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Search results for: “droplet”

  • Bubble Break-Up

    Bubble Break-Up

    When bubbles burst, they spray a myriad of tiny droplets into the air. In general, the older a bubble gets, the thinner it is, thanks to gravity draining its liquid away. When older bubbles burst, they create tinier and more numerous droplets (upper right) compared to a younger bubble (upper left). But there are more forces than just gravity at play.

    Bubbles also undergo evaporation – most effectively at the apex. Evaporation cools the cap of the bubble, increasing its surface tension and triggering a Marangoni flow that helps restore fluid to the bubble film. This stabilizes an aging bubble. 

    Contamination plays a role as well. The bright spots in the bottom image reveal bacteria in the bubble’s cap. Compared to a clean bubble, these contaminated ones can survive far longer and, when burst, produce 10 times as many droplets as a clean bubble of the same age. That has major implications for disease transmission, especially for bacteria that spend a significant portion of their life cycle in liquids. (Image and research credit: S. Poulain and L. Bourouiba; see also Physics Today)

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    Reader Question: Inside a Vortex

    Reader embersofkymillo asks:

    Hey FYFD, could you do some analysis/explanations behind the physics of this vortex stuff? I love when you do spots on Slow Mo Guys vids and figured I’d share a recent one w you 

    I enjoy doing that, too! So let’s talk a little about vortices. What Dan’s tea stirrer is doing is creating a low-pressure core for a vortex. We can see just how strong that low pressure region is by the way it sucks the air-water interface down toward the spinning arms. Eventually the interface and stirrer meet, and what was once a single, smooth(ish) surface gets torn into a myriad of bubbles. (As an aside, those bubbles get loud.) 

    I also like the sequence of sugar cube drops because they make for some very cool splashes. Notice how the orientation of the cube’s edges as it hits determines the shape of the inital splash curtain. The asymmetry borne out of that impact actually follows through all the way through the seal of the cavity behind the cube. It reminds me of this oldie-but-goodie video on drops hitting different shapes. (Video and image credit: The Slow Mo Guys; submitted by embersofkymillo)

  • The Leidenfrost Crack

    The Leidenfrost Crack

    In 1756, Leidenfrost reported on the peculiar behaviors of droplets on surface much hotter than the liquid’s boiling point. Such droplets were highly mobile, surfing on a thin layer of their own vapor and were prone to loud cracking noises.

    More recently, scientists have observed that drops with an initially small radius eventually rocket off the hot surface whereas larger drops end their lives in an explosion (above) – the source of Leidenfrost’s crack. Now researchers have explained why drops of different sizes have such different fates. The key is their level of contamination.

    To reach the take-off radius, the drop has to evaporate a significant portion of its volume. For an initially-large drop, that’s tough because any solid contaminants in the drop will build up along the surface of the drop as it shrinks. Eventually, they restrict the liquid from evaporating, which thins the vapor layer the drop sits on. It sinks until a part of it touches the surface. The sudden influx of heat from the surface explosively destroys whatever remains of the drop. (Image and research credit: S. Lyu et al.; via Brown University; submitted by gdurey)

  • Inside an Evaporating Drop

    Inside an Evaporating Drop

    The evaporation of a simple droplet holds far more complexity than one would expect. If you look closely at the edge of the drop, there’s a tiny, beautiful display at work. It begins with small variations in surface tension at the contact line where solid, liquid, and gas meet. These could be caused by local temperature or concentration differences; either way, the gradient in surface tension creates a flow. It starts out as a series of microjets spaced evenly around the contact line (left). 

    As the microjets strengthen, they merge into larger and larger vortical structures (right). This kind of feature – large structures emerging from smaller ones – is known as an inverse cascade. Fluid dynamicists have traditionally studied the classic (turbulent) energy cascade, where kinetic energy moves from large scales into smaller ones, but researchers are beginning to recognize more situations where the inverse cascade occurs, such as in the storms of Jupiter. (Image and research credit: A. Ghasemi et al., source)

  • How Rain Can Spread Pathogens

    How Rain Can Spread Pathogens

    Rainfall can help spread pathogens from an infected plant to healthy ones. This transfer can happen both through droplets and by dry-dispersal of pathogen spores (top). When a raindrop hits a leaf, its initial spread triggers a vortex ring of air that can lift thousands of dry spores into a swirling trajectory (bottom). That boost in height carries spores beyond the slower wind speeds of the plant’s boundary layer and into faster air streams that disperse it toward healthy plants. (Image and research credit: S. Kim et al.)

  • Granular Instabilities

    Granular Instabilities

    Granular mixtures show surprising similarities to fluids, even though their underlying physics differ. The latest example of this is a Rayleigh-Taylor-like instability that occurs when heavy particles sit atop lighter ones. By combining vertical vibration and an upward gas flow, researchers found that the lighter particles form fingers and bubbles that seep up between the heavier grains (upper left). Visually, it looks remarkably similar to a lava lamp or other Rayleigh-Taylor-driven instability (upper right).

    But the physics behind the two are distinctly different. In the fluid, buoyancy drives the instability while surface tension acts as a stabilizing force. There’s no surface tension in a granular material, though. Instead, the drag force from gas flowing upward provides the vertical impetus while friction between the grains – essentially an effective viscosity – replaces surface tension as a stabilizing influence.

    The similarities don’t stop there, though. When the researchers tested a “bubble” of heavy grains suspended in lighter ones (lower left), they found that, instead of sinking, the granular bubble split in two and drifted downward on a diagonal. Eventually, those daughter bubbles also split. Again, visually, this looks a lot like what happens to a drop of ink or food coloring falling through water (lower right), but the physics aren’t the same at all. 

    In the fluid, the breakup happens when a falling vortex ring splits. In the granular example, gas moving upward tends to channel around the heavy grains because they’re harder to move through. Eventually, this builds up a solidified region under the bubble. When the heavy grains can’t move directly down, they split and sink through the surrounding suspended particles until they build up another jammed area and have to split again. (Image credits: granular RTI – C. McLaren et al.; RTI simulation – M. Stock; bag instability – D. Zillis; research credit: C. McLaren et al.; submitted by Kam-Yung Soh)

  • The Bouncing Drop

    The Bouncing Drop

    For a droplet to bounce, we expect it to hit a wall or a sharp interface of some kind. But in a new study, researchers demonstrate a droplet that bounces with neither. Shown above is an oil droplet sinking through a stratified mixture of ethanol (toward the top) and water (toward the bottom). Because the oil is heavier than ethanol, it initially sinks, dragging some of the ethanol with it as it falls. Over time, some of that ethanol rises again, forming what’s known as a buoyant jet.

    Simultaneously, the gradient of ethanol to water between the top and bottom of the drop creates an imbalance in surface tension. The ethanol near the top of the drop has a lower surface tension than the water at the bottom. This creates a downward Marangoni flow along the drop interface.

    The bounce itself happens quickly after a long, slow sinking period. As the drop’s sinking slows, the buoyant jet weakens until it disappears completely. At the same time, the downward Marangoni flow pulls fresh ethanol-rich fluid toward the top of the drop. That increases the surface tension difference and strengthens the Marangoni flow, creating a positive feedback loop. In less than a second, the Marangoni flow increases by two orders of magnitude, pulling so hard that the drop shoots upward.

    That resets the cycle by weakening the Marangoni flow and strengthening the buoyant jet. The droplet can continue bouncing for about 30 minutes until the concentration gradient is so well-mixed that the cycle can’t continue. (Image and research credit: Y. Li et al.; via APS Physics; submitted by Kam-Yung Soh)

  • Freezing Stains

    Freezing Stains

    When they evaporate, drops of liquids like coffee and red wine leave behind stains with a darker ring along the edges, thanks to capillary action and surface tension pulling particles to that outer edge. In contrast, sublimating a frozen droplet leaves a stain pattern that concentrates at the center (top). When droplets freeze from the surface upward, particles within the droplet are driven toward the center as the freeze front pushes toward the drop apex. The final shape of the stain depends on the initial geometry of the droplet, and the concentration of particles toward the center occurs because of the way that the particle freezes, not how it sublimates (bottom). 

    Since many industrial processes rely on droplet evaporation to spread coatings, this work offers a new way to control the final outcome. (Image and research credit: E. Jambon-Puillet, source)

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    Breaking

    As waves fold over and break, they trap air, creating bubbles of many sizes. The smallest of these bubbles can be only a few microns across and persist for long times compared to larger bubbles. When they burst, they create tiny droplets that can carry sea salt up into the atmosphere to seed rain. Understanding how these bubbles form and how many there are of a given size is key to predicting both oceanic and atmospheric behaviors. Numerical simulations like the one featured in the video above reveal the dynamic collisions that create these tiny bubbles and help researchers learn how to model the tiniest bubbles so that future simulations can be faster. (Image and video credit: W. Chan et al.)

  • Resonating on a Bounce

    Resonating on a Bounce

    When we think of resonance, we often think of it in simple terms: hit the one right note, and the wine glass will shatter. But resonance isn’t always about a one-to-one ratio between a driving frequency and the resonating system. Especially in fluid dynamics, we often see responses that occur at other, related frequencies.

    One of the simplest places to see this is with a droplet bouncing on a bath of fluid. Above you see a liquid metal droplet bouncing on a bath of the same metal. At low amplitude, the pool surface moves at the driving frequency and a droplet bounces simply upon that surface, with one bounce per oscillation. Increase the amplitude, though, and the droplet’s bounce changes. It bounces twice – one large bounce and one small bounce – in the time it takes for the pool surface to go through one cycle. This is called period doubling because the bouncing occurs at twice the driving frequency.

    Turn the amplitude up further, and the system undergoes another change. Faraday waves form on the surface. They resonate at half the driving frequency, and a droplet’s bouncing will sync up with the waves. That means the droplet returns to a one-to-one bounce with the waves, but the waves themselves are no longer reacting at the driving frequency. It’s this kind of complexity that makes fluid systems fertile grounds for studying paths toward chaos. (Image and research credit: X. Zhao et al.)

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