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Summary: Direct Measurement of Sub-Debye-Length Attraction between Oppositely Charged Surfaces
Nir Kampf,1
Dan Ben-Yaakov,2
David Andelman,2
S. A. Safran,1
and Jacob Klein1,*
1
Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel
2
Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv University, Ramat Aviv, Tel Aviv 69978, Israel
(Received 15 May 2009; published 11 September 2009)
Using a surface force balance with fast video analysis, we have measured directly the attractive forces
between oppositely charged solid surfaces (charge densities þ, À) across water over the entire range of
interaction, in particular, at surface separations D below the Debye screening length S. At very low salt
concentration we find a long-ranged attraction between the surfaces (onset ca. 100 nm), whose variation at
D < S agrees well with predictions based on solving the Poisson-Boltzmann theory, when due account is
taken of the independently-determined surface charge asymmetry (þ Þ jÀj).
DOI: 10.1103/PhysRevLett.103.118304 PACS numbers: 82.70.Ày, 07.10.Pz, 82.45.Mp
The Debye-Hu¨ckel, and more generally the Derjaguin-
Landau-Verwey-Overbeek approach, based on lineariza-
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