Application of a dynamic atomic force microscope for the measurement of lubrication forces and hydrodynamic thickness between surfaces bearing adsorbed polyelectrolyte layers

An atomic force microscope, modified to perform dynamic measurements, has been used to measure the lubrication forces between a colloid probe and flat substrate in the normal direction. The hydrodynamic interactions were measured for bare surfaces and for surfaces with an adsorbed layer of the weak polyelectrolyte, poly(2-vinylpyridine) in aqueous solution. The flow of liquid between the surfaces is described in terms of the effective mobility, G. At surface separations far exceeding twice the equilibrium steric layer thickness, the solvent between the surfaces is Newtonian with a viscosity equal to that of the bulk electrolyte solution. However, there is significant deviation from Newtonian behavior and a large increase in the effective viscosity at surface separations less than twice the steric layer thickness, due to the restricted drainage of solvent between overlapping polyelectrolyte chains. Using the dynamic AFM technique, it was possible to determine the hydrodynamic thickness of the adsorbed polyelectrolyte layer.