For those from colder climates, icicles are a familiar part of winter. They come in a multitude of shapes and sizes, many of which have been captured and cataloged in the Icicle Atlas project. The site contains images, videos, and measurements of more than 230 icicles grown in the lab over the course of four years of research into how and why icicles form the way they do. One interesting result of the work was the discovery that the ripples commonly found on icicles are directly related to impurities. Icicles grown with pure water remain smooth, and only those with ionic impurities, like salt, develop ripples. Check out more images and icicle research at the Atlas. (Video credit: A. Chen and S. Morris/The Icicle Atlas; via Discover)
Tag: science
Blast Waves Visualized
Typically, shock waves are invisible to the human eye. Using sensitive optical techniques like schlieren photography, researchers in a lab can visualize sharp density gradients like shock waves or even the slight density variations caused by natural convection. But it takes some special conditions to make shock waves visible to the naked eye. The blast wave of the explosion in the photo above is a great example. The leading edge of the shock wave and the heat of the explosion create a strong, sharp change in density. That density change is accompanied by a change in the air’s refractive index. As light travels from the distance toward the camera, it’s distorted–more specifically, refracted–when it travels through the blast wave and its wake. And, in this case, that visual distortion is strong enough that we can clearly see the outlines of the shock waves moving out from the explosion. The apparent horizontal line through the blast wave is probably the intersection of a weaker secondary shock wave with the initial expanding shock wave. (Image credit: Defense Research and Development Canada; via io9)
Make Your Own Dancing Droplets
As a follow-up to last week’s “dancing droplet” post, here’s a video that describes how to recreate the experiment yourself at home. The droplet motion is driven by the two-component structure of the droplets, where differing evaporation rates and surface tension values between the two fluids in the drop cause the attractions and chasing behavior you see. To demonstrate this at home, you’ll need glass, fire (for sterilization), tweezers, a pipette, water, and food coloring. Looks like a fun way to spend a weekend afternoon! (Video credit: M. Prakash et al.; via io9)
Lab-borne Tornadoes
Conventional wind tunnels are great, but some aerodynamic testing requires facilities of a different nature. The video above is from the WindEEE dome, a hexagonal chamber with sixty fans on one wall, eight directional fans on the other five walls, and six fans in the upper chamber. Each is individually computer controlled, allowing the researchers to create straight flows as well as complex vortical ones. The video shows their tornado flow, which stands 5 m tall and swirls at 30 m/s. They can also move the tornado around the chamber at 2 m/s. This capability enables a kind of scale-model analysis of tornadoes and their impact that’s not possible in most facilities. You can read more about the dome at New Scientist or the WindEEE website. (Video credit: New Scientist/WindEEE; submitted by entropy-perturbation)
Rowing Water Striders
Water strider insects are light enough that their weight can be supported by surface tension. For some time, they were thought to propel themselves by using their long middle legs to generate capillary waves–ripples– that pushed them forward, but juvenile water striders are too small for this technique to work. Instead researchers found that water striders move by using their middle legs like oars. The leg motion creates vortices about 4 mm below the water surface, and this water moving backward propels the insect forward. In the photos above, the scientists visualized the flow by sprinkling thymol blue on the water and letting the striders move freely. You can learn more about the work here or in this Science Friday episode. (Photo credits: J. Bush et al.)
Dancing Droplets
What makes drops of food coloring able to dance, chase, sort themselves, or align with one another? This unexpected behavior is a consequence of food coloring consisting of two mixed liquids: water and propylene glycol. Both have their own surface tension properties and evaporation rates, which ultimately drives the behavior you see in the animations above. Both long-range and short-range interactions are observed. The former are due to vapor from each droplet adsorbing onto the glass around the droplet, thereby changing the local surface tension and causing nearby drops to feel an attractive force. The short-range effects are also surface-tension-driven. Droplets with lower surface tension will naturally try to flow toward areas of higher surface tension, which causes them to “chase” dissimilar adjacent drops. You can learn more about the research in the videos linked below (especially the last two), or you can read about the work in this article or the original research paper. (Image credit: N. Cira et al., source videos 1, 2, 3, 4; GIFs via freshphotons; submitted by entropy-perturbation)
“Jack and the Giant”
This fantastic music video by Kim Pimmel is a beautiful merger of art and fluid dynamics. Using household goods (and some slightly more exotic ferrofluid), the video shows how mesmerizing diffusion, buoyancy, Marangoni flow, and other fluid effects can be up close. It may also be the first time I’ve ever seen fluid dynamics–specifically bubbles–used as characters! Also be sure to check out some of his previous videos, many of which also feature cool fluid dynamics. (Video credit and submission: K. Pimmel)
Encapsulating Drops
Building and manipulating drops containing multiple chemicals is useful in pharmaceutical applications. But it can be a challenge to encapsulate multiple fluids without mixing them immediately. The research poster above describes a clever and simple method of building these compound drops. It uses a crosswise array of fibers, as seen in the top image. Dyed water droplets are placed at each intersection, pinning them in place. Then a larger drop of oil is added to the vertical fiber. As it runs down the fiber, it collects and encapsulates the individual droplets, creating the compound drop seen in the bottom photo. (Photo credit: F. Weyer et al.)
Sand Dunes
Sand dunes form with a gentle incline facing the wind and a steeper slip face pointing away from the wind. Most slip faces are angled at about 30 to 34 degrees–called the angle of repose. The shape is determined by the dune’s ability to support its own weight; add more sand and it will cascade down the slip face in a miniature avalanche. Similarly, if you disturb sand on the slip face by digging a hole at the base, you get the cascading collapse seen in this video. By removing sand, the dune’s equilibrium is broken and it can no longer support its weight. This makes sand flow down the slip face until enough is shifted that the dune can support itself. Being a granular material, the sand itself appears to flow much like a fluid, with waves, ripples and all. (Video credit: M. Meier; submitted by Boris M.)
Reader Question: When Mercury Meets Lava
Reader lucondri asks:
What happens when mercury touches lava?
That’s an interesting thought experiment, but hopefully no one tries it any time soon given mercury’s toxicity. So, what might happen? Mercury has a boiling point just under 630 Kelvin, and, although the temperature of molten lava varies, it’s between 970 and 1470 Kelvin when it first erupts. So mercury would definitely vaporize (i.e. boil) on contact with lava. (Again, this is very bad for anyone nearby.) If you’re curious what boiling liquid mercury looks like, wonder no further.
Molten lava is much, much hotter than the boiling point of mercury, though, so there’s a possibility that the mercury won’t boil away instantly. This is because of the Leidenfrost effect, where a thin layer of vapor forms between a liquid and an extremely hot surface. The vapor has such low friction that the liquid can essentially skate across a surface, and it doesn’t boil away instantly because the vapor insulates it from the extreme heat. After some digging, I found a paper that placed the Leidenfrost temperature of mercury between about 850 and 950 Kelvin, meaning that fresh lava is probably hot enough to generate mercury Leidenfrost drops.
So pouring a lot of mercury on lava will probably result in some boiling, but there’s also a good chance that it will form a bunch of skittering mercury droplets that will stick around awhile before they evaporate into toxic mercury gas. That said, it’s a lot easier and safer to watch awesome Leidenfrost drop videos with other liquids. (Collage credit: N.Sharp; images sources: Z. T. Jackson, and A.Biance)
