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Dynamics of water, hydrated-ions and charged polymers in highly-confined films, and their role in friction modification

  • Jacob Klein (a1) (a2), Uri Raviv (a1), Susan Perkin (a2), Nir Kampf (a1) and Suzanne Giasson (a3)...
Abstract
ABSTRACT

Recent studies have revealed that, in contrast to non-associating liquids such as oils or organic solvents, salt-free water retains a viscosity close to its bulk value even when confined to films thinner than some 3 nm, indeed down to only one or two monolayers thick [1,2]. For the case of high concentration aqueous salt solution compressed down to subnanometer films between charged surfaces, the trapped hydrated ions serve to act as molecular ball-bearings, sustaining a large load while remaining very fluid under shear [3]. This behaviour is attributed to the tenacity of the hydration sheaths together with their rapid relaxation time. Finally, a very recent study [4] has shown that when charged polymer brushes in aqueous media are compressed and slid past each other, they provide a lubrication that is considerably superior to that afforded by neutral brushes: This is attributed on the one hand to the resistance to mutual interpenetration of the chains due to entropic barriers in the good-solvent conditions, and, on the other hand, to the hydration-sheaths on the charged polymer segments which can act – as noted above – as molecular ball-bearings.

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1. U. Raviv , P. Laurat , and J. Klein , Nature, 413, 5154 (2001)

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MRS Online Proceedings Library (OPL)
  • ISSN: -
  • EISSN: 1946-4274
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