Molecular simulation of the water meniscus in dip-pen nanolithography

We report a molecular dynamics simulation of the nanometer water meniscus formed in dip-pen nanolithography (DPN). When an atomic force microscope tip is in contact with a surface, the meniscus is significantly asymmetric around the tip axis. The meniscus as a whole can move away from the tip axis due to surface diffusion. The structure of the meniscus fluctuates and its periphery has a finite thickness as large as 25% of its width. We simulated the transport of nonpolar hydrophobic molecules through a water meniscus. Molecules move on the surface of, not dissolving into the interior of, the meniscus. As a result, an annular pattern forms in DPN. Even if the meniscus is cylindrically symmetric, the molecular flow from the tip and the subsequent pattern growth on the surface are anisotropic at the nanosecond timescale.