Quantitative analysis of fluid interface-atomic force microscopy

Net repulsive interactions between n-hexadecane and a polystyrene microsphere in aqueous solutions are measured with atomic force microscopy and interpreted using the augmented Young-Laplace equation. The true separation between probe and fluid interface is implicitly computed from the force-distance data, providing a more accurate description of drop or bubble deformation. Experiments and theoretical arguments demonstrate that a fluid interface stiffens with increasing deformation and is not accurately treated as a Hookean spring. The unexpected stability of the draining aqueous film between hydrophobic bodies in electrolyte solutions is explained primarily by the deformation of the oil drop in response to the applied normal force, as well as the increased hydrodynamic resistance due to the increased drainage area. (C) 2001 Academic Press.