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1. Stress-activated friction in sheared suspensions probed with piezoelectric nanoparticles

   https://doi.org/10.1073/pnas.2310088120  

   Kim, Hojin; Esser-Kahn, Aaron P.; Rowan, Stuart J.; Jaeger, Heinrich M. (December 2023, Proceedings of the National Academy of Sciences)

   Weitz, David (Ed.)

   A hallmark of concentrated suspensions is non-Newtonian behavior, whereby the viscosity increases dramatically once a characteristic shear rate or stress is exceeded. Such strong shear thickening is thought to originate from a network of frictional particle–particle contact forces, which forms under sufficiently large stress, evolves dynamically, and adapts to changing loads. While there is much evidence from simulations for the emergence of this network during shear thickening, experimental confirmation has been difficult. Here, we use suspensions of piezoelectric nanoparticles and exploit the strong local stress focusing within the network to activate charge generation. This charging can then be detected in the measured ac conductance and serve as a signature of frictional contact formation. The direct link between stress-activated frictional particle interactions and piezoelectric suspension response is further demonstrated by tracking the emergence of structural memory in the contact network under oscillatory shear and by showing how stress-activated friction can drive mechano-transduction of chemical reactions with nonlinear reaction kinetics. Taken together, this makes the ac conductance of piezoelectric suspensions a sensitive in-situ reporter of the micromechanics associated with frictional interactions. 

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2. Scaling Analysis of Shear Thickening Suspensions

   https://doi.org/10.3389/fphy.2022.946221  

   Malbranche, Nelya; Santra, Aritra; Chakraborty, Bulbul; Morris, Jeffrey F. (July 2022, Frontiers in Physics)

   Dense suspensions of particles in viscous liquid often demonstrate the striking phenomenon of abrupt shear thickening, where their viscosity increases strongly with increase of the imposed stress or shear rate. In this work, discrete-particle simulations accounting for short-range hydrodynamic, repulsive, and contact forces are performed to simulate flow of shear thickening bidisperse suspensions, with the packing parameters of large-to-small particle radius ratio δ = 3 and large particle fraction ζ = 0.15, 0.50, and 0.85. The simulations are carried out for volume fractions 0.54 ≤ ϕ ≤ 0.60 and a wide range of shear stresses. The repulsive forces, of magnitude F R , model the effects of surface charge and electric double-layer overlap, and result in shear thinning at small stress, with shear thickening beginning at stresses σ ∼ F R a −2 . A crossover scaling analysis used to describe systems with more than one thermodynamic critical point has recently been shown to successfully describe the experimentally-observed shear thickening behavior in suspensions. The scaling theory is tested here on simulated shear thickening data of the bidisperse mixtures, and also on nearly monodisperse suspensions with δ = 1.4 and ζ = 0.50. Presenting the viscosity in terms of a universal crossover scaling function between the frictionless and frictional maximum packing fractions collapses the viscosity for most of the suspensions studied. Two scaling regimes having different exponents are observed. The scaling analysis shows that the second normal stress difference N 2 and the particle pressure Π also collapse on their respective curves, with the latter featuring a different exponent from the viscosity and normal stress difference. The influence of the fraction of frictional contacts, one of the parameters of the scaling analysis, and its dependence on the packing parameters are also presented. 

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3. Elasto-Inertial Focusing Mechanisms of Particles in Shear-Thinning Viscoelastic Fluid in Rectangular Microchannels

   https://doi.org/10.3390/mi13122131  

   Naderi, Mohammad Moein; Barilla, Ludovica; Zhou, Jian; Papautsky, Ian; Peng, Zhangli (December 2022, Micromachines)

   Growth of the microfluidics field has triggered numerous advances in focusing and separating microparticles, with such systems rapidly finding applications in biomedical, chemical, and environmental fields. The use of shear-thinning viscoelastic fluids in microfluidic channels is leading to evolution of elasto-inertial focusing. Herein, we showed that the interplay between the elastic and shear-gradient lift forces, as well as the secondary flow transversal drag force that is caused by the non-zero second normal stress difference, lead to different particle focusing patterns in the elasto-inertial regime. Experiments and 3D simulations were performed to study the effects of flowrate, particle size, and the shear-thinning extent of the fluid on the focusing patterns. The Giesekus constitutive equation was used in the simulations to capture the shear-thinning and viscoelastic behaviors of the solution used in the experiments. At low flowrate, with Weissenberg number Wi ~ O(1), both the elastic force and secondary flow effects push particles towards the channel center. However, at a high flowrate, Wi ~ O(10), the elastic force direction is reversed in the central regions. This remarkable behavior of the elastic force, combined with the enhanced shear-gradient lift at the high flowrate, pushes particles away from the channel center. Additionally, a precise prediction of the focusing position can only be made when the shear-thinning extent of the fluid is correctly estimated in the modeling. The shear-thinning also gives rise to the unique behavior of the inertial forces near the channel walls which is linked with the ‘warped’ velocity profile in such fluids. 

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4. Emergence of viscosity and dissipation via stochastic bonds

   https://doi.org/10.1016/j.jmps.2021.104660  

   Leadbetter, Travis; Seiphoori, Ali; Reina, Celia; Purohit, Prashant K. (January 2022, Journal of the mechanics and physics of solids)

   “Viscosity is the most ubiquitous dissipative mechanical behavior” (Maugin, 1999). Despite its ubiquity, even for those systems where the mechanisms causing viscous and other forms of dissipation are known there are only a few quantitative models that extract the macroscopic rheological response from these microscopic mechanisms. One such mechanism is the stochastic breaking and forming of bonds which is present in polymer networks with transient cross-links, strong inter-layer bonding between graphene sheets, and sliding dry friction. In this paper we utilize a simple yet flexible model to show analytically how stochastic bonds can induce an array of rheological behaviors at the macroscale. We find that varying the bond interactions induces a Maxwell-type macroscopic material behavior with Newtonian viscosity, shear thinning, shear thickening, or solid like friction when subjected to shear at constant rates. When bond rupture is independent of the force applied, Newtonian viscosity is the predominant behavior. When bond breaking is accelerated by the applied force, a shear thinning response becomes most prevalent. Further connections of the macroscopic response to the interaction potential and rates of bonding and unbonding are illustrated through phase diagrams and analysis of limiting cases. Finally, we apply this model to polymer networks and to experimental data on “solid bridges” in polydisperse granular media. We imagine possible applications to material design through engineering bonds with specific interactions to bring about a desired macroscopic behavior. 

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5. The deformation of marine snow enables its disaggregation in simulated oceanic shear

   https://doi.org/10.3389/fmars.2023.1224518  

   Song, Yixuan; Burd, Adrian B; Rau, Matthew J (July 2023, Frontiers in Marine Science)

   Understanding the effect of hydrodynamics on aggregate size and structure is key to predicting mass transport in the aquatic environment. Aggregation theory of particles is well established but our knowledge of deformation processes, biological bonding forces, and their effects on fragmentation of aquatic aggregates is still limited. To better comprehend fragmentation processes and adhesion forces we implemented breakup experiments with diatom and microplastic aggregates made in the laboratory. We captured a substantial number of events showing deformation and subsequent fragmentation of these aggregates in an oscillatory shear flow. Polystyrene and polyethylene aggregates showed distinct fragmentation strengths and provided comparative upper and lower limits to the biological bonding strength of the diatom aggregates. Additionally, we employed a force balance model to evaluate attractive interactions within clusters of particles using the Lagrangian stress history and morphology. We found that the fractal structures of aggregates led to a power law of breakup strength with size and that time-integrated stress governed the overall fragmentation process. We also found that the weakening of the aggregates through deformation with shear exposure enabled their disaggregation at very low shear rates typical of the ocean environment. 

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