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Search results for: “droplet”

  • Erie Waves

    Erie Waves

    Photographer Dave Sandford braved the cold and turbulent waters of Lake Erie in late fall to capture some remarkable wave action. Like on the ocean, waves in the Great Lakes are largely driven by winds, but lakes don’t develop the constant set of rolling waves that oceans do. Instead their waves are more erratic and unpredictable. Sandford focused on capturing the moment when wind-driven waves coming into shore collided with waves rebounding from piers or rocks along the shore. The results are waves that, through Sandford’s lens, look like exploding mountainsides. Such energetic waves mix sediment and nutrients in the lake, and the spray of droplets can even loft aerosols and pollutants from the water into the atmosphere.   (Photo credit: D. Sandford; via Flow Vis)

  • When Jets Collide

    When Jets Collide

    Two liquids that collide don’t always coalesce. The image above shows two jets of silicone oil colliding. On the left, the jets collide and bounce off one another. On the right, at a slightly higher flow rate, the two jets coalesce. This bouncing, or noncoalescence, observed at lower speeds is due to an incredibly thin layer of air separating the two jets. This air layer is constantly being replenished by air that gets dragged along by the flowing oil. But if the oil flows too quickly, that air layer becomes unstable–in the same way that a droplet that falls too quickly will splash on impact. When the separating air layer becomes unstable and breaks down, the jets collide and merge. (Image credit: N. Wadhwa et al., pdf)

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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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    Liquid Fragmentation

    From spilling coffee to driving through puddles, our daily lives are full of examples of liquids fragmenting into drops. A recently published study describes how this break-up occurs and predicts what the distribution of droplet sizes will be for a given fluid. Viscoelasticity is the property that governs this droplet size distribution. Viscoelasticity describes two aspects of a fluid–its viscosity, which acts like internal friction, resisting motion–and its elasticity, the fluid’s ability to return to its original shape after stretching. Most fluids have a little bit of each of these properties, which makes them somewhat sticky, both in the sense of not-flowing-easily and in the sense of sticking-to-itself. These same properties cause viscoelastic fluids to wind up with a broader droplet size distribution, ultimately creating both more small droplets and more large droplets than a Newtonian liquid like water. (Video credit: MIT News; research credit: B. Keshavarz et al.; submitted by mrvmt)

  • Oil in Alcohol

    Oil in Alcohol

    A drop of oil impacts and falls through a pool of isopropyl alcohol. Momentum, viscosity, and diffusion combine to deform the drop into a shape that is initially like an upside-down wine glass (top image). Because the oil is both denser than the alcohol and soluble in it, the drop sinks and dissolves as it falls. The drop expands rapidly outward, thinning and formed a concave shape around its denser, sinking core (bottom image). Ultimately, the droplet will deform and fragment as it dissolves into the alcohol. (Image credit: R. La Foy et al.)

  • A Particle-Filled Splash

    A Particle-Filled Splash

    A drop of water that impacts a flat post will form a liquid sheet that eventually breaks apart into droplets when surface tension can no longer hold the water together against the power of momentum flinging the water outward. But what happens if that initial drop of water is filled with particles? Initially, the particle-laden drop’s impact is similar to the water’s – it strikes the post and expands radially in a sheet that is uniformly filled with particles. But then the particles begin to cluster due to capillary attraction, which causes particles at a fluid interface to clump up. You’ve seen the same effect in a bowl of Cheerios, when the floating O’s start to group up in little rafts. The clumping creates holes in the sheet which rapidly expand until the liquid breaks apart into many particle-filled droplets. To see more great high-speed footage and comparisons, check out the full video.  (Image credit and submission: A. Sauret et al., source)

  • Surfing on Vapor

    Surfing on Vapor

    Place a drop of liquid on a surface much, much hotter than the liquid’s boiling point, and the portion of the drop that impacts will vaporize immediately. This leaves the droplet hovering on a thin layer of vapor. With a fluid like water, the vapor state is a much more efficient insulator than the liquid state. Thus, the vapor layer actually protects the liquid droplet, enabling it to boil off at a much slower rate than if the drop were touching the heated surface. This is known as the Leidenfrost effect, and it can be used to create self-propelled droplets.  (Image credit: R. Thévenin and D. Soto)

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    Colors in Macro

    Milk, acrylic paints, soap, and oil – all relatively common fluids, but together they form beautiful mixtures worth leaning in to enjoy. Variations in surface tension between the liquids cause much of the motion we see. Soap, in particular, has a low surface tension, which causes nearby colors to get pulled away by areas with higher surface tension, behavior also known as the Marangoni effect. Adding oil creates some immiscibility and lets you appreciate both the coalescence and fragmentation of the fluids. And finally, there’s one of my favorite sequences, where bubbles start popping in slow motion. As the bubble film ruptures, fluid pulls away, breaking into ligaments and then a spray of droplets as the bubble disintegrates. (Video credit: Macro Room; via Gizmodo)

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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)

  • Dripping, Frozen

    Dripping, Frozen

    The simple drip of a faucet is more complicated when frozen in time. Any elongated strand of water tends to break up into droplets due to surface tension and the Plateau-Rayleigh instability. Whenever the radius of the water column shrinks, surface tension tends to drive water away from the narrow region and toward a wider point. This exaggerates the profile, making narrow regions skinnier and wider regions fatter. Eventually, the neck connecting the droplets becomes so thin that it pinches off completely, leaving a string of falling droplets.  (Image credit: N. Sharp)

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