title: A study on the behavior of water droplet confined between an atomic force microscope tip and rough surfaces.
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.
authors.affiliation: School of Mechanical Engineering, Pusan National University, San 30, Jangjeon-dong, Geumjeong-gu, Busan 609-735, Korea.
authors.forename: Jeong-Ahn
authors.initials: JA
authors.lastname: Ko
authors.forename: Ho-Jin
authors.initials: HJ
authors.lastname: Choi
authors.forename: Man-Yeong
authors.initials: MY
authors.lastname: Ha
authors.forename: Seung-Do
authors.initials: SD
authors.lastname: Hong
authors.forename: Hyun-Sik
authors.initials: HS
authors.lastname: Yoon