Free energy analysis of system comprising biased atomic force microscope   tip, water meniscus and dielectric surface
  We are concerned with free energy analysis of the system comprising an AFM tip, water meniscus, and polymer film. Under applied electrostatic potential, the minimum in free energy is at a distance greater than the initial tip--substrate separation in the absence of potential. This equilibrium distance, t_0, mostly depends on the tip bias V and cantilever spring constant k_s, where as variations of t_0 is less pronounced with respect to the dielectric constants, and polymer film thickness. Polarization of water meniscus under the AFM tip appears to be the dominant factor enabling the creation of mechanical work for tip retraction. 
Sergei F. Lyuksyutov, Pavel B. Paramonov, and Richard A. Vaia