Molecular expansion of an individual coiled DNA on a graphite surface

Direct visualization and manipulation of a single polymer on a solid surface in fluid can greatly contribute to our understanding of polymer dynamics and physics relevant to many practical applications. In this paper, the authors report a novel phenomenon that a randomly coiled DNA molecule can be gradually expanded to form a spiral-like pattern by repeatedly rotating a water droplet on a graphite surface where the DNA molecule was deposited. The process of DNA expansion was traced using atomic force microscopy relocation imaging. A simple theoretical model based on the analysis of centrifugal and resistant forces exerted on the DNA molecule during the manipulation was presented to explain the formation of the spiral-like pattern. This study should provide us insight into the movement of polymers on a solid surface under fluid flow.