Microscopic origin of the elastic instabilities during flow of polymer solutions

Polymeric fluids exhibit a number of intriguing flow phenomena such as vortex formation, die swelling, stress overshoot, and drag reduction when the flow rate exceeds a certain critical value (typically when flow rate is larger than reciprocal of relaxation time of polymeric fluid or Wi >1.0). The exact molecular picture behind many of these complex flow responses is not known yet and it is still under debate. This limits the design, control and optimization of the technological processes related to polymer products. To overcome these technological limitations, it is required to develop novel methods that can relate the macroscopic flow with the molecular conformation. Recently, the direct visualization of stained DNA inside microfluidics provides a unique opportunity for us to create a conceptual framework for nonlinear polymer rheology in the fast flow rate regime (Wi>1.0). For instance, we recently employed single molecule experiments to study the necking and pinch-off dynamics of polymeric droplets by combining microfluidics and single DNA observation. We demonstrated that the individual polymer molecules suddenly stretch from their coiled conformation at the onset of necking. The extensional flow inside the neck is strong enough to stretch polymer chains. Furthermore, we have unraveled the molecular process leading to dead-zone formation during flow of shear thinning polymer solutions through porous media by a similar approach. We believe that these single-molecule experiments allow us to develop a realistic theoretical picture of polymer solutions during flow in porous media.