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Search results for: “water droplet”

  • Snowmelt

    Snowmelt

    Much of the rain that falls on Earth began as snow high in the atmosphere. As it falls through warmer layers of air, the snowflakes melt and form water droplets. The details of this melting process have been difficult to capture experimentally, but a new computational model may provide insight. The basic process has a couple stages. As snow begins to melt, surface tension draws the water into concave areas nearby. When those regions fill up, the water flows out and merges with neighboring liquid, forming water droplets around a melting ice core.

    Although this same sequence was observed for many types of snow, scientists also observed some important differences between rimed and unrimed snowflakes. Rime forms when supercooled water droplets freeze onto the surface of a snowflake. Lightly rimed snow still looks light and fluffy, like the animation above, but heavily rimed snow forms denser and more spherical chunks. Because there are lots of porous gaps in heavily rimed snow, water tends to gather there during initial melting. Rimed snow was also more likely to form one large water droplet rather than breaking into multiple droplets like snow with less rime. For more, check out NASA’s video and the Bad Astronomy write-up. (Image credit: NASA, source; research credit: J. Leinonen and A. von Lerber; via Bad Astronomy; submitted by Kam Yung-Soh)

  • Atmospheric Aerosols

    Atmospheric Aerosols

    Recently, NASA Goddard released a visualization of aerosols in the Atlantic region. The simulation uses real data from satellite imagery taken between August and October 2017 to seed a simulation of atmospheric physics. The color scales in the visualization show concentrations of three major aerosol particles: smoke (gray), sea salt (blue), and dust (brown). One of the interesting outcomes of the simulation is a visualization of the fall Atlantic hurricane season. The high winds from hurricanes help pick up sea salt from the ocean surface and throw it high in the atmosphere, making the hurricanes visible here. Fires in the western United States provide most of the smoke aerosols, whereas dust comes mostly from the Sahara. Tiny aerosol particles serve as a major nucleation source for water droplets, affecting both cloud formation and rainfall. With simulations like these, scientists hope to better understand how aerosols move in the atmosphere and how they affect our weather. (Image credit: NASA Goddard Research Center, source; submitted by Paul vdB)

  • The Mist of Champagne

    The Mist of Champagne

    If you’ve ever popped open a chilled bottle of champagne, you’ve probably witnessed the gray-white cloud of mist that forms as the cork flies. Opening the bottle releases a spurt of high-pressure carbon dioxide gas, although that’s not what you see in the cloud. The cloud consists of water droplets from the ambient air, driven to condense by a sudden drop in temperature caused by the expansion of the escaping carbon dioxide. Scientifically speaking, this is known as adiabatic expansion; when a gas expands in volume, it drops in temperature. This is why cans of compressed air feel cold after you’ve released a few bursts of air.

    If your champagne bottle is cold (a) or cool (b), the gray-white water droplet cloud is what you see. But if your champagne is near room temperature ( c ), something very different happens: a blue fog forms inside the bottle and shoots out behind the cork. To understand why, we have to consider what’s going on in the bottle before and after the cork pops.

    A room temperature bottle of champagne is at substantially higher pressure than one that’s chilled. That means that opening the bottle makes the gas inside undergo a bigger drop in pressure, which, in turn, means stronger adiabatic expansion. Counterintuitively, the gas escaping the warm champagne actually gets colder than the gas escaping the chilled champagne because there’s a bigger pressure drop driving it. That whoosh of carbon dioxide is cold enough, in fact, for some of the gas to freeze in that rushed escape. The blue fog is the result of tiny dry ice crystals scattering light inside the bottleneck.

    That flash of blue is only momentary, though, and the extra drop in temperature won’t cool your champagne at all. Liquids retain heat better than gases do. For more, on champagne physics check out these previous posts. (Image and research credit: G. Liger-Belair et al.; submitted by David H.)

  • Elastic Bounces

    Elastic Bounces

    A rigid ball accelerated by a moving surface can only ever move as fast as the surface propelling it. But that’s not true for squishy objects like a water droplet. The composite image above shows the trajectory of a water droplet launched from a moving superhydrophobic surface. As the surface starts rising, it squishes the droplet like a pancake, triggering a deformation cycle where the droplet will squish and extend repeatedly. How quickly the drop changes shape depends on factors like its size and surface tension. The researchers found that a droplet’s launch was strongly affected by the ratio of the droplet’s shape-changing frequency and the frequency of the plate’s motion. When the drop’s shape changed three times faster than the surface’s motion, it would catapult off the surface with 250% of the kinetic energy of a rigid ball!

    Launching elastic balls works the exact same way as droplets, indicating that the phenomenon depends on the way the projectiles deform. The process is similar to jumping on a trampoline. If a trampolinist times her jump just right, she’ll get more energy from the trampoline and fly higher. The droplet does the same when its deformation is properly tuned to its catapult. (Image credit: C. Raufaste et al.; via APS Physics; submitted by Kam-Yung Soh)

  • Rocket Launch Systems

    Rocket Launch Systems

    If you’ve ever watched a rocket launch, you’ve probably noticed the billowing clouds around the launch pad during lift-off. What you’re seeing is not actually the rocket’s exhaust but the result of a launch pad and vehicle protection system known in NASA parlance as the Sound Suppression Water System. Exhaust gases from a rocket typically exit at a pressure higher than the ambient atmosphere, which generates shock waves and lots of turbulent mixing between the exhaust and the air. Put differently, launch ignition is incredibly loud, loud enough to cause structural damage to the launchpad and, via reflection, the vehicle and its contents.

    To mitigate this problem, launch operators use a massive water injection system that pours about 3.5 times as much water as rocket propellant per second. This significantly reduces the noise levels on the launchpad and vehicle and also helps protect the infrastructure from heat damage. The exact physical processes involved – details of the interaction of acoustic noise and turbulence with water droplets – are still murky because this problem is incredibly difficult to study experimentally or in simulation. But, at these high water flow rates, there’s enough water to significantly affect the temperature and size of the rocket’s jet exhaust. Effectively, energy that would have gone into gas motion and acoustic vibration is instead expended on moving and heating water droplets. In the case of the Space Shuttle, this reduced noise levels in the payload bay to 142 dB – about as loud as standing on the deck of an aircraft carrier. (Image credits: NASA, 1, 2; research credit: M. Kandula; original question from Megan H.)

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    Quad Copter Schlieren

    Schlieren photography is a classic method of flow visualization that utilizes small variations in density (or temperature) to make otherwise unseen air motion visible. Because changing air’s density or temperature changes its index of refraction, variations in either quantity show up as dark and light regions. Here researchers use it to reveal some of the airflow around a small quadcopter, including the vortices that spiral off each propeller and help generate the lift necessary for take-off. The full video includes a couple of neat demos, including what happens when the blades are wet (shown below). In that case, the wingtip vortices are somewhat disrupted by strings of water droplets being flung off the blades by centrifugal force. Beautiful!  (Video and image credit: K. Nolan et al., source; submitted by J. Stafford)

  • Icy Spikes

    Icy Spikes

    Water is one of those strange materials that expands when it freezes, which raises an interesting question: what happens to a water drop that freezes from the outside in? A freezing water droplet quickly forms an ice shell (top image) that expands inward, squeezing the water inside. As the pressure rises, the droplet develops a spicule – a lance-like projection that helps relieve some of the pressure. 

    Eventually the spicule stops growing and pressure rises inside the freezing drop. Cracks split the shell, and, as they pull open, the cracks cause a sudden drop in pressure for the water inside (middle image). If the droplet is large enough, the pressure drop is enough for cavitation bubbles to form. You can see them in the middle image just as the cracks appear. 

    After an extended cycle of cracking and healing, the elastic energy released from a crack can finally overcome surface energy’s ability to hold the drop together and it will explode spectacularly (bottom image). This only happens for drops larger than a millimeter, though. Smaller drops – like those found in clouds – won’t explode thanks to the added effects of surface tension. (Image credit: S. Wildeman et al., source)

    ETA: A previous version of this post erroneously said this was freezing from the “inside out” instead of “outside in”.

  • Freezing Impact

    Freezing Impact

    When a water droplet hits a frozen surface, what happens depends significantly on the temperature of the substrate. At relatively high temperatures (-20 degrees C), the droplet freezes without any cracking (upper left). As the surface gets colder, drops begin to crack. At first the cracks are relatively large and unstructured (upper right), but at lower temperatures, they grow in a network of smaller cracks with more distinctive structure (lower left). Cold temperatures can also affect the contact line where water, air, and substrate meet. This can cause droplets to splash even as they’re freezing (lower right). (Image credit: V. Thievenaz et al.; see also E. Ghabache et al.)

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    Freezing Drops

    A water droplet deposited on a cold surface freezes from the bottom up. As anyone who has made ice cubes knows, water expands when it freezes. But watch the outline of the drop carefully. The drop isn’t expanding radially outward while it freezes. Instead the remaining liquid part of the drop forms what’s known as a spherical cap, a shape like the sliced-off top of a sphere. Surface tension creates that spherical shape, but the water still has to expand when it freezes. The result? The last bit of the drop freezes into a point! This means that surface tension maintains the drop’s spherical shape, for the most part, and all the expansion the water does takes place vertically. (Video credit: D. Lohse et al.)

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    Avoiding Coalescence

    If you watch closely as you go about your day, you may notice drops of water sometimes bounce off a pool of water instead of coalescing. Fluid dynamicists have been fascinated by this behavior since the 1800s, but it was Couder et al. who explained that these droplets can bounce indefinitely as long as the thin air layer separating the drop and pool is refreshed by vibrating the pool. In this video, Destin teams up with astronaut Don Pettit to film the phenomenon in beautiful high-speed. My favorite part of the video starts around 8:18, where Destin shows Don’s experiments with this effect in microgravity. It turns out that the cello produces just the right frequencies to create a cascade of bouncing water droplets, much like a Tibetan singing bowl turned back on itself! (Video credit: Smarter Every Day; submitted by Destin and effyeahjoebiden)

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