Yielding Is Gradual: New Insights from Large Amplitude Oscillatory Shear Rheology

Elastoviscoplastic fluids, such as toothpaste, uncured concrete, and drilling fluids, are classified as soft materials as they transition between solid-like and liquid-like behavior under the application of sufficient stress/deformation. Despite their ubiquity in applications ranging from consumer products to industrial systems, a method that can quantitatively describe yielding in these semisolid materials has eluded researchers. The lack of a well-defined characterization method for yielding severely limits the ability of researchers to develop materials with precision yield conditions and obscures the pursuit of a deeper understanding of the phenomenon.

To rectify this, a number of measurement approaches have been proposed to study yielding. The majority of these have been marred with inaccuracies, particularly when measuring the low shear and transient flow behavior of these soft materials. Further studies have been pursued to prove the existence of the yielding behavior. Despite the wide range of available tests for characterizing the yielding transition in soft materials, the accurate determination of the yield point remains inconclusive. The search for an acceptable approach has pushed researchers into employing various techniques and models, including: the soft glassy rheology model, shear transformation zone (STZ) theory and the large amplitude oscillatory shear (LAOS) approach. The LAOS approach is quite auspicious; however, more studies are still necessitated.

Recently, researchers at the University of Illinois at Urbana-Champaign, University of Western Ontario and Massachusetts Institute of Technology developed a rigorously-defined method that could enable mapping and quantification of the time-resolved dynamical yielding process of elastoviscoplastic materials. In particular, they aspired to build on the foundations of linear viscoelastic theory for oscillatory deformations to develop their novel technique for determining the yielding transition under strain-controlled LAOS. The research team included Professor Simon Rogers and graduate student Gavin Donley from Illinois, Professor John R. de Bruyn from Western Ontario and Professor Gareth McKinley from MIT and their study was recently published in Journal of Non-Newtonian Fluid Mechanics.

Rheological experiments were carried out on prepared samples, followed by steady shear measurements and oscillatory measurements. The team then proceeded to process the obtained data. For this, a MATLAB-based software utilizing a Sequence of Physical Processes (SPP) analysis was employed to process the time-resolved data. The SPP analysis was performed on data reconstructed via Fourier-domain filtering, using all odd harmonics identifiable above the noise floor. A Carbopol 980 microgel, a model soft material known to exhibit yielding behavior, was used for the study.

The authors outlined that the analysis of time-resolved rheological data was necessary to gain a full picture of yielding dynamics in a soft material. Secondly, they reported that yielding in a soft elastoviscoplastic solid such as Carbopol gels was a gradual transition. In addition, they revealed that the instantaneous phase angle was an effective metric to discern the macroscopic yield state of a material.

In summary, the study presented a novel technique for determining the yielding transition under strain-controlled LAOS. It was observed that the gradual nature of the yielding process seen in LAOS not only provided insight into the dynamics of how a material yields, but also explained why more traditional rheological measures of yielding have the specific form they do. Remarkably, the phase angle velocity was shown to be an effective measure of the yielding transition. Altogether, insights gained from this work will enable progress toward the goal of forming structure-property relationships that map the microstructural dynamics of a soft solid to the rheological response observed during yielding.