Modeling the Effect of the Relative Humidity on the Manipulation of Nanoparticles with an Atomic Force Microscope

In the manipulation of nanoparticles, different behaviors are typically observed including sliding, rolling and rotation. Most of investigations in this field have so far focused on describing the interaction forces under vacuum (dry air) environmental condition, while the effect of the relative humidity has been poorly considered. In this work we developed a model for simulating the dynamic nanoparticle motion (rolling and sliding) in an AFM-based manipulation of nanoparticles in a humid environment. In our method, the interaction forces include the adhesion force, mainly consisting of the capillary force and van der Waals force, the normal force and friction forces. We calculated the adhesion force by considering the contributions from the wet and dry portions of the particle. Our stimulations show that nanoparticles smaller than the AFM tip tend to slide before rolling, while in large nanoparticles the rolling occurs first. The particle motion is achieved if the applied force exceeds a critical value and the direction of the rolling movement depends on the applied force angle. Furthermore, small nanoparticles are more easily manipulated by the tip in low-humidity conditions while the manipulations with large nanoparticles need high-humidity conditions. Preliminary results can be used to adjust proper handling force for the accurate and successful assembly of particles.