Abstract
Formation and breakup of fluid threads is pervasive in nature and technology, where high extensibility of fluid filaments and extended filament lifetimes are commonly observed as a consequence of fluid viscoelasticity. In contrast, threads of low viscous Newtonian fluids like water rupture quickly. Here, we demonstrate that a unique banding instability during filament thinning of model surfactant solutions, with a viscosity close to water and no measurable elasticity, leads to extremely long filament lifetimes and to the formation of remarkably long threads. Complementary measurements in planar extension as well as in shear reveal that this flow instability is characterized by a multivalued stress, arising beyond a critical strain rate, irrespective of flow kinematics. Our work reports the first observation of such phenomena during extensional deformation and provides a unifying view on instabilities in complex flow fields.
Highlights
When you put a drop of saliva between your thumb and index finger, slowly pull your fingers apart, a slender fluid thread forms
We demonstrated that a heterogeneous distribution of the axial velocity component in the whole filament cross-section, with multiple bands of flow in opposite filament direction, results in global filament lifetimes of several minutes and remarkably long filaments of dilute surfactant solutions during uniaxial elongation
We find this heterogeneous flow behavior in the surfactant solutions investigated by Sachsenheimer et al.[9] and Omidvar et al.[10], who attributed the observation of such long filament lifetimes to the formation of elongation-induced structures (EIS) without further experimental evidence
Summary
When you put a drop of saliva between your thumb and index finger, slowly pull your fingers apart, a slender fluid thread forms. In this study we report a surprising phenomenon, as shown, for very dilute surfactant solutions with a low shear viscosity close to water and no measurable viscoelasticity (see Supplementary Information Figure S1) This solution forms remarkably long filaments when (a) a fluid drop drips from a nozzle, and when (b) pulling a rod out of a fluid reservoir (see Supplementary Information Movie S1). Many studies investigated the capillary thinning of viscoelastic, shear-thinning surfactant solutions[4,5,6,7,8], only two examined the uniaxial flow behavior of inelastic, shear-thickening surfactant solutions[9,10] They reported an unexpectedly slow thinning process during CaBER measurements, where fluid threads exhibit unusually long filament lifetimes on the order of several minutes - another striking feature of the flow behavior of dilute surfactant solutions in extension. The type of flow instability during uniaxial elongation observed for this model system turned out to occur in biopolymeric solutions such as saliva and hagfish slime
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