Vibrating particles or granular materials can produce many fluid-like behaviors. In this video, researchers demonstrate how a granular gas made up of particles of two sizes behaves at different conditions. By tweaking the amplitude of the vibration, they alter how the particles cluster in a divided container. At large vibrational amplitudes, the particles behave much like a gas–energetic and spread out. At lower amplitudes, though, the particle density and the number of particle collisions increases. Each collision dissipates some of a particle’s energy; more collisions means less energy available to escape. As a result, the particles cluster, forming an attractor that draws in additional particles over time. (Video credit: R. Mikkelson et al.)
Tag: granular material
Breaking Up Falling Beads
In a stream of falling liquid, surface tension instabilities cause the fluid to break up into droplets. This video shows a similar experiment with a stream of glass beads, a granular material. The whole system is housed under a vacuum to eliminate the effects of air drag on the stream, and a camera rides alongside the stream to track the evolution of the falling material in a Lagrangian fashion. As with a liquid stream, we see the granular flow develop undulations as it falls, ultimately breaking up into clusters of beads. The authors suggest that nanoscale surface roughness and van der Waals forces may be responsible for the clustering behavior in the absence of surface tension. (Video credit: J. Royer et al.)
Sandy Jets
When a fluid is vibrated, instabilities can form along its surface. With a sufficient amplitude, voids form inside the fluid and their collapse leads to a jet that shoots out from the fluid. A very different process leads to air cavities forming in a vibrated granular medium, but the jets produced are remarkably similar, as seen in this video. (Video credit: M. Sandtke et al.)
Frozen Powder Drops
Droplet impacts on granular surfaces and water interactions with superhydrophobic surfaces are not unfamiliar topics for FYFD. But this behavior of water droplets falling on a superhydrophobic powder is unusual, to say the least. When the droplets impact in powder, they rebound with a partial coating of powder. In the case of the superhydrophobic powder, the shape of the droplet is “frozen” by the powder. A satellite droplet is ejected from the region not coated in powder and the resultant main drop falls back to the surface and comes to rest with little to no deformation. The researchers report a critical velocity at which the behavior is observed. (Video credit: J. Marston et al.; via Physics Buzz)
Labyrinth
A labyrinthine pattern forms in this timelapse video of a multiphase flow in a Hele-Shaw cell. Initially glass beads are suspended in a glycerol-water solution between parallel glass plates with a central hole. Then the fluid is slowly drained over the course of 3 days at a rate so slow that viscous forces in the fluid are negligible. As the fluid drains, fingers of air invade the disk, pushing the beads together. The system is governed by competition between two main forces: surface tension and friction. Narrow fingers gather fewer grains and therefore encounter less friction, but the higher curvature at their tips produces larger capillary forces. The opposite is true of broader fingers. Also interesting to note is the similarity of the final pattern to those seen in confined ferrofluids. (Video credit and submission: B. Sandnes et al. For more, see B. Sandes et al.)
Dancing Sands
Here a collection of dry grains are vertically vibrated, creating a series of standing waves on the surface of the sand. The shapes of these Faraday waves are dependent upon the frequency of the vibration. Despite the solid nature of sand particles, this behavior is much the same as the behavior of a vibrated fluid.
Granular Eruptions
Granular flows, which are made up of loose particles like sand, often display remarkably fluid-like behavior. Here researchers explore the behavior of granular flows when a solid impacts them at high speed. The sand, unlike a fluid, does not have surface tension, yet we still observe many of the same behaviors. Like a fluid, the sand splashes and creates cavities and jets as it deforms around the fallen object. The sand even “erupts” as submerged pockets of air make their way back to the surface.
Particle Patterning
Here a container filled with a suspension of neutrally buoyant polystyrene beads and fluid is rotated. As the container rotates, a thin layer of fluid and bunches of particles get drawn up onto the wall by capillary forces capable of holding the particles in place even if the container stops rotating. The density and patterning of the particles on the wall depends on the container’s rotation speed and the volume fraction of particles. (Video credit: J. Kao and A. Hosoi)
Water Drops on Sand
This high-speed video captures the impact of liquid droplets onto a granular surface. While there is some similarity to liquid-solid and liquid-liquid impacts, the permeability of the granular surface helps to “freeze” the splash rather quickly. Energy is dissipated in the initial impact, causing a splash of grains. Then the surface tension, viscosity and inertia of the droplet compete in causing the deformations seen in the video. The deformation appears strongly dependent on the kinetic energy with which the droplet hits the surface (i.e. proportional to the height from which it is dropped). (Video credit: G. Delan et al)
Stick-Slip Bubbles
Varying the rate of injection of air into a wet granular mixture contained in a Hele Shaw cell results in very different flow patterns. At low injection rates, stick-slip bubbles form. As the injection rate increases, patterns are affected by “temporal intermittency” where continuous motion is occasionally interrupted by jamming. Increasing the injection rate still further results in Saffman-Taylor-like fingering. #