Extensional rheology and flow-induced structure formation of wormlike micelle solutions
Experimental and Theoretical Physics of Polymers
Polymer Materials
Final Report Abstract
In this work, we examine the elongational flow behavior of a series of commonly studied surfactant solutions covering a broad range of surfactant and salt concentration, focusing on dilute solutions at low salt/surfactant ratio. Employing flow velocimetry on horizontally stretched filaments, we discovered a new banding phenomenon, governing the flow in so-called CaBER experiments. This flow phenomenon is characterized by a heterogeneous distribution of the axial velocity component π£π§ with multiple bands of flow in both negative and positive filament direction. These bands occur randomly in the whole filament cross-section and lead to an additional flow from the reservoir at the filament ends back into the filament besides the expected surface tension-driven flow from the filament towards the reservoirs. The additional flow back into the filament results in global filament lifetimes π‘πππ of several minutes, i.e. up to six orders of magnitude larger than expected based on the Newtonian shear viscosity of the respective solution. Moreover, in filament stretching and falling drop experiments, we are able to create surfactant filaments with lengths of more than 10 cm. Our results disclose the nature of the recently proposed elongation-induced structure (EIS) phenomenon of low concentrated surfactant solutions on a mesoscopic length scale. Upon increase of surfactant concentration and/or salt/surfactant ratio, the flow profile in the filament cross-section turns homogeneous as expected for viscoelastic fluids. By carefully investigating the flow behavior inside surfactant filaments covering a surfactant concentration regime of 5 β€ ππ π’ππ β€ 100 mM, we are able to construct uniaxial extensional flow stability maps for some of the most commonly studied surfactant systems CTAB, CPyCl, CTAC and CTAT. Further, we use a microfluidic optimized-shape cross-slot rheometer (OSCER), combined with flow velocimetry and birefringence imaging, to explore the critical conditions in terms of elongation rate π' and strain π required for the onset of banding instabilities in planar elongation. While the flow behavior of surfactant solutions with higher concentration resembles elastic instabilities as previously reported for polymer and wormlike micellar solutions, a striking difference appears for solutions exhibiting heterogeneous flow during uniaxial deformation as described above. Here, we find a timedependent transition from a symmetric flow towards a large-scale symmetry-breaking of the flow field at a critical elongation rate. Prior to this transition, a single localized birefringent strand emerges along the outflow direction. However, this strand splits into several individual strands that start to fluctuate laterally indicating multiple stresses at one global rate. A minimum elongation rate π'π , depending on sample composition, and a critical total strain ππ , independent of strain rate, are required for this phenomenon to arise. Similar strain rates and total strains are also experienced by the fluids during the initial step stretch in CaBER experiments. The dilute surfactant systems exhibiting these unusual flow instabilities in extensional deformation show non-monotonic shear stress π versus shear rate Γ½Μ curves in rotational rheometry, previously related to banding in vorticity direction and flow-induced structure buildup. The critical shear rate required to induce vorticity banding with different values of stress at a constant shear rate is in the same range as the critical elongation rate causing the instability in planar extension. By relating the results in elongation to the hysteretic flow curve, we propose a heterogeneous flow-induced structure buildup similar to the non-homogeneous, banded flow. However, the total strain required to trigger this phenomenon is larger in shear than in elongation. Although we focus on elongational flow behavior of dilute surfactant solutions as a model system in this study, preliminary results indicate that low viscous biopolymeric fluids such as saliva and hagfish mucus also show banded flow during filament thinning. However, further investigations will be required to elucidate the origin for the heterogeneous flow phenomenon in these fluids, which might not be necessarily the same as for the low concentrated surfactant solutions. Our experimental method can be readily extended to study the flow inside other low viscous fluid filaments and our data complement a unique flow phenomenon to the rich variety of banding instabilities in complex fluids. Thus our study fills the remaining knowledge gap regarding such phenomena occurring in extensional flows, ubiquitous in nature and technology.
Publications
- Heterogeneous Flow Inside Threads of Low Viscosity Fluids Leads to Anomalous Long Filament Lifetimes, Scientific Reports (2019)
S. M. Recktenwald, S. J. Haward, A. Q. Shen, and N. Willenbacher
(See online at https://doi.org/10.1038/s41598-019-43590-z)
