A Structural and Optical Study of Evaporated Droplets Containing DNA

Amy Lytle

Droplets of sonicated calf thymus DNA in a Tris-EDTA solution, with a range of fragment lengths from 300-2000 bp, were evaporated on a glass substrate. DNA forms several liquid crystalline phases with increasing concentration before reaching the completely evaporated crystalline phase, so the droplets are birefringent both during and after evaporation. Dynamics within the droplet during evaporation due to contact line pinning and the majority of the evaporation occurring at the edge cause an outward radial flow of material in the droplet. The evaporated droplets consequently possess a raised ring at the edge, smaller concentric rings further in, and a depression in the center. In this study, the effects of varying temperature, vapor pressure, and induced alignment at the substrate on the microscopic and macroscopic structure of the droplets were investigated. Due to the characteristic rings and central well, the evaporated droplets possess lensing and diffraction properties. The diffraction pattern in the Fraunhofer regime was measured and compared to an initial model of the droplet structure as sets of multiple slits separated by an opaque obstacle. Parameters for the theory were measured using optical microscopy.