ABSTRACT
The atomic force microscope (AFM) is used for imaging, measuring, and manipulating matter at the nanoscale. It is well-known that water condenses between an AFM tip and a solid surface, thereby generating a pull-off force acting on the tip. We investigated the behavior of a water meniscus between the tip and a solid surface using molecular dynamics simulation. We considered ideally smooth surfaces and rough surfaces that are regularly structured and randomly generated with a standard deviation of 2 A. The characteristic energy values of the solid surfaces used in the study are 0.1, 0.5, 1.0, 1.5, 2.0, and 2.5 kcal/mol, and the tip-to-surface distance considered is in the range from 1.5 to 3.7 nm. The behavior of water confined between the tip and a solid surface depends on the characteristic energy of the solid surface, the tip-to-surface distance, and the shape of the solid surface. The contact angle, neck radius of the water meniscus, and absolute value of capillary force decreases as the tip-to-surface distance increases, regardless of the pattern of the solid surface. Compared to an ideally smooth surface, the effect of regularly structured roughness on the behavior of a water meniscus on a solid surface is significant, whereas the effect of randomly generated roughness is relatively small.
