Molecular Imaging of Shear Inhomogeneity (Wall slip/Shear Banding) in Entangled Fluids

In all standard rheometric measurements, the fluid of interest is sandwiched between two solid surfaces, and no-slip boundary condition (NSBC)/uniform shear is assumed during simple shear between the two solid walls. Violation of these premises has been noticed in entangled liquids at both macro/microscale gaps. However, the molecular origin of shear inhomogeneity (slip/shear banding) still lacks experimental verification. Using combination of spin-disk confocal microscopy and rotational rheometer, molecular images were captured in non-linear response regime of entangled DNA solutions with simultaneous rheometric and velocimetric measurements. At low shear rates with Weissenberg number Wi < 1.0, the growth of stress as function of time is monotonic. As we expected, the velocity profile is linear across the gap at all time of shearing. DNA chains remained coiled in this terminal flow regime. When the Wi > 1.0, the change of the boundary condition from no-slip to slip produces stress overshoot. Specifically, adsorbed DNA chains remained unperturbed till after the stress maximum when the molecules start to stretch and elongate at the surface after disentanglement from coiled bulk chains. At higher Wi, the DNA molecules close to surface exhibit tumbling behavior to indicate that they have become fully disentangled

The amount of wall slip was reduced considerably by changing solvent from water to glycerol, allowing bulk shear banding to develop with heterogonous topological conformation from partial coiled to fully stretched across the gap. These new experimental findings provide new insight into the phenomenology of entangled fluids in presence of fast external deformation.