(589c) Fate of Fluids in, on, and between Phobic Fiber Networks

Fiber networks constitute a core class of porous media and include materials such as nonwoven fabrics, electrospun mats, and paper. These networks display desirable properties: mechanical strength, flexibility, permeable structure, and scalable manufacture, that have led to applications in batteries, apparel, functional fabrics, liquid-liquid separations, fluid barriers, and industrial processes. In each of these settings, fluid transport within the fiber network, as well as between adjacent networks, determines the performance of engineered materials. Wetting of paper is particularly interesting because its highly complex structure has limited the ability to predict behavior, relative to that of other porous media. In this work, we address that knowledge gap by elucidating the effects of fiber geometry, surface characteristics, and liquid properties.

We investigate how strongly phobized porous substrates repel liquids by determining the critical Laplace pressure (CLP) needed to force liquid into the pores for a variety of substrates and liquids. Comparing these critical pressures to values predicted by the cylindrical pore model demonstrates the inadequacy of that approach; the model fundamentally cannot capture the variation in wetting resistance of liquids with different surface tension and solid-liquid contact angles. Using a model that accounts for the fibers’ reentrant geometry tracks the experimental trends more closely. Further adjustment of this model to account for stochastic structural effects yields quantitative prediction of the measured barrier performance for arbitrary pressures and liquids.

Liquid transport between fiber networks is investigated by examining droplet transfer behavior between non-absorbing papers. By modifying nano-scale fiber roughness, porous substrates were created with nearly identical static contact angles, but different degrees of adhesion (contact angle hysteresis). Thus, the effect of adhesion on the transfer of a droplet positioned between fiber networks that are being separated, can be determined. When the separation velocity is sufficiently small, surface forces dominate, drawing the droplet toward the more highly adhesive paper (static regime). For faster separations (dynamic regime), adhesion has a muted effect. Also interesting is the length scale for which fiber geometry becomes relevant.

Through direct measurement, insight was gained into the complex wetting behavior of an arbitrary fluid in, on, and between phobic papers. This study examines the effects of fiber width, fiber spacing, variation in fiber spacing, liquid surface tension, solid-liquid contact angle, and contact angle hysteresis on interactions in this solid-liquid system. From a design perspective, understanding the influence of these parameters is critical to engineering materials in a wide range of applications.