Colloidal glasses are suspensions of small solid particles in a liquid. When placed under stress, they transition from a solid-like to fluid-like state. These properties make colloidal suspensions, including emulsions, foams, and microgels, well-suited for a variety of applications such as spreadable foods and personal care products.
The development of additional commercial products based on these materials is hampered by a lack of knowledge regarding the direct connections between the macroscopic flow/deformation of colloidal glasses and suspensions and their microscopic structures. To fill in this knowledge gap, researchers used simultaneous stress-controlled rheology and X-ray photon correlation spectroscopy at the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Argonne National Laboratory. By probing the onset of yielding in a common concentrated colloidal suspension, they determined that the transition from recoverable to unrecoverable deformation is strongly linked to the loss of structural memory and the acceleration of nanoscale fluctuations within the glass.
A wide array of soft materials including concentrated colloidal suspensions and emulsions, foams, and microgels exhibit glass-like properties. These materials can undergo yielding—i.e., transitioning from solid-like to liquid-like state—if enough stress is applied to them. In order to precisely design novel colloidal systems that exhibit on-demand yielding, it is critical to understand the specific physical processes that occur during this transition.
This development is poised to bring advances to various materials engineering challenges, ranging from the formulation of better 3D printing inks to the construction of wearable flexible electronics and sensors, the accurate printing of biomedical implants, and even improving the textures of processed foods and personal care products.
Yielding has long been thought of as an instantaneous transition to a flowing state at a fixed-stress threshold. More recent studies have shown both the importance of elasticity to the process of yielding and the gradual nature of the transition. Additional work has also linked the processes of yielding and unyielding to the loss and formation of material memory in glassy systems. Yet no study has been able to determine how yielding evolves, nor has any research distinguished between elastic deformation and plastic or viscous flow in a time-resolved manner.
Recovery tests, however, have recently been used to study the onset of the yielding transition by directly probing the reversibility of any deformation. These tests have shown that materials begin to acquire strain unrecoverably once the linear regime is exceeded, and then transition to steady flow at larger applied deformations. Despite these advances, a precise understanding of when the yielding transition occurs, and the relationship between the unrecoverable strain and underlying microstructural changes, has remained elusive.
Researchers at the University of Illinois at Urbana-Champaign, Georgetown University, the National Institute of Standards and Technology, Argonne National Laboratory, Sookmyung Women’s University, and the University of Ottawa performed X-ray experiments on beamline 8-ID-I at the Advanced Photon Source (APS) to investigate the microstructural evolution of the concentrated colloidal suspension Ludox TM-50 using persistent mechanical stress and recovery protocols. The team used the previously mentioned technique of combining stress-controlled rheology and X-ray photon correlation spectroscopy (rheo-XPCS).
The researchers’ rheo-XPCS tests demonstrated that the macroscopic flow behavior of colloidal suspensions is affected by both the magnitude of the persistent stress and the time spent under stress, demonstrating that the yielding transition is more complicated than an abrupt transition at a constant stress threshold. The rheo-XPCS measurements show this complex dependence on shear history extends to the structural memory and nanoscale dynamics of the material.
Specifically, the researchers found that the unrecoverable strain is associated with microscopically heterogeneous changes, such that surprisingly large correlations in the microstructure persist before and after yielding. Additionally, although slowly applied stress is observed to slow the intrinsic dynamics in the glass, small unrecoverable strains somehow act to constrain positional fluctuations of the particles in the glassy matrix. These observations suggest that the yielding transition in these systems manifests first as a localized hardening with applied stress followed by the onset of heterogeneous, irreversible flow.
While the researchers focused on connecting the rheo-XPCS correlations to simple stress and recovery experiments, future experiments with more complex flow phenomena such as oscillatory shear may shed even more light on the coupling of structural memory and dynamics to applied deformation, and further guide the design of future products based on colloidal suspensions.
The in-progress upgrade to the APS will dramatically improve the coherence of the X-ray beams one-hundred-fold, which will extend XPCS to higher energies enabling more direct observations of materials under real operating conditions. - Chris Palmer
See: G.J. Donley1,2,3,, S. Narayanan4, M.A. Wade1, J.D. Park5, R.L. Leheny6, J.L. Harden7, S.A. Rogers1,, “Investigation of the yielding transition in concentrated colloidal systems via rheo-XPCS,” Proceedings of the National Academy of Sciences, 120 (18) e2215517120 (2023).
Author affiliations: 1University of Illinois Urbana-Champaign; 2Georgetown University; 3National Institute of Standards and Technology; 4Argonne National Laboratory; 5Sookmyung Women’s University; 6Johns Hopkins University; 7University of Ottawa
The authors thank Anton Paar for the use of the MCR 702 TwinDrive rheometer for offline measurements through their academic VIP program, and the Zhao lab at UIUC for assistance with sample preparation. The authors also thank Dr. Piyush Singh and Jiho Choi of UIUC for assistance with experiments. This research was performed at beamline 8-ID-I of the Advanced Photon Source and the Center for Nanoscale Materials, US Department of Energy (DOE) Office of Science User Facilities operated for the DOE Office of Science by ANL (contract No. DE-AC02-
382 06CH11357). This material is based on work supported by NSF Grant No. 1847389 and the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the US Department of Energy’s National Nuclear Security Administration Contract No. DE-NA0003525. R.L.L. acknowledges funding from NSF Grant No. CBET-1804721. J.L.H. acknowledges the support of the Natural Sciences and Engineering Research Council of Canada (NSERC) through the Discovery Grant Program.
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