Decoupling Wetting from Swelling in Electrospun Fibrous Mats: Predicting Saturation Time, Swelling Time, and...

Electrospun mats are used wet, yet the transition from the characterized dry mat to the functioning wet one is rarely quantified. The Hansen solubility parameters of a polymer and a liquid generate two distinct metrics rather than one, and so two independent gates: the Flory-Huggins parameter follows from the Hansen distance, while the polymer-liquid interfacial tension collapses exactly onto a second squared distance on volume-scaled coordinates, weighting dispersion differently by 4β ≈ 2.5. Wetting and swelling are consequently separable, and four regimes exist rather than two. The pore-scale Laplace pressure and a Kozeny-Carman permeability reduce capillary imbibition to one coefficient, Dimb = ε0Rfσ cos θ/[kK(1 - ε0)μ], which saturates a mat through its thickness in 10 to 50 ms. Fiber orientation makes in-plane wicking anisotropic, so an aligned mat wets into an ellipse of axis ratio √2. A closed gate instead imposes a 14 kPa intrusion pressure; the mat stays dry unless the interior drive is favorable, in which case it resists a droplet yet imbibes once entry has occurred. Fiber swelling completes within 102 s, and a jamming argument closes the dimensional-change problem, giving a porosity linear in the fiber swelling ratio and an exact decomposition of gravimetric uptake into network and void terms. Against published data, a cellulose acetate (CA) mat-and-film pair returns a mat porosity of 0.90 with no adjustable parameter, and a cross-linked poly(vinyl alcohol) (PVA) series a monotonic cross-link density reproducing the reported cross-linker ranking. A swelling ratio reported without the dry porosity is not interpretable: for one hydrophilic mat, 54% of the uptake is capillary water between the fibers, not water in the polymer. Two independent datasets test the two branches separately: the wetting gate against poly(ε-caprolactone) (PCL) mats that absorb a water droplet completely despite an apparent contact angle of 137.9°, and the Kozeny-Carman baseline against absolute permeability measurements on poly(lactic acid) (PLA) scaffolds several times coarser in fiber diameter. The framework applies unmodified to microfluidic and tissue-engineering scaffolds as well as to wound dressings, drug-releasing membranes, separators and filtration media.