FYFD

Year: 2017

  • Detergency

    Detergency

    Have you ever wondered just how detergents are able to get grease and oil off a surface? This simple example demonstrates one method. In the top image, a drop of oil sits attached to a solid surface; both are immersed in water. An eyedropper injects a surfactant chemical near the oil drop. This lowers the surface tension of the surrounding water and allows the mixture to better wet the solid. That eats away at the oil drop’s contact with the surface. It takes awhile – the middle animation is drastically sped up – but the oil droplet maintains less and less contact with the surface as the surfactant works. Eventually, in the bottom image, most of the oil drop detaches from the surface and floats away.   (Image credits: C. Kalelkar and A. Sahni, source)

  • Sunglinting Seas

    Sunglinting Seas

    Sunlight reflecting off the Earth can reveal a remarkably rich picture of our planet’s activity. The silver-gray areas seen in this satellite image are sunglint, where lots of light is reflected back to space. Sunglint occurs in regions with very few waves; more waves – like in the bluer areas – mean more directions in which light can be scattered. The reason for these rough and smooth waters is atmospheric: the prevailing summer winds blow across the Aegean from the north. In open water, that wind drives up the waves, but rocky islands disrupt the flow, leaving “wind shadows” on their southern, leeward sides where the waves are smaller. (Image credit: J. Schmaltz; via NASA Earth Observatory)

  • Surfing Mists

    Surfing Mists

    Watch your hot cup of coffee or tea carefully, and you may notice a white mist of tiny micron-sized droplets hovering near the surface. These microdroplets are a little understood part of evaporation. They form over a heated liquid, levitating on vapor that diffuses out from them and reflects off the liquid surface. (This is similar to the Leidenfrost effect, but the authors note it occurs at much lower temperatures. Unrelated research has suggested the Leidenfrost effect can occur at lower temperatures when there is very little surface roughness.)

    One of the particularly peculiar behaviors of these tiny levitating microdroplets is that they can exist over dry surfaces as well. The image above shows microdroplets migrating from a liquid surface (right) to a dry surface (center and left). When the droplets near the contact line, they encounter a strong upward flow due to increased evaporation there. This launches the droplets upward and they sail to the dry area. There, their vapor layers continue creating levitation and provide a cushion between them and their neighbors, causing the drops to self-organize into arrays. (Image credit: D. Zaitsev et al.; via Physics World; submitted by Kam-Yung Soh)

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    Pelican Diving

    Pelicans, like many sea birds, are aerial divers. They spot their prey from high above, bank, and dive into the water to catch the fish. Although they hit the water at high speeds, pelican diving techniques differ somewhat from plunge divers like gannets or boobies. Pelicans are only aiming for a shallow dive, so they have features – like their expandable neck pouch – that help them decelerate quickly instead of taking a full-body plunge. The goal is to increase drag after the head enters, slowing everything down. That can add more stress to the bird’s neck – the rest of the body is still moving quickly even after the head begins to slow. To counter this compression, the birds must have strong neck muscles to stabilize their spines during the impact process. (Video and image credit: Deep Look)

  • Controlling Leidenfrost Drops

    Controlling Leidenfrost Drops

    On a surface much hotter than their boiling point, droplets can surf on a layer of their own vapor due to the Leidenfrost effect. Recent research has shown that textured surfaces like ratchets can create corrals, traps, and mazes for such droplets. Here, researchers manipulate the propulsion of Leidenfrost drops using non-parallel grooves instead. When placed between two non-parallel plates, the droplet is squeezed by side forces perpendicular to the walls, with the resultant force in the direction where the gap widens. In most states, friction forms an opposition to this squeeze, but for Leidenfrost droplets that frictional force is negligible. Instead, the squeezing from the plates launches droplets toward the wider end of the groove, allowing researchers to design repellers (top) and traps (bottom) for the fast-moving drops. (Image credits: C. Luo et al., source)

  • Convection

    Convection

    Blue paint in alcohol forms an array of polygonal convection cells. We’re accustomed to associating convection with temperature differences; patterns like the one above are seen in hot cooking oil, cocoa, and even on Pluto. In all of those cases, temperature differences are a defining feature, but they are not the fundamental driver of the fluid behavior. The most important factors – both in those cases and the present one – are density and surface tension variations. Changing temperature affects both of these factors, which is why its so often seen in Benard-Marangoni convection.

    For the paint-in-alcohol, density and surface tension differences are inherent to the two fluids. Because alcohol is volatile and evaporates quickly, its concentration is constantly changing, which in turn changes the local surface tension. Areas of higher surface tension pull on those of lower surface tension; this draws fluid from the center of each cell toward the perimeter. At the same time, alcohol evaporating at the surface changes the density of the fluid. As it loses alcohol and becomes denser, it sinks at the edges of the cell. Below the surface, it will absorb more alcohol, become lighter, and eventually rise at the cell center, continuing the convective process. (Image credit: Beauty of Science, source)

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    Dam Failure

    In a recent video, Practical Engineering tackles an important and often-overlooked challenge in civil engineering: dam failure. At its simplest, a levee or dam is a wall built to hold back water, and the higher that water is, the greater the pressure at its base. That pressure can drive water to seep between the grains of soil beneath the dam. As you can see in the demo below, seeping water can take a curving path through the soil beneath a dam in order to get to the other side. When too much water makes it into the soil, it pushes grains apart and makes them slip easily; this is known as liquefaction. As the name suggests, the sediment begins behaving like a fluid, quickly leading to a complete failure of the dam as its foundation flows away. With older infrastructure and increased flooding from extreme weather events, this is a serious problem facing many communities. (Video and image credit: Practical Engineering)

  • Self-Healing Bubbles

    Self-Healing Bubbles

    Soap films have the remarkable property of self-healing. A water drop, like the one shown above, can pass through a bubble (repeatedly!) without popping it. This happens thanks to surfactants and the Marangoni effect. Surfactants are molecules that lower the surface tension of a liquid and congregate along the outermost layer of a soap film. When water breaks through the soap film, its lack of surfactants causes a higher surface tension locally. This triggers the Marangoni effect, in which flow moves from areas of low surface tension toward ones of high surface tension. That carries surfactants to the region where the drop broke through and helps stabilize and heal the soap film. Incidentally, the same process lets you stick your finger into a bubble without popping it as long as your hand is wet! (Image credit: G. Mitchell and P. Taylor, source)

  • Creating Clouds

    Creating Clouds

    Despite their ubiquity and importance, we know surprisingly little about how clouds form. The broad strokes of the process are known, but the details remain somewhat fuzzy. One challenge is understanding how nucleation – the formation of droplets that become clouds or rain – works. A recent laboratory experiment in an analog cloud chamber suggests that falling rain drops may help spawn more rain drops.

    The experiment takes place in a chamber filled with sulfur hexafluoride and helium. The former acts like water in our atmosphere, appearing in both liquid and vapor forms, while the latter takes the place of dry components of our atmosphere, like nitrogen. The bottom of the chamber is heated, forming a liquid layer of sulfur hexafluoride, seen at the bottom of the animation above. The top of the chamber is cooled, encouraging sulfur hexafluoride vapor to condense and form droplets that fall like rain. A top view of the same apparatus during a different experiment is shown in this previous post.

    When droplets fall through the chamber, their wakes mix cold vapor from near the drop with warmer, ambient vapor. This changes the temperature and saturation conditions nearby and kicks off the formation of microdroplets. These are the cloud of tiny black dots seen above. Under the right conditions, these microdroplets grow swiftly as more vapor condenses onto them. In time, they grow heavy enough to fall as rain drops of their own. (Image credits: P. Prabhakaran et al.; via APS Physics; submitted by Kam-Yung Soh)

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    Flames in Freefall

    Gravity is such an omnipresent force in our lives that we frequently forget how strongly it affects our daily experiences and how differently nature behaves without it. A wonderful example of this is the simple flame of a candle. On Earth, a candle flame is tear-drop-shaped and elongated, burning hotter near the bottom and glowing yellow from soot at the top. But, as Dianna demonstrates with her free-fall experiment, this shape is due entirely to the effects of gravity. Buoyant forces make the hot air near the candle rise, pulling in cooler air and fresh oxygen at the base while stretching out the flame. In microgravity – or free-fall – flames are instead spherical, their shape driven by molecular and chemical diffusion. Check out the full video to see more effects of acceleration on flames. (Video credit: Physics Girl)