We use dynamic atomic force microscopy (AFM) to investigate the forces involved in squeezing out thin films of aqueous electrolyte between an AFM tip and silica substrates at variable pH and salt concentration. From amplitude and phase of the AFM signal we determine both conservative and dissipative components of the tip sample interaction forces. The measured dissipation is enhanced by up to a factor of 5 at tip–sample separations of ≈ one Debye length compared to the expectations based on classical hydrodynamic Reynolds damping with bulk viscosity. Calculating the surface charge density from the conservative forces using Derjaguin–Landau–Verwey–Overbeek (DLVO) theory in combination with a charge regulation boundary condition we find that the viscosity enhancement correlates with increasing surface charge density. We compare the observed viscosity enhancement with two competing continuum theory models: (i) electroviscous dissipation due to the electrophoretic flow driven by the streaming current that is generated upon squeezing out the counterions in the diffuse partof the electric double layer, and (ii) visco-electric enhancementof the local water viscosity caused by the strong electric fieldswithin the electric double layer. While the visco-electric model correctlycaptures the qualitative trends observed in the experiments, a quantitativedescription of the data presumably requires more sophisticated simulationsthat include microscopic aspects of the distribution and mobilityof ions in the Stern layer.
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