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1. Dynamics and memory of boundary stresses in discontinuous shear thickening suspensions during oscillatory shear

   https://doi.org/10.1039/D0SM01917H  

   Rathee, Vikram ; Blair, Daniel L. ; Urbach, Jeffrey S. ( February 2021 , Soft Matter)

   null (Ed.)

   We report direct measurements of spatially resolved surface stresses of a dense suspension during large amplitude oscillatory shear (LAOS) in the discontinuous shear thickening regime using boundary stress microscopy. Consistent with previous studies, bulk rheology shows a dramatic increase in the complex viscosity above a frequency-dependent critical strain. We find that the viscosity increase is coincident with that appearance of large heterogeneous boundary stresses, indicative of the formation of transient solid-like phases (SLPs) on spatial scales large compared to the particle size. The critical strain for the appearance of SLPs is largely determined by the peak oscillatory stress, which depends on the peak shear rate and the frequency-dependent suspension viscosity. The SLPs dissipate and reform on each cycle, with a spatial pattern that is highly variable at low frequencies but remarkably persistent at the highest frequency measured ( ω = 10 rad s −1 ). 

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2. Mechanical properties of polymeric microfiltration membranes

   https://doi.org/10.1016/j.memsci.2019.117351  

   Elele, E. ; Shen, Y. ; Tang, J. ; Lei, Q. ; Khusid, B. ; Tkacik, G. ; Carbrello, C. ( August 2019 , Journal of membrane science)

   The influence of the pore topology and polymer properties on mechanical characteristics of asymmetric polyethersulfone (PES) and symmetric polyvinylidene fluoride (PVDF) microfiltration membranes was investigated by conducting elongation, creep, stress relaxation, small-amplitude oscillatory and bubble point pressure tests. The main aspects of the membrane stress-strain curves were found to be similar despite significant differences in the pore topology and polymer properties. While the Kelvin-Voigt model for solid polymers described the membrane viscoelastic response below the transition to ductile yielding, the stress-strain curves of membranes and solid polymers above the yield point appeared to be drastically different. All tested membranes demonstrated weak strain hardening, low sensitivity to strain rate, significant elastic recovery, stress relaxation and reduction of the bubble point pressure with accumulation of plastic deformation. Therefore, tensile stresses exerted on a membrane under assembling and process conditions should be smaller than the yield stress to assure that they will not impair filter performance. The novelty of our approach is the use of models for perforated plates to evaluate membrane mechanical properties as ductile yielding for both proceeds via localized plastic deformation around pores. Presented results provide a reliable framework for development of membranes with properties tailored to applications. 

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3. Strain shift measured from stress-controlled oscillatory shear: Evidence for a continuous yielding transition and new techniques to determine recovery rheology measures

   https://doi.org/10.1122/8.0000756  

   Griebler, James J ; Donley, Gavin J ; Wisniewski, Victoria ; Rogers, Simon A ( May 2024 , Journal of Rheology)

   Understanding the yielding of complex fluids is an important rheological challenge that affects our ability to engineer and process materials for a wide variety of applications. Common theoretical understandings of yield stress fluids follow the Oldroyd–Prager formalism in which the material behavior below the yield stress is treated as solidlike, and above the yield stress as liquidlike, with an instantaneous transition between the two states. This formalism was built on a quasi-static approach to the yield stress, while most applications, ranging from material processing to end user applications, involve a transient approach to yielding over a finite timescale. Using stress-controlled oscillatory shear experiments, we show that yield stress fluids flow below their yield stresses. This is quantified through measuring the strain shift, which is the value about which the strain oscillates during a stress-controlled test and is a function of only the unrecoverable strain. Measurements of the strain shift are, therefore, measurements of flow having taken place. These experimental results are compared to the Herschel–Bulkley form of the Saramito model, which utilizes the Oldroyd–Prager formalism, and the recently published Kamani–Donley–Rogers (KDR) model, in which one constitutive equation represents the entire range of material responses. Scaling relationships are derived, which allow us to show why yield stress fluids will flow across all stresses, above and below their yield stress. Finally, derivations are presented that show strain shift can be used to determine average metrics previously attainable only through recovery rheology, and these are experimentally verified.

    

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4. Time-resolved microstructural changes in large amplitude oscillatory shear of model single and double component soft gels

   https://doi.org/10.1122/8.0000486  

   Donley, Gavin J. ; Bantawa, Minaspi ; Del Gado, Emanuela ( November 2022 , Journal of Rheology)

   Soft particulate gels can reversibly yield when sufficient deformation is applied, and the characteristics of this transition can be enhanced or limited by designing hybrid hydrogel composites. While the microscopic dynamics and macroscopic rheology of these systems have been studied separately in detail, the development of direct connections between the two has been difficult, particularly with regard to the nonlinear rheology. To bridge this gap, we perform a series of large amplitude oscillatory shear (LAOS) numerical measurements on model soft particulate gels at different volume fractions using coarse-grained molecular dynamics simulations. We first study a particulate network with local bending stiffness and then we combine it with a second component that can provide additional cross-linking to obtain two-component networks. Through the sequence of physical processes (SPP) framework, we define time-resolved dynamic moduli, and by tracking the changes in these moduli through the period, we can distinguish transitions in the material behavior as a function of time. This approach helps us establish the microscopic origin of the nonlinear rheology by connecting the changes in dynamic moduli to the corresponding microstructural changes during the deformation including the nonaffine displacement of particles, and the breakage, formation, and orientation of bonds.

    

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5. Rheology of Candida albicans fungal biofilms

   https://doi.org/10.1122/8.0000427  

   Beckwith, Joanne K. ; Ganesan, Mahesh ; VanEpps, J. Scott ; Kumar, Anuj ; Solomon, Michael J. ( July 2022 , Journal of Rheology)

   Fungi such as Candida albicans exist in biofilm phenotypes, which present as viscoelastic materials; however, a method to measure linear viscoelastic moduli, yield stress, and yield strain is lacking. Characterization methods for fungal materials have been limited to techniques specific to particular industries. Here, we present a method to measure the shear stress, strain amplitude, and creep of C. albicans BWP17 biofilms. Our method includes features tailored to the analysis of fungi including an in vitro growth protocol attuned to the slow growth rates of C. albicans biofilms and a resultant cultured biofilm that has sufficient integrity to be transferred to the rheometer tooling without disrupting its structure. The method's performance is demonstrated by showing that results are insensitive to gap, evaporative sealant, length of experiment, and specimen radius. Multiscale imaging of the fungal biofilm showed complex entanglement networks at the hundred-micrometer scale. For a wild-type strain cultivated for 14 days, using small-amplitude oscillatory rheology, we found that the elastic (G′) and viscous (G″) moduli were nearly independent of frequency over the range 0.1–10 s −1 , with magnitudes of [Formula: see text] and [Formula: see text], respectively. The yield stress was approximately [Formula: see text]. We modeled the linear creep response of the fungal biofilm and found that C. albicans has a characteristic relaxation time of [Formula: see text] and a viscosity of [Formula: see text]. We applied this method to probe the effects of altered chitin deposition in the C. albicans cell wall. Differences between the biofilm's phenotypic cell shape and rheological properties in mutants with altered chitin synthase activity were resolved. Discovering how genotypic, phenotypic, and environmental factors impact the material properties of these microbial communities can have implications for understanding fungal biofilm growth and aid in the development of remediation strategies. 

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