Oscillatory Forces of Nanoparticle Suspensions Confined between Rough Surfaces Modified with Polyelectrolytes via the Layer-by-Layer Technique

摘要

This paper addresses the systematic study of surfacenroughness effects on the internal structuring of silica nanoparticlensuspensions under confinement. The confining surfaces are modified bynphysisorption of layers of oppositely charged polyelectrolytes with the socallednlayer-by-layer technique. The layer-by-layer technique modifies thensurface roughness without changing the surface potential of a multilayernwith the same outermost layer, by increasing the number of constituentnlayers and ionic strength of the polyelectrolyte solutions and by selectingnan appropriate pair of polyelectrolytes. The oscillatory forces ofnnanoparticle suspensions with a particle diameter of 26 nm are measured by a colloidal-probe atomic force microscope (CPAFM).nThe characteristic lengths of the oscillatory force, i.e., wavelength, which indicates interparticle distance, and decay length,nor particle correlation length, are not affected by the surface roughness. The corresponding reduction in the oscillatory amplitudenand the shift in the phase correlate with an increase in surface roughness. Increasing surface roughness further induces andisappearance of the oscillations, and both confining surfaces contribute to the effect of surface roughness on the force reduction.nIn order to show an oscillatory force, the particles have to show positional correlation over a reasonably long range perpendicularnto the surface, and the correlation function should be the same over a larger lateral area. This requires that both the particles andnthe surfaces have a high degree of order or symmetry; otherwise, the oscillation does not occur. A roughness of a few nanometersnon a single surface, which corresponds to about 10% of the nanoparticle diameter, is sufficient to eliminate the oscillatory force.