Direct Observation of polymeric Dynamics in Flow


Many fluids in our everyday life like blood, paint or mayonnaise are complex fluids , which means that their constituents built up at least two phases. These fluids usually show a quite different mechanical behavior in comparison to „normal“ newtonian fluids like water. A simple two-component complex fluid, consisting of water and polymers, can already show complex effects like shear-thinning, rod-climbing or drag reduction. This behavior can be traced back to the complex conformational dynamics of polymer molecules in flow, meaning the change of its orientation and spacial arrangement. However, the investigation and description of polymeric conformational dynamics poses a major challenge to theory and experiment alike due to the inherently large number of degrees of freedom.


Detailed informations about the interaction between the fluid‘s flow field and the polymer conformation is essential to fully understand its behavior. Former experiments with DNA-molecules indicated complex internal motions of the molecules without uncovering the exact nature of deformation. To this end, our team has investigated the flow behavior of individual actin polymers as they travel through a microchannel. With a self developed camera setup, analog to the pit-lane cameras used in motor-sports, we were able observe the motion of a single flowing polymer. From the obtained movies we observed exactly how the flow field deforms the polymer‘s shape, leading to quasi-cyclic and successive motion: a tumbling motion. 


Video 1: Shows the tumbling motion of 8 micron actin filament in microchannel flow.

Video 2: Shows the tumbling motion of  a 16 micron actin filament in microchannel flow.

We analytically modeled the motion of the polymer using force balance ansatz coupled with the diffusive behavior of a stiff rod. Although the polymer is bent quite drastically during the tumbling motion, we could show that its basic orientational dynamics is the same as for a stiff rod. Also for longer filaments, which have even more degrees of freedom, the dynamic can be well described by this model. This is even more surprising since we resolved for the first time multiple tumbling motions. 

Video 3: Shows the tumbling motion of a  4 micron actin filament in microchannel flow.

Video 4: Shows the tumbling motion of a 34 micron actin filament in microchannel flow.