquid transport exponentially depends on saturation levels.

Needle-punching reorients fibers, creating pathways that enhance through-thickness flow.

Increased needling intensity improves liquid transport despite reduced permeability.

Abstract

Nonwoven fibre networks underpin filtration, insulation and geotextiles, where liquid uptake, redistribution and release govern performance. In needle-punched felts, barbed needles mechanically entangle fibres and partially reorient them toward the thickness direction ($z$), creating out-of-plane "pillars" and heterogeneity. While mechanical and structural consequences of needling are well documented, dynamic $z$-direction transport in partly saturated networks remains difficult to access due to opacity and sub-second timescales. Here we combine micro-CT ($\mu$CT) of dry structure with time-resolved X-ray radiography during droplet addition to quantify through-thickness transport as a function of saturation and needling intensity, using a compact Washburn-type descriptor for dynamics. Results show an exponential dependence of $z$-directional liquid transport on saturation, consistent with previous models for in-plane relative permeability of nonwoven networks. Additionally, increased needle-punch intensity reorients fibres toward the $z$-direction, forming preferential flow pathways that enhance through-thickness transport, even as single-phase permeability decreases. These findings underscore needle-punch as a key design parameter for tuning liquid transport in nonwoven fibre networks. The approach provides an experimental and modelling framework for dynamic, capillarity-driven transport in opaque fibrous materials.