Visualization of the flow profile inside a thinning filament during capillary breakup of a polymer solution via particle image velocimetry (PIV) and particle tracking velocimetry (PTV)

TL;DR

This study visualizes the flow inside a thinning polymer filament during capillary breakup, revealing inhomogeneous flow profiles and predominantly extensional flow in the thinning regime using advanced velocimetry techniques.

Contribution

It introduces a combined PIV and PTV approach to accurately measure flow profiles in thinning polymer filaments, correcting optical aberrations for detailed analysis.

Findings

Flow at filament formation is highly inhomogeneous with flow mainly in outer regions.

Flow during exponential thinning is predominantly extensional.

Flow profiles vary with filament thickness and thinning stage.

Abstract

We investigated the flow profile of a polymer solution in a thinning capillary bridge. Fluorescent tracer particles with a diameter of 3$\mu$m were used to visualize the flow. The cylindrical shape of the filament introduced strong optical abberations that could be corrected for, and we were able to characterize the flow in filaments with a thickness ranging from 150 to 30 $\mu$m. In the first regime when the filament was still sufficiently large, we used a PIV algorithm to deduce the flow field. At later stages when the number of particles in the observation plane decreased a PTV algorithm was used. The main two results of our measurements are as follows. First, the flow profile at the formation of the cylindrical filament is highly inhomogeneous and there is only flow in the outer parts of the filament. Second, we find that in most parts of the regime, where the temporal radius of the thinning filament can be fitted with an exponential law the flow indeed is purely extensional.