Poro-elasto-capillarity of cellulose sponges

It is a mundane experience that cellulose sponges swell while absorbing water. Fluid flows in deformable porous media, such as soils and hydrogels, are classically described based on the theories of Darcy and poroelasticity, where the expansion of the media arises due to increased pore pressure. However, the situation is qualitatively different in cellulosic porous materials like sponges because the pore expansion is driven by wetting of the surrounding cellulose walls rather than by increase of the internal pore pressure. The observation using the environmental scanning electron microscopy reveals the coalescence of microscale wall pores, which allows us to build a mathematical model for pore size evolution and the consequent wicking dynamics. The scaling law constructed through this work agrees well with the experimental results. Our study sheds light on the physics of water absorption in hygroscopically responsive porous materials, which have far more implications than everyday activities (e.g. cleaning, writing and painting) carried out with cellulosic materials (paper and sponge), including absorbent hygiene products, cooking, and soft robotics.