What controls filament thinning in uniaxial extension?

Abstract

Breakup of fluid threads is omnipresent in nature and highly relevant for technical processes such as atomization, printing, coating, or spinning. We discuss how to control the filament lifetime of shear-thinning, viscoelastic fluids during uniaxial extension without affecting their shear viscosity. Two commercial acrylic thickeners differing with respect to the co-polymerized hydrophobic monomers, and hence different hydrophobic intra- and intermolecular association, are mixed to obtain aqueous solutions with similar shear viscosity. The elongational relaxation time as determined by capillary breakup extensional rheometry, however, varies by almost two orders of magnitude. Filament lifetime of these solutions can be varied upon adding non-Brownian, plate-shaped particles, again without affecting shear viscosity. A trace amount of particles increases elongational relaxation time by about a factor of four. Car bodies are usually coated using high-speed rotary bell atomizers guaranteeing high transfer efficiency and high-quality appearance. We use the solutions and suspensions described above to investigate the effect of extensional viscosity on ligament formation at the bell edge as a decisive intermediate step prior to droplet formation. High-speed imaging reveals a logarithmic scaling of ligament length with extensional relaxation time for pure thickener solutions. In contrast, ligament length monotonically decreases with increasing particle concentration, i.e. extensional viscosity. Plate-shaped particles obviously act as defects promoting ligament breakup. Extended filament lifetimes are commonly observed during atomization and spraying as a consequence of fluid viscoelasticity. On the other hand, low viscosity fluid threads rupture quickly. Here, we demonstrate that a unique banding instability during extension of a low viscosity surfactant solution with no measurable elasticity leads to extremely long filament lifetimes and to the formation of remarkably long threads. Combining filament stretching and particle image velocimetry we found an unexpected heterogeneous, banded flow in opposing directions. This phenomenon is not limited to surfactant solutions but can also occur in biopolymer solutions, thus broadening the view on instabilities in complex elongational flows.