FYFD

Tag: granular material

  • In Search of a Better Espresso

    In Search of a Better Espresso

    Of specialty coffee drinks, espresso has the most cup-to-cup variation in quality. For those who are not coffee aficionados — such as yours truly — espresso is made by forcing hot water through a packed bed of coffee grains. Many factors can affect the final output, including the amount of dry coffee used, the fineness of the grind, water temperature and pressure, and how tightly packed the granular bed is.

    Conventional wisdom suggests that a fine grind is best since it increases the exposed surface area of coffee, but researchers found this is not, in fact, ideal. At very fine grinds, the bed of coffee becomes so tightly packed that water cannot pass through some sections, meaning that the coffee there is completely wasted since nothing is extracted.

    Instead, a slightly coarser grind provided better and more consistent extraction because water passed through the entire bed of grains. The researchers point out that this not only produces a good, consistent cup of espresso, but it does so with less waste, something that is becoming more and more important as the climate crisis affects coffee growers. (Image credit: K. Butz; research credit: M. Cameron et al.; via Cosmos; submitted by Kam-Yung Soh)

  • Self-Assembly Under Stratification

    Self-Assembly Under Stratification

    Sometimes mistakes lead to great discoveries. After leaving a failed outreach demo overnight, researchers discovered a new mechanism for self-assembling particles. In the initial set-up, a layer of fresh water is poured atop a layer of denser, saltier water. This creates what’s known as a stably stratified fluid, with progressively denser mixtures of salt water as one moves downward. If you pour in particles of an intermediate density (heavier than fresh water and lighter than salt water), they’ll form a layer at one height, and, if you wait overnight, those particles will slowly form a disk-like raft.

    A spheroidal particle causes attractive flow at its equator and repulsive flow at its poles.

    This self-assembly is driven by fluid dynamics — not by any attraction between the particles. Because the particles are unable to absorb salt, their boundaries distort the concentration gradients in the surrounding fluid. This generates subtle currents at the particle boundaries, like in the picture above, where flow moves toward the particle at the equator and away at the poles. Larger particle clusters generate stronger flows, allowing them to attract even more particles.

    Although the speeds involved are quite slow, this mechanism may play an important role in nature, where stratified flows are common. The researchers speculate, for example, that the effect could be important in the clustering of microplastics in the ocean. (Image and research credit: R. Camassa et al.; see also R. McLaughlin; submitted by Kam-Yung Soh)

  • Recreating Volcanic Lightning

    Recreating Volcanic Lightning

    Some natural phenomena, like volcanic eruptions or tornado formation, don’t lend themselves to fieldwork — at least not at the height of the action. The danger, unpredictability, and destructiveness of these environments is more than our equipment can survive. And so researchers find clever ways to recreate these phenomena in controllable ways. The latest example comes from a lab in Germany, where researchers are recreating volcanic lightning.

    To do so, they heat and pressurize actual volcanic ash in an argon environment and let the mixture decompress into a jet, like a miniature eruption. The lightning that accompanies the jet is thought to depend on friction between ash particles, which build up electric charges when rubbed, just like a balloon rubbed against one’s hair. When the charges get large enough, lightning discharges the build-up.

    Small-scale experiments like this one allow researchers to vary the temperature and water content of the ash and observe how this changes the lightning. Drier ash generates more lightning, but it’s hard to distinguish whether this is inherent to the ash or the result of the denser jets that form without the added eruptive force of steam. (Image credit: eruption – M. Szeglat, lab lightning – Sönke Stern/Ludwig-Maximilians-Universität München/Gizmodo; research credit: S. Stern et al.; via Gizmodo)

  • Martian Landslides

    Martian Landslides

    Sometimes there are advantages to studying planetary physics beyond Earth. Mars does not have plate tectonics, vegetation, or the level of erosion we do, allowing geological features like those left behind by landslides to persist undisturbed for millions of years. And, thanks to a suite of orbiters, we’ve mapped most of Mars at a resolution better than many parts of our own planet. All together, this gives researchers a treasure trove of geological data from our nearest neighbor.

    One peculiar feature of many landslides is their long runout. Even over relatively flat ground, some landslides cover extreme distances from their point of origin. On Earth, we often see this behavior near glaciers, so scientists theorized that the presence of ice was somehow necessary for the landslide to cover such a long distance. But previous laboratory experiments with dry, ice-free grains showed the same behavior: long runouts marked with ridges running parallel to the flow. The mechanism behind the ridges is still somewhat unclear, but it seems to be connected to fluid dynamical instabilities that form between fast-flowing particles of differing density. But such results have been confined to lab-scale experiments and numerical simulations.

    A new report, however, shows that landslides on Mars share the same characteristic spacing and thickness between their ridges. This evidence suggests that the same ice-free mechanism could account for the long run-out of landslides on Mars and other planets. (Image credit: NASA/JPL-Caltech/University of Arizona; research credit: G. Magnarini et al.; via The Conversation; submitted by Kam-Yung Soh)

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    Drops That Dig

    On extremely hot surfaces, droplets will skitter on a layer of their own vapor, thanks to the Leidenfrost effect. This keeps the liquid insulated from contact with the hot surface. But what if the surface isn’t solid?

    That situation is what we see above. Instead of soaking into a granular material like a room temperature droplet (left), a drop falling onto a very hot bed of grains digs a hole! As with a typical drop on a super hot surface, the heat vaporizes part of the droplet. As the vapor escapes, it carries sand with it, allowing the boiling drop to burrow its way into the material. As the temperature difference between the sand and droplet changes, the digging slows. Eventually, the drop comes to a rest and boils away. (Video and image credit: J. Zou et al.)

  • Shearing Grains

    Shearing Grains

    Granular materials, like beads and sand, demonstrate both solid and fluid-like behaviors, which makes them difficult to study. Traditionally, one method for studying how fluids respond to deformation places the fluid in a ring-shaped cell with a rotating outer wall. That creates a uniform shear, as indicated by the red arrows above. For granular materials, though, this classic set-up usually breaks the grains up into two separate regions, one that behaves solidly and the other that behaves fluidly.

    To get past that issue and study grains under truly uniform shear, researchers built a new version of the classic apparatus. In this new ring-shaped cell, the outer wall moves but so do independent concentric rings beneath the grains. This allows researchers to see how grains move under uniform shear (left) and what kinds of forces develop between jammed grains in the system (right). (Image and research credit: Y. Zhao et al.; via APS Physics; submitted by Kam-Yung Soh)

  • Crowds as a Fluid

    Crowds as a Fluid

    At a low density, crowds of people can behave like a fluid, which has led to numerous hydrodynamically-based crowd models. At higher densities, though, crowds are more like a soft solid, and researchers are adapting models developed for granular materials like sand to describe these crowds. In granular materials, these models help scientists identify how vibrations move through the complex network of grains and what circumstances might cause sudden reorganizations. In a large crowd, this could tell scientists the difference between the innocuous shuffle at a rock concert and the trigger for a deadly stampede. Getting real-world data for comparison is tough – obviously, it’s unethical to intentionally cause a crowd to panic – so thus far the models remain relatively untested. (Image credit: M. Lebrun; research credit: A. Bottinelli and J. Silverberg)

  • What Controls an Avalanche?

    What Controls an Avalanche?

    In an avalanche, grains spontaneously flow when a slope reaches a critical angle, and they continue flowing until they settle at a new, lower angle. Scientists have long debated why this angle mismatch occurs, and, in recent years, the general opinion was that the avalanche’s inertia kept it flowing long enough to settle at a lower angle. But a new experiment, using a slowly-rotating drum similar to the one above*, shows that friction, not inertia, is the key player. 

    The researchers used silica beads suspended in water, which allowed them to cleverly control the interparticle friction. In water, silica beads build up negative electrostatic charges, which push the grains apart and eliminate friction. In that frictionless state, the researchers found that the beads tumbled smoothly; their starting and ending angles were always the same. 

    By adding salt to the water, the researchers were able to eliminate some of the electrostatic charge and thereby tune the friction. When they did that, the difference between starting and stopping angles came back and grew more substantial as the friction increased. All in all, the results indicate that friction between particles is what makes an avalanche avalanche. (Image credit: J. Gray and V. Chugunov, source; research credit: H. Perrin et al.; via APS Physics; submitted by Kam-Yung Soh)

    * If you’re curious about the patterns in the image, I explain them in this previous post.

  • Capillary Action and Sand Castles

    Capillary Action and Sand Castles

    Capillary action – or capillarity – is the ability of liquids to flow through narrow constrictions. It results from intermolecular forces between fluids and solids. It’s a combination of surface tension – which creates cohesion within the liquid – and adhesion, which allows the liquid and solid to hold to one another. Together, these forces propel the liquid to flow through narrow gaps.

    In the video below, a saturated mixture of sand and water is poured into a mold on a bed of dry sand. When left to settle, much of the water flows from the mold into the dry sand bed through capillary action. When the mold is removed (top), the sand holds its shape, something it can’t do without a porous bed to soak in the excess liquid. (Image and video credit: amàco et al.)

  • Giving Chocolate that Smooth Finish

    Giving Chocolate that Smooth Finish

    Anyone who’s tried to make chocolate confections at home can tell you that achieving that perfect smooth consistency isn’t easy. It was only after Rodolphe Lindt invented the process of conching in 1879 that anyone enjoyed smooth chocolate. Conching is what allows granular solids like sugar, milk and cocoa powders to mix with liquid cocoa butter into a smooth, homogeneous liquid. Although the process has been known for more than a century, it’s only recently that researchers have unraveled the underlying physics that enables it.

    One of the key parameters to conching is the a mixture’s jamming volume fraction; in other words, the point where the fraction of solid particles in the mixture is too high for it to flow freely. In the first stage of conching, the solid particulates and a small amount of liquid are stirred and slowly heated. The mechanical action of stirring breaks up aggregates and raises the jamming volume fraction. By the end of the dry conche, the mixture could flow, in theory, except that it fractures at a lower stress than what’s necessary to flow.

    At this point, chocolatiers add the remainder of the liquid ingredients. That infusion of moisture decreases the friction between solid particles and further raises the jamming volume fraction. With the system now far below that jamming point, the mixture transforms into a freely-flowing, smooth fluid. By understanding the intricacies of the process, scientists hope to reduce the energy necessary in chocolate production and similar industrial processes.  (Image credit: A. Stein; research credit: E. Blanco et al.; via Physics World; submitted by Kam-Yung Soh)