Ocean currents play a major role in the weather and climate of our planet. This video shows a simulation of the surface ocean currents in the Mediterranean and Atlantic over an 11-month period. Each second corresponds to 2.75 days. You’ll see many swirling eddies in the Mediterranean and more flow along the coastlines in the Atlantic. One observation worth noting: near the end of the video, you’ll notice that flow through the Strait of Dover between England and France changes its direction, flowing back and forth depending on tidal forces. In contrast, flow through the Strait of Gibraltar is always into the Mediterranean (within the timescale of the simulation, at least). This net in-flow to the Mediterranean is due in part to the warm waters there evaporating at a higher rate than the cooler Atlantic. (Video credit: NASA; via Flow Viz; h/t to Ralph L)
Tag: evaporation
Pinning a Drop
The shape of a droplet sitting on a surface depends, in part, on its surface tension properties but also on the nanoscale roughness of the surface. Small variations in the height and shape of the surface will change the area a drop contacts as well as the contact angle the edge of the drop makes with the surface. If the contact line between the drop and surface stays the same as a droplet evaporates into the surrounding gas or dissolves into the surrounding liquid, then we say the drop is pinned. A pinned drop’s contact angle will decrease as the drop’s volume decreases. This strains the ability of the nanoscale roughness to keep the drop’s edge pinned. As individual points of contact fail, the drop’s edge may jump inward to a new contact point. This set of discrete jumps between pinned states is called a stick-jump or stick-slip mode. (Image credit: E. Dietrich et al., source; see also: E. Dietrich et al. 2015)
Mushrooms Make Their Own Breeze
Plants and other non-motile organisms have developed some clever methods to disperse their seeds and spores for reproduction. Some plants use vortex rings for dispersal; others make their seeds aerodynamic. Low ground-dwellers like mushrooms must contend with a lack of wind to lift their spores and carry them away. Instead, they use evaporative cooling to generate their own air currents.
Mushroom caps contain a lot of water and, as that water evaporates, it cools air near the mushroom, just as sweat evaporating off your skin cools you. That cooler, denser air tends to spread, carrying the spores outward. At the same time, the freshly evaporated water vapor is less dense than the surrounding air, so it rises. This combination of rising and spreading is capable of carrying spores tens of centimeters into the air, where the wind is stronger and able to carry spores further. (Image credit: New Atlantis, source; research credit: E. Dressaire et al.)
Dancing Droplets
The seemingly-alive dancing droplets are back in a new video from Veritasium. These droplets of food coloring attract, merge, and chase one another due to evaporation and surface tension interactions between their two components: water and
propylene glycol. Because the droplets are constantly evaporating, they are surrounded by a cloud of vapor that helps determine a drop’s surface tension. These localized differences in surface tension are what causes the drops to attract. The chasing is also surface-tension-driven. Like any liquid, the drops will flow from areas of low surface tension to those of higher surface tension due to the Marangoni effect. Thus drops of different concentration appear to chase one another. This is a relatively simple experiment to try yourself at home, and Derek outlines what you need to know for it. (Video credit: Veritasium; research credit: N. Cira et al.; submitted by @g_durey)
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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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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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)
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)
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)
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)
