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.