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

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.