Figure 1.

Force on an electric dipole p as it is translated over a dielectric sphere. The dots indicate force from the sphere, the gray line indicates force from the substrate, and the black line indicates their sum. Inset: schematic of the model in which the dipole p rests a distance z above and ρ from the center of a dielectric sphere of radius a and dielectric permittivity ε_P. The sphere rests on a substrate of dielectric permittivity ε_M. An image dipole p^′ is generated in the substrate. For the force calculation,z=a and all materials are silica, ε_P=ε_S=3.8ε₀.