Direct Measurement of Sub-Debye-Length Attraction between Oppositely Charged Surfaces

Using a surface force balance with fast video analysis, we have measured directly the attractive forces between oppositely charged solid surfaces (charge densities σ₊, σ_-) across water over the entire range of interaction, in particular, at surface separations D below the Debye screening length λS. At very low salt concentration we find a long-ranged attraction between the surfaces (onset ca. 100 nm), whose variation at D<λS agrees well with predictions based on solving the Poisson-Boltzmann theory, when due account is taken of the independently-determined surface charge asymmetry (σ₊≠|σ_-|).