Search results for: “droplet”

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.)
March 25, 2015
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)
March 20, 2015
Asteroid Impact
I often receive questions about how fluids react to extremely hard and fast impacts. Some people wonder if there’s a regime where a fluid like water will react like a solid. In reality, nature works the opposite way. Striking a solid hard enough and fast enough makes it behave like a fluid. The video above shows a simulated impact of a 500-km asteroid in the Pacific Ocean. (Be sure to watch with captions on.) The impact rips 10 km off the crust of the Earth and sends a hypersonic shock wave of destruction around the entire Earth. There’s a strong resemblance in the asteroid impact to droplet impacts and splashes. Much of this has to do with the energy of impact. The asteroid’s kinetic (and, indeed, potential) energy prior to impact is enormous, and conservation of energy means that energy has to go somewhere. It’s that energy that vaporizes the oceans and fluidizes part of the Earth’s surface. That kinetic energy rips the orderly structure of solids apart and turns it effectively into a granular fluid. (Video credit: Discovery Channel; via J. Hertzberg)
March 6, 2015
The Milk Crown
This frequently imitated photograph of a drop of milk splashing was taken by engineer Harold Edgerton in 1934. Edgerton pioneered the application of stroboscopic photography to everyday objects, allowing him to capture images with an effective shutter speed much faster than could be mechanically achieved. The photo captures the crown or coronet of a splash. The momentum of the incoming drop flings a thin sheet of liquid radially outward. The rim of this sheet breaks down into thin ligaments that eject tiny droplets at their tips when surface tension can no longer hold the milk together. (Image credit: H. Edgerton, via The Art Reserve; submitted by Vince G)
February 23, 2015
Laser-Made Superhydrophobics
Superhydrophobic surfaces are so repellent to water that liquids often cannot wet them. Today these surfaces are usually created with chemical coatings or deliberate manufacturing to create micro- and nanoscale structures that trap air between the drop and the surface in order to prevent adhesion. Researchers recently announced they’ve made metals superhydrophobic with laser treatments. The process is still time-consuming, but they hope it can be scaled up for wider applications. Because drops bounce so readily off the treated surfaces, it takes very little water to clean them, which may be especially useful for sanitation purposes in the developing world. Superhydrophobic materials are also good for preventing icing on aircraft wings. To learn more about the research, check out the University of Rochester’s video explanations. (Image credit: C. Guo et al., source videos 1,2; submitted by entropy-perturbation and buckitdrop)
January 23, 2015
How Rain Gets Its Smell
Light rain after a dry spell often produces a distinctive earthy scent called petrichor that is associated with plant oils and bacteria products. How these chemicals get into the air has been unclear, but new research suggests that the mechanism may come from the rain itself. When water falls on a porous surface like soil, tiny air bubbles get trapped beneath the drop. These bubbles rise rapidly due to buoyancy and, upon reaching the surface, burst and release tiny droplets known as aerosols. Depending on the surface properties and the drop’s impact speed, a single drop can produce a cloud of aerosol droplets. The research team is now investigating how readily bacteria or pathogens in the soil can spread through this mechanism. Other human-focused research has already shown that these tiny aerosol droplets can persist in the air for remarkably long periods and may help spread diseases. (Video credit: Massachusetts Institute of Technology; research credit: Y. Joung and C. Buie; submitted by Daniel B and entropy-perturbation)
January 19, 2015
Hand Dryers and Atomization
Some newer electric hand dryers, like the Dyson Airblade, use jets of high-speed air to dry hands faster than traditional models. Much of their effectiveness comes from the rapid atomization–or break-up into tiny droplets–of water on one’s hands. This is demonstrated in the animation above, which comes from a high-speed video of a water drop falling through the jets of a homemade dryer. Breaking up the water quickly disperses the microdroplets but it also speeds up evaporation by greatly increasing the exposed surface area of the water. This is similar to how you can get instant snow from throwing boiling water if it’s cold enough outside. (Image credit: tesla500, source video; submitted by Nick)
January 9, 2015
Grow Your Own Snowflakes
If your Christmas holiday was a little too green (like mine was), Science Friday has just the activity for you – grow your own snowflakes! With a few materials you probably already have and some dry ice from the store, you can grow and observe ice crystals at home. Although these crystals form from water vapor instead of water droplets like proper snowflakes, they do exhibit different structures depending on temperature and humidity, just the way natural snowflakes do. (Video credit: Science Friday/F. Lichtman)
December 30, 2014
Manipulating Fluids
Combining water-repelling superhydrophobic surfaces with water-loving hydrophilic surfaces allows scientists and engineers to manipulate common fluids. Here a hydrophilic track surrounded by a superhydrophobic background collects and distributes drops of dyed water. The wetting characteristics of the surface combined with surface tension in the liquid drives the flow. No pumping or power input is necessary. This kind of manipulation of droplets can be especially useful in biomedical applications where fast-acting, low-cost devices could be used to diagnose diseases or measure blood glucose levels. (Image credit: A. Ghosh et al., via NSF; see also source video)
December 29, 2014
Beverage Bubbles Bursting
Fizzy drinks like soda and champagne have many bubbles which rise to the surface before bursting. When the film separating the bubble and the air drains and bursts, it leaves a millimeter-sized cavity that collapses on itself. That collapse creates an upward jet of fluid which can break into tiny aerosol droplets that disperse the aroma and flavor of the drink. Similar bubble-bursting events occur in sea spray and industrial applications, too. Researchers find that droplet ejection depends on bubble geometry and fluid properties such as viscosity. More viscous liquids, for example, generate smaller and faster droplets. Learn more and see videos of bubble-bursts at Newswise. (Image credit: E. Ghabache et al.)
December 3, 2014

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