Microscopic dissipation in a cohesionless granular jet impact

From splashing to bouncing: The influence of viscosity on the impact of suspension droplets on a solid surface

We experimentally investigated the splashing of dense suspension droplets impacting a solid surface, extending prior work to the regime where the viscosity of the suspending liquid becomes a significant parameter. The overall behavior can be described by a combination of two trends. The first one is that the splashing becomes favored when the kinetic energy of individual particles at the surface of a droplet overcomes the confinement produced by surface tension. This is expressed by a particle-based Weber number We_{p}. The second is that splashing is suppressed by increasing the viscosity of the solvent. This is expressed by the Stokes number St, which influences the effective coefficient of restitution of colliding particles. We developed a phase diagram where the splashing onset is delineated as a function of both We_{p} and St. A surprising result occurs at very small Stokes number, where not only splashing is suppressed but also plastic deformation of the droplet. This leads to a situation where droplets can bounce back after impact, an observation we are able to reproduce using discrete particle numerical simulations that take into account viscous interaction between particles and elastic energy.

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.

Impact dynamics of granular jets with noncircular cross sections

Using high-speed photography, we investigate two distinct regimes of the impact dynamics of granular jets with noncircular cross sections. In the steady-state regime, we observe the formation of thin granular sheets with anisotropic shapes and show that the degree of anisotropy increases with the aspect ratio of the jet's cross section. Our results illustrate the liquidlike behavior of granular materials during impact and demonstrate that a collective hydrodynamic flow emerges from strongly interacting discrete particles. We discuss the analogy between our experiments and those from the Relativistic Heavy Ion Collider, where similar anisotropic ejecta from a quark-gluon plasma have been observed in heavy-ion impact.

Still water: dead zones and collimated ejecta from the impact of granular jets

When a dense granular jet hits a target, it forms a large dead zone and ejects a highly collimated conical sheet with a well-defined opening angle. Using experiments, simulations, and continuum modeling, we find that this opening angle is insensitive to the precise target shape and the dissipation mechanisms in the flow. We show that this surprising insensitivity arises because dense granular jet impact, though highly dissipative, is nonetheless controlled by the limit of perfect fluid flow.

Simulation of granular jets: is granular flow really a perfect fluid?

We perform three-dimensional simulations of the impact of a granular jet for both frictional and frictionless grains. Small shear stress observed in the experiment [X. Cheng et al., Phys. Rev. Lett. 99, 188001 (2007)] is reproduced through our simulation. However, the fluid state after the impact is far from a perfect fluid, and thus the similarity between granular jets and quark gluon plasma is superficial because the observed viscosity is finite and its value is consistent with the prediction of the kinetic theory.