Plasticization by Water Governs the Hydration-Adhesion Relationship of Cellulose Mucilage

Hydration modulates the adhesion of cellulosic plant-seed mucilage to substrates. Using coarse-grained many-body dissipative particle dynamics (MDPD), we show the synergistic adhesion of hydrated cellulose; the adhesion of the water-cellulose mixture is stronger than the weighted average of the two components. At low to moderate hydration, water plasticizes the stiff cellulose chains, making them more flexible and better able to adapt to the substrate during retraction. We show this by artificially modifying dry cellulose so that its flexibility matches that of hydrated cellulose. Thus, softened dry cellulose has the same pull-off force as the unchanged, but hydrated cellulose. More support for the softening mechanism being responsible for increased adhesion is that it can also be brought about by small molecules other than water: Mixing in cellulose monomers or dimers improves adhesion, but the admixture of longer oligomers does not. For comparison, we also investigated pectin mucilages, pectin being the other important component of seed mucilage. Here, there is no synergy between water and pectin, with the effect being additive at best. Blending cellulose with pectin increases adhesion because pectin is inherently more flexible and has a higher baseline stickiness, yet the effect remains additive.