A study on the behavior of water droplet confined between an atomic force microscope tip and rough surfaces

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.