Spontaneous imbibition in cellulose: an absorption-mediated non-Lucas-Washburn phenomenon

Hypothesis: The classical Lucas–Washburn (LW) model, which assumes a saturated front progressing uniformly with a height scaling as the square root of time, has long been used to describe liquid seepage in various porous media. However, for hygroscopic cellulose-based materials, a fraction of water can be absorbed in the form of (nanoconfined) bound water into the amorphous regions of cellulose microfibrils thus inducing cellulose swelling. Thus, water imbibition in cellulose fiber networks might not be solely governed by capillary effects but could be significantly impacted by bound water absorption, leading to a coupled two-phase transport behavior. Experiments: We examined the dynamics of water imbibition in porous cellulose stacks using a combination of optical imaging, gravimetry, NMR and MRI experiments, under conditions of unlimited and limited water supply. These techniques allowed us to independently track both free water and bound water transport and follow sample swelling, providing a detailed view of the complex processes occurring in such materials. Findings: Although the macroscopic rising height still follows a square-root time dependence, the actual imbibition is one to two orders of magnitude slower than LW predictions. MRI results show that the free water saturates the porosity behind the front but the bound water progressively saturates the cellulose fibers ahead of the free water front. This slower dynamics and the separation between bound and free water fronts indicate that capillary forces alone cannot account for the observed behavior. These results finally challenge the direct applicability of standard concepts to interfacial phenomena in hygroscopic porous materials.