Depletion of λ-DNA near moving contact line

An understanding of the contact line dynamics of viscoelastic liquids is essential for the successful application of liquid processing methods in many industries. In this article, the contact line motion of a λ-DNA solution is considered both experimentally and theoretically to understand the contact line motion of polymeric solutions. Experimentally, by using fluorescently tagged λ-DNAs in glycerin, the depletion of λ-DNA near the contact line is observed. The depletion length increases with time in a range near 10 µm. DNA fragments in glycerin did not show such a depletion region. The geometric exclusion of λ-DNA from the region cannot be the proper reason for the depletion because λ-DNA is in the stretched state. It is suggested that the depletion region might be developed by the migration of polymers from the solid wall to the bulk due to the elasticity of polymers. In the theoretical study, the Stokes flow near the contact line region is solved analytically for a drop-spreading problem to obtain the inhomogeneous flow field. Then, the motions of DNAs are considered in the flow using the FENE-P dumbbell model at the dilute limit. Two independent mechanisms of migration of the dumbbells were considered: migration due to wall-bead hydrodynamic interactions and the migration of stretched dumbbells in the inhomogeneous flow. It was found that the migration of stretched dumbbells in the inhomogeneous flow appears to be the dominant reason for the presence of the depletion region. The effects of the severe extension of dumbbells on migration are also discussed for both mechanisms.