The Interfacial Physics Group

Carnegie Mellon University, Department of Physics

Dynamic Wetting of Viscous and Vicoelastic Fluids

by Yuli Wei



Dynamic wetting poses a complex fluid mechanical problem of both fundamental and technological importance.  This complexity arises because the fundamental geometry of the wedge-like region near the moving contact line requires that fluid elements experience ever increasing deformation rates as they pass into and out of that region.  Thus, somewhere in the flow field, relaxation modes caused by segmental and chain motions in a viscous polymeric fluid may be unable to relax, leading to nonlinear stresses and possible changes in the shape of the deformable free surface.  In this thesis, we investigate the wetting behavior of model viscoelastic fluids, Boger fluids, and the viscous oligomeric fluids which are the base solvents in these Boger fluids.  We determine that the dominant relaxation mode in Boger fluids has only small effects on both viscous bending and dynamic contact angles even approaching the air entrainment limit.  A lubrication analysis, used to examine the impact of the linear elasticity due to dominant relaxation mode, also shows only small effects on the interface curvature and on the dynamic contact angle.  The viscous oligomeric base fluids, which have only very short relaxation times, also show deviations from the viscous bending that a Newtonian fluid exhibits near the contact line.  Our experiments prove that these deviations do not arise due to the interactions between the fluid and the solid surface.  It is very possible that this behavior arises from the influence of the non-Newtonian behavior at a small length scale on the Newtonian hydrodynamics at larger distances from the contact line.  Finally, a new relaxation regime has been discovered as the final static contact angle is approached in the spontaneous relaxation of the viscous fluids with non-zero static contact angles.  This new regime may be due to slow rearrangements of the polymeric molecules altering the static force balance at the contact line.   predictions, and recent theoretical papers support this position.