More friction for polyelectrolyte brushes

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Science  29 Jun 2018:
Vol. 360, Issue 6396, pp. 1399-1400
DOI: 10.1126/science.aat5343

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Skeletal joints must provide lubrication under considerable load. Lubrication between two sliding surfaces in aqueous environments can be greatly enhanced by polyelectrolyte brushes (1): Long macromolecular chains that bear charges at each repeating unit are grafted densely to a planar or curved surface. In the so-called osmotic limit (low salt concentrations), a large fraction of the counterions are confined within the brush layer, thus creating an enormous osmotic pressure (see the figure, left). Surface forces apparatus (SFA) (2) studies revealed a marked repulsion between such surfaces (3), and Klein and co-workers (4) showed that this effect very efficiently lubricates polyelectrolyte brush interfaces. It is now generally believed that lubrication in many biological systems works according to the same principle. On page 1434 of this issue, Yu et al. (5) add a twist to this story by studying the same problem in a SFA and finding that traces of di- and trivalent ions can increase the frictional force between two polyelectrolyte brush layers, dramatically so for Y3+ ions.