Slide 6 of 12
Notes:
Let’s look at the system properties for a repulsive interaction, ignoring any hydrophobic effect for the time. To accurately model the sphere-drop interaction, we must know the drop curvature since it is related to the effective stiffness of the oil interface. The oil drop is first flattened at the apex and eventually dimpled inward by the pressing sphere. The particle is significantly smaller than the oil droplet and limits the interaction area.
The interfacial tension is obviously the most important parameter for the extent of drop deformation, and surfactants may be added to see this. If the surfactants are ionic, they will also affect the long-ranged electrostatics by compressing the double-layer or altering the surface charge densities. A systematic addition of salt, sometimes called a force or electrolyte titration, is useful for deconvoluting component interactions. The approach velocity is an indirectly controlled variable. And the list goes on as other solution additives change the medium and the interfaces.
The sphere-drop interactions will not always be repulsive, although this would seem to be the more common situation since most materials are negatively charged in water. But if the film thins enough for van der Waals attraction to dominate, it will drain and rupture. One unfortunate consequence of a fluid interface is its mechanical instability to attractions. The drop actually gets weaker as it’s pulled upward; but it conversely stiffens when being indented. This is the singular most important factor for interpreting FI-AFM data.