Abstract
Owing to the size effect, the physical behaviors of objects will greatly diverge from the knowledge in the macroscopic world. This study investigates the adhesion phenomenon and its physical behavior between contact bodies in the mesoscopic scale by molecular dynamics (MD) simulation and by the experiment of atomic force microscope (AFM). The system model is constructed as the tip and substrate system of the AFM, which consists of about 7000 Au atoms and utilizes the Morse potential function to simulate the intermolecular forces between atoms. Five tip models with different size of the contact area are built. The simulation results reveal that the jump-to-contact phenomenon at the atomistic level is apparent between gold tip and gold substrate. Larger contact area triggers earlier jump-to-contact between the tip and substrate. In addition, owing to the atom migrations caused by adhesion, an extended neck is gradually formed upon the retraction of the tip from contact, and an island shape structure is formed on the substrate after the neck breaking. The size of these nanostructures (neck and island) has positive correlation with the contact area. Results also show that the mechanical contact between the tip and substrate closely correlates to the formation of the nanowire and nanodot due to the adhesion phenomenon. In the experimental study, the Au-coated Si tip and substrate are utilized to plot the force curve. The adhesion force is measured by the force curve plotting. By comparing with the adhesion forces estimated from the classical adhesion theories, the experimental results exhibit a huge variance. This variance on the adhesion force is attributed to the applicability of the continuum theory for the mesoscopic bodies, the uncertainty on the size of the contact area between the tip and substrate, and some influences of the experimental environment (ex. the moisture).