Long-ranged forces between surfaces in a liquid control effects from colloid stability1 to biolubrication2, and can be modified either by steric factors due to flexible polymers3, or by surface charge effects4. In particular, neutral polymer ‘brushes’ may lead to a massive reduction in sliding friction between the surfaces to which they are attached5,6,7, whereas hydrated ions can act as extremely efficient lubricants between sliding charged surfaces8. Here we show that brushes of charged polymers (polyelectrolytes) attached to surfaces rubbing across an aqueous medium result in superior lubrication compared to other polymeric surfactants. Effective friction coefficients with polyelectrolyte brushes in water are lower than about 0.0006–0.001 even at low sliding velocities and at pressures of up to several atmospheres (typical of those in living systems). We attribute this to the exceptional resistance to mutual interpenetration displayed by the compressed, counterion-swollen brushes, together with the fluidity of the hydration layers surrounding the charged, rubbing polymer segments. Our findings may have implications for biolubrication effects, which are important in the design of lubricated surfaces in artificial implants, and in understanding frictional processes in biological systems.
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We thank J. Frey for the STAI synthesis, and R. Tadmor, T. Witten, D. Lukatsky and P. Pincus for discussions. We also thank the Eshkol Foundation (U.R.), the Canadian Friends of the Weizmann Institute (Charpak-Vered Fellowship, S.G.), the US-Israel BSF, the Deutsche-Israelische Program, and the Israel Science Foundation for their support of this work. J.F.G. (Chargé de Recherches by FNRS) and R.J. thank the SSTC for financial support.
The authors declare that they have no competing financial interests.
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Raviv, U., Giasson, S., Kampf, N. et al. Lubrication by charged polymers. Nature 425, 163–165 (2003). https://doi.org/10.1038/nature01970
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