Poro-elastic bending of cellulose sponges: Inspired by curling leaves

Structural changes in plant leaves are a major indicator of the state of plants. Under high heat stress or insufficient water supply conditions, the leaves tend to curl to reduce water loss through transpiration and evaporation. However, the underlying mechanical reasons for leaf curling are unclear. Here, we study changes in out-of-plane curvature in leaf-inspired sponges, regarding the difference in contractility and rigidity of materials. In general, different types of plant cells with varying stiffness are distributed inside the leaf: soft parenchyma cells in the upper surface and middle region, and rigid veins with lignified sclerenchyma cells at the lower part. Inspired by this feature, we built a sponge with a polycarbonate sheet attached to its lower surface to introduce a difference in stiffness. When this sponge is soaked in water and allowed to dry naturally, its moisture content decreases, resulting in the edges contracting and the curvature increasing. This phenomenon is theoretically interpreted using the diffusion equation, which accounts for the change in water distribution caused by evaporation, as well as the changes in strain, force, and moment resulting from water loss-induced contraction. The relationship between moisture content and curvature can be explained by balancing structural stresses by curvature with local stresses induced by water evaporation. This result holds significant potential for smart materials or soft robotics related to moisture-induced shrinkage.