Characteristics of the Water Meniscus at AFM Tip for Various Surface Energy

The atomic force microscope (AFM), invented by Binning et al [1], has become a useful tool for its application in various fields of research. Even though AFM shows outstanding performance, one problem is the large adhesion forces due to the formation of a water meniscus in ambient condition. Therefore, it is important to understand the properties of the water meniscus. In previous studies, a theoretical method or Monte Carlo method was used to model the water meniscus problem with the assumption of thermodynamic equilibrium. But the physical phenomenon, occurs in a real AFM environment, is difficult to reach the thermodynamic equilibrium state due to a continuous variation of the water meniscus structure. Through two methods, addressed upon, only shows the equilibrium state, they have many difficulties to make clear the mechanism of a water meniscus formation. Because it is also troublesome to simulate the formation process of a water meniscus, most of researches have only investigated the properties of the water meniscus. Molecular Dynamics Simulation (MD) is the most suitable method for our needs. We can not only obtain the capillary force of the water meniscus, but also visualize the forming mechanism of the water meniscus. For these reasons, the water meniscus is expected to be understood easier. We performed a molecular dynamics simulation of the water meniscus that forms between an atomic force microscope (AFM) tip and a flat solid surface. We obtained the density profile and the capillary force of the nano-scale meniscus. We also examined the structure change in the meniscus by scanning the AFM tip at various distances between the AFM tip and the flat surface. In the case of a hydrophilic tip and a hydrophobic flat surface, the water meniscus changes from convex to concave as the surface energy of the flat surface increases. Using Young-Laplace equation, we obtain the capillary force of the water meniscus. In the case of a hydrophilic tip and a hydrophilic flat surface, the capillary force decreases as the distance between the AFM tip and the flat surface increases. We also examined the radius of the water meniscus. As the distance increases, the radius gets smaller. At a distance over d = 2.5nm, the radius of the water meniscus starts to fluctuate due to the instability of the water meniscus becoming narrow.