Viscosity of cohesive granular flows

Mechanical Properties of Conducting Printed Nanosheet Network Thin Films Under Uniaxial Compression

Thin film networks of solution processed nanosheets show remarkable promise for use in a broad range of applications including strain sensors, energy storage, printed devices, textile electronics, and more. While it is known that their electronic properties rely heavily on their morphology, little is known of their mechanical nature, a glaring omission given the effect mechanical deformation has on the morphology of porous systems and the promise of mechanical post processing for tailored properties. Here, this work employs a recent advance in thin film mechanical testing called the Layer Compression Test to perform the first in situ analysis of printed nanosheet network compression. Due to the well-defined deformation geometry of this unique test, this work is able to explore the out-of-plane elastic, plastic, and creep deformation in these systems, extracting properties of elastic modulus, plastic yield, viscoelasticity, tensile failure and sheet bending vs. slippage under both out of plane uniaxial compression and tension. This work characterizes these for a range of networks of differing porosities and sheet sizes, for low and high compression, as well as the effect of chemical cross linking. This work explores graphene and MoS2 networks, from which the results can be extended to printed nanosheet networks as a whole.

Rheology of debris flow materials is controlled by the distance from jamming

Debris flows are fast-flowing and dangerous slurries of soil and water that often form when intense rainfall soaks hillsides burned by wildfire. As climate change intensifies this hazard, models capable of predicting failure and flow behaviors are needed. Here we capitalize on recent progress in the physics of dense suspensions, to determine how the physical and chemical composition of natural hillslope soils controls the viscosity and yield stress of debris flows. We show how a simple flow model—previously developed for idealized suspensions—can be extended to highly heterogeneous, natural debris flow materials. This model reconciles previously contradictory observations and could help to improve computer models that assess the hazard potential of debris flows in the field.

Debris flows are dense and fast-moving complex suspensions of soil and water that threaten lives and infrastructure. Assessing the hazard potential of debris flows requires predicting yield and flow behavior. Reported measurements of rheology for debris flow slurries are highly variable and sometimes contradictory due to heterogeneity in particle composition and volume fraction (ϕ) and also inconsistent measurement methods. Here we examine the composition and flow behavior of source materials that formed the postwildfire debris flows in Montecito, CA, in 2018, for a wide range of ϕ that encapsulates debris flow formation by overland flow. We find that shear viscosity and yield stress are controlled by the distance from jamming, Δϕ=ϕm−ϕ, where the jamming fraction ϕm is a material parameter that depends on grain size polydispersity and friction. By rescaling shear and viscous stresses to account for these effects, the data collapse onto a simple nondimensional flow curve indicative of a Bingham plastic (viscoplastic) fluid. Given the highly nonlinear dependence of rheology on Δϕ, our findings suggest that determining the jamming fraction for natural materials will significantly improve flow models for geophysical suspensions such as hyperconcentrated flows and debris flows.

How cohesion controls the roughness of a granular deposit

Cohesive granular materials often form clusters of grains, which alter their flowing properties. How these clusters form and evolve is difficult to visualize in the bulk of the material, and thus to model. Here, we use a proxy to investigate the formation of such clusters, which is the rough surface of a cohesive granular deposit. We characterize this roughness and show how it is related to the cohesion between beads. Specifically, the size of this roughness increases with the inter-particle cohesion, and the profile exhibits a self-affine behaviour, as observed for crack paths in the domain of fractography. In addition to providing a simple method to measure the inter-particle cohesion from macroscopic parameters, these results give better comprehension of the formation of clusters in cohesive granular materials.

Arctic soil patterns analogous to fluid instabilities

Slow-moving arctic soils commonly organize into striking large-scale spatial patterns called solifluction terraces and lobes. Although these features impact hillslope stability, carbon storage and release, and landscape response to climate change, no mechanistic explanation exists for their formation. Everyday fluids-such as paint dripping down walls-produce markedly similar fingering patterns resulting from competition between viscous and cohesive forces. Here we use a scaling analysis to show that soil cohesion and hydrostatic effects can lead to similar large-scale patterns in arctic soils. A large dataset of high-resolution solifluction lobe spacing and morphology across Norway supports theoretical predictions and indicates a newly observed climatic control on solifluction dynamics and patterns. Our findings provide a quantitative explanation of a common pattern on Earth and other planets, illuminating the importance of cohesive forces in landscape dynamics. These patterns operate at length and time scales previously unrecognized, with implications toward understanding fluid-solid dynamics in particulate systems with complex rheology.

Inertial Force Transmission in Dense Granular Flows

Dense granular flows are well described by several continuum models; however, their internal dynamics remain elusive. This study explores the contact force distributions in simulated steady and homogenous shear flows. The results demonstrate the existence of high magnitude contact forces in faster flows with stiffer grains. A proposed physical mechanism explains this rate-dependent force transmission. This analysis establishes a relation between contact forces and grain velocities, providing an entry point to unify a range of continuum models derived from either contact forces or grain velocity.