Poroelastic shape relaxation of hydrogel particles

We describe a straightforward way to determine the poroelastic diffusion coefficient of a hydrogel particle by first indenting it via swelling in a granular packing, and then monitoring how its indented shape relaxes after it is removed.

Micromechanics of slow granular flows

It has been contemplated for a long time that dense granular materials flow in a stick-slip manner, and large fluctuations in the stresses are associated with it. However, the particle scale mechanics for this type of macroscopic motion has not been understood so far. We have analyzed the time evolution of contact networks from particle dynamics simulations and found that the rate of change of elastic energy of the packing can distinguish the stick regimes and the slip events. The isostatic criterion (number of contacts for a minimally stable particle) has been used to construct a cascade failure mechanism which reveals that the effect of the random breaking of contacts due to applied shear can be system-spanning for some cases. The size of the cascade failures follows a power law that explains experimentally observed large fluctuations is stresses. We expect that this power law distribution can connect the microstructure of a granular packing to its mechanical response.

Deep spontaneous penetration of a water droplet into hot granular materials

The contact between droplets and granular materials is of practical importance for many processes, such as spraying cooling (to cool down the soil) and the wet dusting (to collect the grains). While the phenomenon is commonly known in nature and industry, our knowledge of the interaction between the water drop and hot grains is still very limited. Here, we experimentally investigated the drop behaviours released on a heated granular bed. Surprisingly, we found that the drops start digging the granular material as deep as 15 times the diameter of the droplet. Hot particles are absorbed into the drop and vaporise the liquid. The vapour production is so intense that the vapour is able to blow away the particles underneath the drop. The drop can then move downwards under the action of the gravity. In order to inspect this digging behaviour, two kinds of setups were designed: a 3D granular packing inside a cylinder and a quasi-2D packing inside a Hele-Shaw cell. The first allows the observation of the droplet with the heated material, while the second provides the direct observation from the side view to uncover the drop behaviour in the deep bed. One proposes a mechanism based on the Leidenfrost effect considering a rough surface that models the surface of the granular material. This model allows to explain why the droplet can dig on a range of temperatures between the boiling temperature of the cooling liquid and the Leidenfrost temperature relative to the granular material. In this range of temperatures, the cooling of the granular material is then rather efficient since the droplets vaporise deeply in the heart of the material.

The critical state line of nonplastic tailings

The probability of failure of tailing dams and associated risks demand improvements in engineering practice. The critical state line provides a robust framework for the characterization of mine tailings. New experimental data for nonplastic platinum tailings and a large database for tailings and nonplastic soils (grain size between 2 and 500 μm) show that the critical state parameters for nonplastic tailings follow the same trends as nonplastic soils as a function of particle-scale characteristics and extreme void ratios. Critical state lines determined for extreme tailings gradations underestimate the range of critical state parameters that may be encountered in a tailings dam; in fact, mixtures with intermediate fines content exhibit the densest granular packing at critical state. The minimum void ratio emin captures the underlying role of particle shape and grain size distribution on granular packing and emerges as a valuable index property to inform sampling strategies for the assessment of spatial variability. Mineralogy does not significantly affect the intercept Γ100, but it does affect the slope λ. The friction coefficients M of tailings are similar to those of other nonplastic soils; while mineralogy does not have a significant effect on friction, more angular grains lead to higher friction coefficients.