Slide 8 of 12
Notes:
From the model, we can understand how adjusting the physical parameters previously mentioned affects the drop stiffness and final film thickness—the two most important features of the fluid interface. A bigger drop is weaker because the Laplace pressure to resist deformation is initially smaller, but the minimum film thickness is unaffected because the rigid probe fixes the final curvature. A larger particle deforms more of the drop to a smaller degree, so that the apparent stiffness increases and so does the film thickness.
The interfacial tension is not usually large enough to totally squeeze out the film. Of course, if the surfaces have opposite charges or if the hydrophobic interaction is large enough, there won’t be stable film. Increasing the approach velocity has similar effect but to a lesser extent—increasing the hydrodynamic probe radius. Stiffness, of course, increases with interfacial tension and the stable film is correspondingly thinned. While electrostatic conditions have little or no effect on drop stiffness, they can be critical in determining the film thickness.
For a reasonable set of parameters corresponding to an electrolyte titration between an oil drop and polystyrene sphere in water, the force profiles are similar to those of solid-solid interactions. The only obvious difference is the critical film thickness, even at 0.1 M NaNO3.