FYFD

Search results for: “water droplet”

  • Boiling Water to Snow

    Boiling Water to Snow

    When it’s really cold outside–to the tune of -40 degrees (Fahrenheit or Celsius)–physics can get a little crazy. In this photo, boiling-hot water from a thermos turns into an instant snowstorm when tossed. How is this possible? It turns out there are a combination of factors that affect this. Firstly, the rate of heat transfer between two objects depends on the magnitude of the temperature difference between them. The bigger the difference in temperature, the faster the hot object cools. Of course, as the hot object cools down, the temperature difference between it and its surroundings is smaller and the rate of heat transfer decreases.

    The second important factor here is that the water is being tossed. When you throw water, it breaks into droplets, and droplets have a large surface area compared to their volume. As it turns out, the rate of heat transfer also depends on surface area. By breaking the hot water into smaller droplets, you increase the surface area exposed to the cold air, allowing the hot water to freeze faster. (Image credit: M. Davies et al.; via Gizmodo)

    Also: Since there are a few events scheduled around the country over the next couple months, I’ve added an events page where you can find details for those appearances. And as always, if you’re interested in scheduling a talk or event, feel free to contact me directly.

  • Collecting Water in the Desert

    Collecting Water in the Desert

    Desert-dwelling plants like cactuses have to be efficient collectors of water. Many types of cactus are particularly good at gathering water from fog that condenses on their spines. Droplets that form near a spine’s tip move slowly but inexorably toward the base of the spine so that the cactus can absorb them. The secret to this clever transport lies in the microstructure of the spine’s surface. The

    Gymnocalycium baldianum cactus, for example, has splayed scales along its spines. Capillary interactions with the scales result in differences in curvature on either side of the droplet. Curved fluid surfaces generate what’s known as Laplace pressure, with a tighter radius of curvature causing a larger Laplace pressure. Because the curvature of the droplet varies from the base side to the tip side of the spine, the difference in Laplace pressures across the droplet creates a force that drives the droplet toward the spine’s base. (Image credit: C. Liu et al., source)

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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)

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    The Droplet Slide

    One of the joys of science is the sense of discovery that can come even from looking at something seemingly simple. Take, for example, a water droplet sitting on a plate. If you slowly tilt the plate, the droplet’s shape will shift until a critical angle where it starts sliding down the plate. But what happens to two initially different droplets? As this video shows, tilting two droplets of initially different shapes and returning them to horizontal causes the droplets to assume the same shape. There’s a universal behavior at work here–like nature has a kind of reset button that makes gravity and surface tension work together such that a droplet will assume a preferred shape. For an experimentalist, it’s certainly a handy way to create repeatable experiments! (Video credit: M. Musterd et al.)

  • Boiling Water in Oil

    Boiling Water in Oil

    Most people know that throwing water into hot oil is a bad idea. But, as dramatic as the results can be, the boiling of a water droplet submerged in oil is remarkably beautiful, as seen in the animations above. The initial water droplet expands as it shifts from liquid to vapor (top). At a critical volume, the expansion occurs explosively (middle), causing the bubble to overexpand relative to the pressure of the surrounding fluid. The higher pressure of the oil around it collapses the drop, which then re-expands, creating the cycle we see in the final two animations. This oscillation triggers a Rayleigh-Taylor type instability along the bubble’s interface, causing the surface corrugations observed. The vapor bubble will continue to rise through the oil, eventually breaking the surface and scattering hot oil droplets.  (Image credits: R. Zenit, source)

  • Controlling Droplet Bounce

    Controlling Droplet Bounce

    Water repellent, or hydrophobic, surfaces are common in nature, including lotus leaves, many insects, and even some geckos. These hydrophobic surfaces typically gain their water-repelling ability from extremely tiny nanoscale structures in the form of tiny hairs or specially textured surfaces. But, while the nanoscale structures impart superhydrophobicity, researchers have found that larger macroscale structures can improve water-repellent characteristics by reducing a drop’s time of contact with the surface. A smaller contact time means less chance of contamination on self-cleaning surfaces. It’s also helpful in preventing water from freezing on contact to cold surfaces – valuable, for example, in protecting airplane wings’ leading edges from icing over. This combination of nanoscale and macroscale, water-repelling structures can be found in nature, too, such as on the wings of butterflies, which must quickly shed water in order to fly. (Image credits: K. Hounsell et al.; A. Gauthier et al., source video)

  • Alligators Water Dancing

    Alligators Water Dancing

    Amorous alligators call to mates with a behavior known as water dancing. Their audible bellows are accompanied by infrasonic sound–vibrations below the 20 Hz limit of human hearing. These vibrations from their lungs excite Faraday waves in the water near the alligator’s back and make the surface explode in a dance of jets and atomized droplets. I’ve seen similar results in other instances of vibration, but this may be the only example of this I’ve seen in the wild. Researchers studying the phenomenon noted that the frequency of sound the alligators emit corresponds to a wavelength equal to the spacing of the raised scales, or scutes, on the alligators’ backs. They hypothesize that the shape of the scutes helps males create the display.  (Image credit: N. Marven, source; research credit: P. Moriarty and R. Holt; h/t to io9)

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    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)

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    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)

  • Dancing Droplets

    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)

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