FYFD

Tag: porous media

  • Ricequakes

    Ricequakes

    Rockfill dams, sinkholes, ice shelves, and other geological features often consist of brittle, porous materials that are partially submerged. Over time, pressure and chemical reactions with the fluid around them can cause these structures to collapse, but it can take many, many years. 

    To study the physics behind this, researchers have turned to a new model: puffed rice cereal. Like their counterparts in nature, puffed rice grains contain micropores that slowly soften and get crushed after being wetted. Researchers filled their test container with puffed rice and put it under pressure to give the whole stack a constant stress. Then they injected milk in the bottom section of the container. After an immediate collapse in the wet material (lower left), the remaining grains collapsed slowly in a series of “ricequakes”. 

    As the micropores compacted, the cereal let out audible cracks that corresponded with the motion of a crushing wavefront (lower right). The time between ricequakes increased linearly and depended on pore size. The relationship was so consistent, researchers found, that they could predict how long the puffed rice stack had been wet simply by listening to the time between crackles! Experiments like these offer scientists an exciting chance to understand geological physics that would otherwise take up to millions of years to observe. (Image and research credit: I. Einav and F. Guillard; via Physics World; submitted by Kam-Yung Soh)

  • Bubble Trains in a Microchannel

    Bubble Trains in a Microchannel

    Trains of bubbles flowing through a microchannel get distorted by periodic expansions and constrictions. In these images, flow is from left to right, and the narrow point of the channel is about 250 microns across. In narrow regions, the front of the bubble tends to move faster, while in wider areas, the back of the bubble speeds up. This causes the distinctive shape changes we see. Microfluidic channels with these exaggerated shifts in geometry allow researchers to study the physics behind liquids and gases seeping through the interstitial gaps of a porous media, like when water and gases move through rock and soil. (Image and research credit: M. Sauzade and T. Cubaud)

  • Flowing Through Tight Spaces

    Flowing Through Tight Spaces

    Fluid flow through porous media inside confined spaces can be tough to predict but is key to many geological and industrial processes. Here researchers examine a mixture of glass beads and water-glycerol trapped between two slightly tilted plates. As liquid is drained from the bottom of the cell, air intrudes. Loose grains pile up along the meniscus and get slowly bulldozed as the air continues forcing its way in. The result is a labyrinthine maze formed by air fingers of a characteristic width. The final pattern depends on a competition between hydrostatic pressure and the frictional forces between grains. Despite the visual similarity to phenomena like the Saffman-Taylor instability, the authors found that viscosity does not play a major role. For more, check out the video abstract here. (Image and research credit: J. Erikson et al., source)

  • Peering Between Particles

    Peering Between Particles

    Turbulence is not the only way to mix fluids. Even a steady, laminar flow can be an effective mixer if geometry lends a hand. Above, two dyes, fluorescein (green) and rhodamine (red), are injected into a porous flow through packed spheres. The flow runs from bottom to top in both images. Seeing the flow in such a crowded geometry is challenging. Here researchers used spheres with an index of refraction that matches water – that helps them avoid refraction that would prevent them from looking through spheres to the flow on the other side. They also lit a narrow plane of the flow using a laser sheet to isolate it. Together, this allowed the researchers to track the mixing of the two initially separate streaks of dye as they randomly mix in the spaces between spheres. (Image and research credit: M. Kree and E. Villermaux)

  • Oreo Dunking Physics

    Oreo Dunking Physics

    As most people know, cookie dunking is serious business. Everyone has their own preference for cookie saturation and stiffness. Happily, scientists have examined this problem and have advice to offer those seeking cookie dunk perfection. Previously, we discussed Len Fisher’s Ig Nobel Prize-winning work on the physics of cookie dunking. In that work, Fisher found that Washburn’s equation for flow through cylindrical pores worked well to describe the uptake of tea or milk into a cookie.

    More recently, Splash Lab researchers have investigated just how much milk several common American cookies – including Oreos – take up in a given dunk. Because these cookies are quite dry, they take up liquid quickly, soaking in about 80 percent of the liquid weight within the first 2 seconds when dipped in 2% milk. Within five seconds, the cookies take on 99% of their liquid weight capacity, so there’s no point to a longer dunk – unless you like your cookie to disintegrate into the milk. The fat and sugar content of the dunking liquid does affect how quickly capillary action can whisk fluid into the cookie’s pores, but, overall, the research shows that milk users should be well-served by a three second dunk. If you like your cookie softer than that, simply pull it out of milk and let it sit for a bit while the milk soaks in. That way, your cookie doesn’t crumble! (Image credits: A. Melton; research credit: R. Hurd et al.; h/t to Randy H. and Mental Floss)

  • Watching Flow Inside Rock

    Watching Flow Inside Rock

    Flow through porous substances has been a major interest in fluid dynamics for the last hundred years because rocks are porous. For most of that period, we’ve used Darcy’s law to calculate how a fluid flows through pores in a rock. (Incidentally, it can also be used for determining the perfect length of time for dunking a cookie in milk.) Often, however, there is more than one fluid in a pore – for example, both a liquid and a gas could be trapped there. In that case, researchers made a few assumptions that allowed them to extend Darcy’s law for these multiphase situations. For a long time, that was the best anyone could do because it was impossible to observe what’s actually happening in the pores inside an actual rock.

    Recently, however, scientists have begun observing these multiphase flows inside sandstone pores using x-ray imaging. They’re only able to take an image every 45 seconds or so, but even that is frequent enough to show that the flow is surprisingly unsteady. An example image is shown above. The colored areas show pores filled with nitrogen inside the rock. Brine is also being injected into the rock but not being shown. The colors indicate how connected the nitrogen-filled pores are to other pores nearby. Red areas are highly connected; blue have moderate connections; and green areas are smaller and have fewer connections. The network connections inside the rock change relatively rapidly, even with steady-state injection conditions. That varying connectivity implies that some of the injection energy is going into shifting interfaces around rather than actually moving the fluids through the pores. More work will be needed to unravel what’s really happening inside the porous network, but the results have far-reaching implications for understanding groundwater filtration, fossil fuel extraction, and, in the future, the possibility of carbon sequestration. (Image credit: C. Reynolds et al., source; submitted by Simon D.)