Tag: evaporation

Drying Blood Can Reveal Anemia
Blood is a remarkably complicated fluid, thanks in part to its many constituents. What we see here is an animation of a drop of blood evaporating at several times normal speed. As water from the blood evaporates, it causes relative changes in surface tension. These surface tension gradients cause convection inside the drop and carry red blood cells toward the outer portion of the drop. As the blood evaporates further, it leaves behind different patterns that depend on which parts of the whole blood mixture were deposited in each region. Interestingly, the final desiccation patterns can indicate the healthiness of a patient. Below are images of dried blood patterns from (left) a healthy individual and (right) an anemic individual. (Image credits: D. Brutin et. al., source)
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February 15, 2016
Tears of Wine
Give your wine glass a swirl and afterward you may notice little rivulets of wine along the side of your glass. These so-called “tears of wine” or “wine legs” are caused by a combination of evaporation, surface tension, and gravity. After the glass has been swirled, alcohol from the thin layer of wine on the glass wall quickly evaporates, leaving behind a fluid that is more watery than the wine in the glass. Since water has a higher surface tension than alcohol or wine, it pulls more fluid up the wall via the Marangoni effect. This carries on until enough wine is pulled up to form a droplet that’s heavy enough to slide down the glass. This up-and-down exchange of fluid is nicely illustrated in the video above, where the tiny particles in the wine help show how flow gets drawn up even as your eye follows the drops sliding down. (Video credit: A. Athanassiadis and K. Khalil; submitted by Thanasi A.)
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February 9, 2016
Oil Film on Water
This award-winning short film features a thin layer of volatile oil on water. The oil evaporates quickest from shallow pools only microns deep, which appear bluish in the video. Surface instabilities along the edge of the pool create flow that draws oil in, generating the iridescent droplets seen floating among the evaporation pools. The droplets combine and coalesce as they come in contact with one another. Since droplets have a larger volume per surface area than the shallow pools, they evaporate more slowly. The behaviors observed here are important to applications like oil and fuel spills, which can persist longer if the floating fluid layer tends to form droplets. (Video credit: J. Hart; via txchnologist)
November 20, 2015
Water in Oil
Pouring water on an oil fire is a quick way to cause almost explosive results. Since water is denser than oil, it quickly sinks to the bottom of a container, heating up as it does. When the water reaches its boiling point, it evaporates and expands as steam. That phase change involves a huge change in volume, a fact made especially clear in the video below. The steam expands and rises, throwing droplets of oil upward and outward. These smaller atomized droplets are easier to combust, which, in the case of the video above, causes a veritable cloud of flames if a fire has already started.
(Video credits: The Slow Mo Guys and N. Moore)
June 15, 2015
Cloud Formation
Clouds are so ubiquitous here on Earth that it’s easy to take them for granted. But there’s remarkable complexity in the mechanics of their formation. This great video from Minute Earth steps through the processes of evaporation and condensation that drive basic cloud formation. After evaporation, buoyancy lifts warm, moist air upward. That warm air expands and cools until it reaches an altitude where water droplets can condense onto dust particles in the atmosphere. These droplets form the wispy cloud we see. Turbulence mixes these droplets and helps them collide and grow. Interestingly, although we understand the basic process of cloud formation, relatively little is understood about the details, and the subject is still very much an area of active research. (Video credit: Minute Earth; via io9)
April 22, 2015
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)
April 6, 2015
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)
March 30, 2015
How Eyelashes Work
New research shows that eyelashes divert airflow around the eye, serving as a passive filter that reduces dust collection and controls evaporation. Mammal hairs in places like the nose act as ram filters that trap the particles that hit them and which require regular cleaning via sneezing. Eyelashes, on the other hand, prevent dust collection by altering airflow at the surface of the eye. At the optimal length of roughly 1/3rd the width of an eye, eyelashes create a stagnation zone near the eye surface that forces air to travel above rather than through the eyelashes. This results in lower shear stress and lower flow speeds at the eye surface, both of which help reduce evaporation and shield the eye from dust. Lashes can get too long, though; the researchers found that longer lashes tended to channel higher flow speeds toward the eye surface, leading to faster evaporation rates. Thus, donning longer fake eyelashes may dry out your eyes. (Image credit: G. Diaz Fornaro; research credit: G. Amador et al.; via skunkbear)
March 2, 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
Zesty Fireballs
Zesting the skin of a citrus fruit like oranges releases a spray of tiny oil droplets. Citrus oil has several volatile components, meaning that it evaporates quickly at room temperature. It is also a liquid with a relatively low flash point, meaning that only modest temperatures (~40-60 degrees Celsius) are needed to generate enough vapor to ignite a vapor/air mixture. With volatile and flammable liquid fuels, a spray of droplets is an ideal platform for combustion because the essentially spherical droplets have a high surface area from which they can evaporate and provide vaporous fuel. (Video credit: ChefSteps)
October 13, 2014