Passive non-linear microrheology for determining extensional viscosity

Abstract

Extensional viscosity is a key property of complex fluids that greatly influences the non-equilibrium behavior and processing of polymer solutions, melts, and colloidal suspensions. In this work, we use microfluidics to determine steady extensional viscosity for polymer solutions by directly observing particle migration in planar extensional flow. Tracer particles are suspended in semi-dilute solutions of DNA and polyethylene oxide, and a Stokes trap is used to confine single particles in extensional flows of polymer solutions in a cross-slot device. Particles are observed to migrate in the direction transverse to flow due to normal stresses, and particle migration is tracked and quantified using a piezo-nanopositioning stage during the microfluidic flow experiment. Particle migration trajectories are then analyzed using a second-order fluid model that accurately predicts that migration arises due to normal stress differences. Using this analytical framework, extensional viscosities can be determined from particle migration experiments, and the results are in reasonable agreement with bulk rheological measurements of extensional viscosity based on a dripping-onto-substrate method. Overall, this work demonstrates that non-equilibrium properties of complex fluids can be determined by passive yet non-linear microrheology.

Macromolecular relaxation, strain, and extensibility determine elastocapillary thinning and extensional viscosity of polymer solutions

“…Each measurement is repeated at least five times, and a good reproducibility is observed. Further details about the DoS rheometry setup, contrast with other techniques that use capillary-driven thinning and breakup measurements, and measurement of concentrationdependent extensional relaxation time of neutral and charged polymer solutions can be found in our previous publications (22)(23)(24)(25)(26).…”

“…We have established that the DoS rheometry protocols that rely on the visualization and analysis of the radius evolution of a thinning fluid neck formed between a nozzle and a sessile drop on a substrate allow measurements of extensional relaxation time and pinch-off dynamics of complex fluids (22)(23)(24)(25)(26), including low viscosity/elasticity fluids, deemed inaccessible in CaBER measurements. In this paper, using the radius evolution data for semidilute aqueous PEO [poly(ethylene oxide)] solutions of two distinct molecular weights acquired using the DoS rheometry protocols, we report systematic characterization and analysis of both EC and TVEC regimes as well as the two transitions, IC/VC-EC and EC-TVEC, that are typically inaccessible in the CaBER measurements (3,58,59,(70)(71)(72)(73)(74)(75)(76)(77) and were observed, but not characterized, in the previous DoS rheometry studies (22)(23)(24)(25)(26)(80)(81)(82)(83)(84). In addition to quantifying the conventional measures including λ E and η ∞ E here we introduce and evaluate three additional measures: the scaled EC span, Δt EC =λ E , the variation in scaled transient extensional viscosity…”

“…polymer physics | rheology | processing | extensional rheology | interfacial flows Q uantitative understanding of the role played by material properties in determining capillary-driven thinning and breakup is critically important for jetting (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13), dripping (14)(15)(16)(17)(18)(19)(20) and dispensing (21)(22)(23)(24)(25)(26), liquid-bridge breakup in printing (27)(28)(29), atomization and spraying (30)(31)(32), drop fission under strong extensional flows (33,34), and drop and emulsion formation in microfluidics or from membranes (35)(36)(37)(38)(39). Longer-lived viscoelastic filaments are also accredited with the stickiness of saliva (40,41) and deadly fluids produced by carnivorous plants (42,43), as well as increasing fiber spinnability (44)(45)…”

“…Understanding and controlling the response of polymer solutions to extensional flows, manifested as delayed thinning and pinch-off, requires characterization and analysis of extensional viscosity, extensional relaxation time, and finite extensibility effects, as well as knowledge of the influence of macromolecular properties on pinch-off dynamics. Such properties cannot be captured by conventional shear rheology characterization or by the use of conventional extensional rheology techniques in which free surface flows are absent, as detailed elsewhere (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(21)(22)(23)(24)(25)(26). In this contribution, we carry out a detailed and quantitative analysis of neck-thinning dynamics using dripping-onto-substrate (DoS) rheometry protocols we developed recently (22)(23)(24)(25)(26) to probe and report the hitherto unexplored correlations between macromolecular properties (relaxation, strain, and finite extensibility) and the visco-elastocapillary thinning dynamics, advancing our ability to carry out macromolecular engineering of formulations that need to be jetted, printed, sprayed, or dispensed.…”

Universal dynamics of dilute and semidilute solutions of flexible linear polymers

“…The quantification of the extensional viscosity of semidilute polymer solutions has only recently been made possible due to developments in solution preparation, and in measurement techniques [94,[108][109][110][111]. The universal aspects of the behaviour, have however, not yet been examined experimentally or theoretically because the field is in an early stage of development.…”