Quick-Drying Foam

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Science  07 Jan 2005:
Vol. 307, Issue 5706, pp. 19
DOI: 10.1126/science.307.5706.19a

Sandcastle worms build shelters for themselves by gathering sand grains and gluing them together into a sturdy tube, using a rather sophisticated construction material. Stewart et al. have analyzed the structure and composition of this glue, which contains three highly charged proteins: two are basic, whereas the third, acidic component accounts for the 30 mol % of phosphoserine in the cement. Concentrating these proteins (along with Ca2+ and Mg2+ to neutralize charge) within low-pH secretory granules in the cement gland initiates a process of complex coacervation. Phase separation occurs, yielding an emulsion-like blend of dehydrated proteins and cations along with water-rich droplets. When this mixture is daubed onto a sand grain, several changes occur, due in part to the higher pH and different ionic composition of seawater. The cation-phosphate interactions become ionic or salt-like in character, and the solvation of charges acts to soak up water from the cement/sand interface, improving contact as the cement sets. The hardened cement displays a cellular foam morphology, reflecting the separated phases, which also confers benefits in terms of an economy of material and a gradient of elasticity ideally suited to life in the intertidal zone. — GJC

J. Exp. Biol. 207, 4727 (2004).

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