Impact dynamics of granular jets with noncircular cross sections

General theory of the viscosity of liquids and solids from nonaffine particle motions

A microscopic formula for the viscosity of liquids and solids is derived rigorously from a first-principles (microscopically reversible) Hamiltonian for particle-bath atomistic motion. The derivation is done within the framework of nonaffine linear response theory. This formula may lead to a valid alternative to the Green-Kubo approach to describe the viscosity of condensed matter systems from molecular simulations without having to fit long-time tails. Furthermore, it provides a direct link between the viscosity, the vibrational density of states of the system, and the zero-frequency limit of the memory kernel. Finally, it provides a microscopic solution to Maxwell's interpolation problem of viscoelasticity by naturally recovering Newton's law of viscous flow and Hooke's law of elastic solids in two opposite limits.

Estimation ofsp-dexchange constants revisited

We present a new computational method for estimating thesp-dexchange constant,Jeffsp-d, applicable to transition metal doped diluted magnetic semiconductors, transition metal oxides, and 2D- and 3D- dichalcogenides. The proposed method is based on results describing the variation of the magnetic features of a doped system with the variation of its magnetization density (M). The results forJeffsp-d(M)obtained with the proposed method are compared with the corresponding results,Jeffsp-d(ΔEVBM), obtained from estimations of the spin electron orbital splitting, ΔE^VBM, at the valence band maximum (VBM). The latter is estimated in two ways; either directly from plots of the band structure calculations or by calculating the energy difference between the band-centers of the spin-up and spin-down electron density of states of the doped systems. Despite the inherent drawbacks in these two estimation methods for ΔE^VBM, they lead to equivalent results and the correspondingJeffsp-d(ΔEVBM)are in good agreement with theJeffsp-d(M)ones.Ab initioresults obtained for the 2D-MoS₂doped with 3d-series transition metals are presented to demonstrate the validity and applicability of the proposed computational schemes for obtainingJeffsp-d. The proposed methods can be utilized as useful tools in the search of new materials for spintronics and valleytronics applications.

Explosion cratering in 3D granular media

Sudden release of energy in an explosion creates craters in granular media. In comparison with well-studied impact cratering in granular media, our understanding of explosion cratering is still primitive. Here, we study low-energy lab-scale explosion cratering in 3D granular media using controlled pulses of pressurized air. We identify four regimes of explosion cratering at different burial depths, which are associated with distinct explosion dynamics and result in different crater morphologies. We propose a general relation between the dynamics of granular flows and the surface structures of the resulting craters. Moreover, we measure the diameter of explosion craters as a function of explosion pressure, duration and burial depth. We find that the size of the craters is non-monotonic with increasing burial depth, reaching a maximum at an intermediate burial depth. In addition, the crater diameter shows a weak dependence on explosion pressure and duration at small burial depths. We construct a simple model to explain this finding. Finally, we explore the scaling relations of the size of explosion craters. Despite the huge difference in energy scales, we find that the diameter of explosion craters in our experiments follows the same cube root energy scaling as explosion cratering at high energies. We also discuss the dependence of rescaled crater sizes on the inertial number of granular flows. These results shed light on the rich dynamics of 3D explosion cratering and provide new insights into the general physical principles governing granular cratering processes.

Granular rotor as a probe for a nonequilibrium bath

This study numerically and analytically investigates the dynamics of a rotor under viscous or dry friction as a nonequilibrium probe of a granular gas. In order to demonstrate the role of the rotor as a probe for a nonequilibrium bath, the molecular dynamics (MD) simulation of the rotor is performed under viscous or dry friction surrounded by a steady granular gas under gravity. A one-to-one map between the velocity distribution function (VDF) of the granular gas and the angular distribution function for the rotor is theoretically derived. The MD simulation demonstrates that the one-to-one map accurately infers the local VDF of the granular gas from the angular VDF of the rotor, and vice versa.

Subharmonic instability of a self-organized granular jet

Downhill flows of granular matter colliding in the lowest point of a valley, may induce a self-organized jet. By means of a quasi two-dimensional experiment where fine grained sand flows in a vertically sinusoidally agitated cylinder, we show that the emergent jet, that is, a sheet of ejecta, does not follow the frequency of agitation but reveals subharmonic response. The order of the subharmonics is a complex function of the parameters of driving.

Head-on collisions of dense granular jets

We study head-on impacts of equal-speed but unequal-width incompressible jets in two dimensions with a focus on dense granular jets. We use discrete particle simulations to show that head-on impact of granular jets produces a quasi-steady-state where a fraction of the excess incident momentum from the larger jet is captured by an impact center that drifts steadily over time. By varying the dissipation in our discrete particle simulations and through additional analogous continuum jet impacts of different rheologies, we show that this central drift speed is remarkably dependent primarily on the total dissipation rate to the power 1.5, and largely independent of the dissipation mechanism. We finish by presenting a simple control volume analysis that qualitatively captures the emergence of the drift speed but not the scaling.