Dielectric exchange: The key repulsive or attractive transient forces between atomic force microscope tips and charged surfaces

Attractive as well as repulsive forces between electrically neutral tips and charged surfaces are measured directly with an atomic force microscope. The exchange of the volume of a region of the electric double layer of a mica surface immersed in water with a dielectric constant εDL by the tip with a dielectric constant εTip is responsible for the repulsion at large distances from the surface (starting at ∼100 nm, diffuse layer) and followed by an attraction when the tip is immersed in the Stern layer (∼2 nm). The force versus distance measured curves for high approaching velocities (⩾30 μm/s) were fitted to the expression of the dielectric exchange force derived by using a continuum theory for a sharpened pyramidal tip immersed in a spatially variable dielectric constant double-layer electric field. The dielectric exchange effect gives a consistent description of the force acting on the tip by assuming a double-layer region of disorganized water with εDL∼80 at distances far away from the surface followed by a region of lower dielectric constant resulting from the alignment of the water molecules due to the mica surface charge.