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1. Investigation of the yielding transition in concentrated colloidal systems via rheo-XPCS

   https://doi.org/10.1073/pnas.2215517120  

   Donley, Gavin J.; Narayanan, Suresh; Wade, Matthew A.; Park, Jun Dong; Leheny, Robert L.; Harden, James L.; Rogers, Simon A. (May 2023, Proceedings of the National Academy of Sciences)

   We probe the microstructural yielding dynamics of a concentrated colloidal system by performing creep/recovery tests with simultaneous collection of coherent scattering data via X-ray Photon Correlation Spectroscopy (XPCS). This combination of rheology and scattering allows for time-resolved observations of the microstructural dynamics as yielding occurs, which can be linked back to the applied rheological deformation to form structure–property relations. Under sufficiently small applied creep stresses, examination of the correlation in the flow direction reveals that the scattering response recorrelates with its predeformed state, indicating nearly complete microstructural recovery, and the dynamics of the system under these conditions slows considerably. Conversely, larger creep stresses increase the speed of the dynamics under both applied creep and recovery. The data show a strong connection between the microstructural dynamics and the acquisition of unrecoverable strain. By comparing this relationship to that predicted from homogeneous, affine shearing, we find that the yielding transition in concentrated colloidal systems is highly heterogeneous on the microstructural level. 

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2. Characterizing emergent multiscale dynamics in colloidal nanoparticle gels

   https://doi.org/10.1103/xn5z-cwjz  

   Brackett, William D; Sherman, Zachary M; Lehmkühler, Felix; Truskett, Thomas M; Milliron, Delia J (June 2026, Physical Review E)

   Colloidal gels assembled from nanoparticles (NPs) are a versatile class of soft network-based materials capable of rich dynamic, mechanical, and even optical or magnetic responses to stimuli. Their behaviors are governed by dynamics of heterogeneous structures coupled across multiple length and timescales. Observable dynamics range from nanoparticle diffusion and clustering to mesoscopic cluster dynamics and interactions to localized or collective network relaxations. Understanding how these hierarchically organized processes relate to macroscopic network properties remains a broad and unresolved problem in soft matter physics. The mechanisms of gel formation can depend sensitively on the pathway and the nature of NP interactions, thus far preventing a unified theoretical bridge between nanoscopic interactions, structural evolution, and network dynamics. Indirect measurement of dynamics using light-scattering techniques provides an experimental means to quantify underlying particle and network motion. The rich dynamic behavior of NP gels warrants consideration of a broad range of models to help interpret nonlinear relaxation phenomena such as anomalous diffusion, nonergodicity, and intrinsically nonequilibrium or mechanically driven dynamics. X-ray photon correlation spectroscopy (XPCS) has emerged as a powerful tool for probing nanoscopic motion in nanoparticle gels but alone cannot resolve the full spatiotemporal spectrum of dynamics that drive gelation, aging, and network mechanical properties. While rheo-XPCS enables simultaneous probing of nanoscale and bulk mechanical responses, complementary light scattering, microscopy, or simulations can extend spatiotemporal characterization and, consequently, understanding of NP gel network physics. Implementing a modular model platform with tunable primary nanoparticle features allows systematic variation of nanoscopic characteristics that drive emergent gel responses and inform the development of theoretical models for a wide range of soft, dynamic, nanostructured materials. Gels formed from particles with unique structural proxies, such as electromagnetic coupling in plasmonic NPs, provide additional metrics for model validation and offer opportunities to develop computational methods for the efficient and accurate replication of NP gel properties. The rapid expansion of XPCS capabilities at fourth-generation light sources, combined with complementary tools and robust model systems, positions the field to move beyond descriptive fundamental studies toward the design of nanoparticle gels with adaptive and programmable behaviors. 

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3. Scaling of relaxation and excess entropy in plastically deformed amorphous solids

   https://doi.org/10.1073/pnas.2000698117  

   Galloway, K. Lawrence; Ma, Xiaoguang; Keim, Nathan C.; Jerolmack, Douglas J.; Yodh, Arjun G.; Arratia, Paulo E. (June 2020, Proceedings of the National Academy of Sciences)

   null (Ed.)

   When stressed sufficiently, solid materials yield and deform plastically via reorganization of microscopic constituents. Indeed, it is possible to alter the microstructure of materials by judicious application of stress, an empirical process utilized in practice to enhance the mechanical properties of metals. Understanding the interdependence of plastic flow and microscopic structure in these nonequilibrium states, however, remains a major challenge. Here, we experimentally investigate this relationship, between the relaxation dynamics and microscopic structure of disordered colloidal solids during plastic deformation. We apply oscillatory shear to solid colloidal monolayers and study their particle trajectories as a function of shear rate in the plastic regime. Under these circumstances, the strain rate, the relaxation rate associated with plastic flow, and the sample microscopic structure oscillate together, but with different phases. Interestingly, the experiments reveal that the relaxation rate associated with plastic flow at time t is correlated with the strain rate and sample microscopic structure measured at earlier and later times, respectively. The relaxation rate, in this nonstationary condition, exhibits power-law, shear-thinning behavior and scales exponentially with sample excess entropy. Thus, measurement of sample static structure (excess entropy) provides insight about both strain rate and constituent rearrangement dynamics in the sample at earlier times. 

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4. Brittle and ductile yielding in soft materials

   https://doi.org/10.1073/pnas.2401409121  

   Kamani, Krutarth M; Rogers, Simon A (May 2024, Proceedings of the National Academy of Sciences)

   Many soft materials yield under mechanical loading, but how this transition from solid-like behavior to liquid-like behavior occurs can vary significantly. Understanding the physics of yielding is of great interest for the behavior of biological, environmental, and industrial materials, including those used as inks in additive manufacturing and muds and soils. For some materials, the yielding transition is gradual, while others yield abruptly. We refer to these behaviors as being ductile and brittle. The key rheological signatures of brittle yielding include a stress overshoot in steady-shear-startup tests and a steep increase in the loss modulus during oscillatory amplitude sweeps. In this work, we show how this spectrum of yielding behaviors may be accounted for in a continuum model for yield stress materials by introducing a parameter we call the brittility factor. Physically, an increased brittility decreases the contribution of recoverable deformation to plastic deformation, which impacts the rate at which yielding occurs. The model predictions are successfully compared to results of different rheological protocols from a number of real yield stress fluids with different microstructures, indicating the general applicability of the phenomenon of brittility. Our study shows that the brittility of soft materials plays a critical role in determining the rate of the yielding transition and provides a simple tool for understanding its effects under various loading conditions. 

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5. Elucidating the G″ overshoot in soft materials with a yield transition via a time-resolved experimental strain decomposition

   https://doi.org/10.1073/pnas.2003869117  

   Donley, Gavin J.; Singh, Piyush K.; Shetty, Abhishek; Rogers, Simon A. (September 2020, Proceedings of the National Academy of Sciences)

   null (Ed.)

   Materials that exhibit yielding behavior are used in many applications, from spreadable foods and cosmetics to direct write three-dimensional printing inks and filled rubbers. Their key design feature is the ability to transition behaviorally from solid to fluid under sufficient load or deformation. Despite its widespread applications, little is known about the dynamics of yielding in real processes, as the nonequilibrium nature of the transition impedes understanding. We demonstrate an iteratively punctuated rheological protocol that combines strain-controlled oscillatory shear with stress-controlled recovery tests. This technique provides an experimental decomposition of recoverable and unrecoverable strains, allowing for solid-like and fluid-like contributions to a yield stress material’s behavior to be separated in a time-resolved manner. Using this protocol, we investigate the overshoot in loss modulus seen in materials that yield. We show that this phenomenon is caused by the transition from primarily solid-like, viscoelastic dissipation in the linear regime to primarily fluid-like, plastic flow at larger amplitudes. We compare and contrast this with a viscoelastic liquid with no yielding behavior, where the contribution to energy dissipation from viscous flow dominates over the entire range of amplitudes tested. 

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