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Tag: bouncing droplets

  • Hot Droplets Bounce

    Hot Droplets Bounce

    In the Leidenfrost effect, room-temperature droplets bounce and skitter off a surface much hotter than the drop’s boiling point. With those droplets, a layer of vapor cushions them and insulates them from the hot surface. In today’s study, researchers instead used hot or burning drops (above) and observed how they impact a room-temperature surface. While room-temperature droplets hit and stuck (below), hot and burning droplets bounced (above).

    In this case, the cushioning air layer doesn’t come from vaporization. Instead, the bottom of the falling drop cools faster than the rest of it, increasing the local surface tension. That increase in surface tension creates a Marangoni flow that pulls fluid down along the edges of the drop. That flow drags nearby air with it, creating the cushioning layer that lets the drop bounce. In this case, the authors called the phenomenon “self-lubricating bouncing.” (Image and research credit: Y. Liu et al.; via Ars Technica)

    A room temperature droplet strikes and sticks to a scratched glass surface.
  • A Game of Toss

    A Game of Toss

    Over the past few years, we’ve seen lots of droplets bouncing and walking on waves. But today’s example is a little different. In this set-up, the wave is a large standing wave that sloshes from side-to-side in a narrow container. As it does, the wave catches and tosses a large ~3mm water droplet. The system is surprisingly stable, with this game of catch lasting for tens of thousands of cycles and up to 90 minutes before the droplet coalesces. The researchers found that, if the droplet tries to wander from its spot, the oscillating surface wave corrects it, guiding the droplet back to the optimal position. (Image and research credit: C. Sandivari et al.; via APS Physics; submitted by Kam-Yung Soh)

  • Hydrodynamic Spin Lattices

    Hydrodynamic Spin Lattices

    Droplets bouncing on a fluid bath display some strikingly quantum-like behaviors thanks to the interactions between a drop and its guiding surface wave. Here, researchers use submerged wells beneath the drop to confine each droplet into a space where it bounces in a clockwise or anticlockwise trajectory.

    (a) An illustration of the experimental set-up and (b) top-down image of the spin lattice.

    With an array of these wells, the droplets form a lattice. Each drop remains in its well, but its wave travels beyond and interacts with nearby wells. Through this interaction, the researchers found that lattices tended to synchronize, similar to the way groups of fireflies will synchronize their flashing. This sort of behavior is also observed in quantum systems, and the researchers hope that further studying their bouncing droplets will give insight into quantum spin systems and their behaviors. (Image and research credit: P. Saenz et al.; via Nature; submitted by Kam-Yung Soh)

  • Bouncing Off Hydrophilic Surfaces

    Bouncing Off Hydrophilic Surfaces

    Droplets typically bounce off hydrophobic surfaces due to air trapped beneath the liquid that prevents contact between the drop and surface. But even extremely smooth, hydrophilic surfaces can elicit a bounce under the right circumstances, as shown in a new study.

    The key is that the droplet must bounce at exactly the right speed. If the bounce has too much momentum, it will squeeze the nanometer-sized air cushion too thin, allowing contact. Too slow and the Van der Waals attraction between the droplet molecules and wall molecules will have time to act. But between those lies a sweet spot where the dimple and cushion of air beneath the drop keep it from impacting. (Image credit: droplets – klickblick, drop bounce – J. Kolinski, bounce sim – J. Sprittles et al.; research credit: M. Chubynsky et al.; submitted by James S.)

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    Superwalking Droplets

    Vibrate a pool of silicone oil and you can generate walking droplets. Drive the vibration at two simultaneous frequencies and you can support much larger droplets, known as superwalkers. These superwalkers have their own intriguing dynamics, a few of which are featured in this video.

    Superwalkers can create promenading pairs, chase one another, orbit, and even form ordered and disordered crystals. They can even generate stop-and-go traffic patterns. As with regular walkers, these complex behaviors come from the interaction of bouncing droplets with their ripples and those of their neighbors. (Image, video, and research credit: R. Valani 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.)

  • Bouncing Off a Moving Wall

    Bouncing Off a Moving Wall

    There are many ways to repel droplets from a surface: water droplets will bounce off superhydrophobic surfaces due to their nanoscale structures; a vibrating liquid pool can keep droplets bouncing thanks to its deformation and a thin air layer trapped under the drop; and heated surfaces can repel droplets with the Leidenfrost effect by vaporizing a layer of liquid beneath the droplet. But all of these methods will only work for certain liquids under specific circumstances. 

    More recently, researchers have begun looking at a different way to repel droplets: moving the surface. The motion of the plate drags a layer of air with it; how thick that layer of air is depends on the plate’s speed. (Faster plates make thinner air layers.) Above a critical plate speed, a falling droplet will impact without touching the plate directly and will rebound completely. This works for many kinds of liquids – the researchers used silicone oil, water, and ethanol – across many droplet sizes and speeds. The key is that the air dragged by the plate deforms the droplet and creates a lift force. If that lift force is greater than the inertia of the droplet, it bounces. (Image and research credit: A. Gauthier et al., source)

  • Growing Droplets on a Trampoline

    Growing Droplets on a Trampoline

    Droplets on a liquid surface will typically coalesce, thanks to gravity and the low viscosity of the air layer between them and the pool. In certain cases, droplets will partially coalesce, producing smaller and smaller droplets until they finally coalesce completely. Vibrating the liquid surface can help prevent this coalescence but only when droplets are small.

    In fact, if the pool is more viscous than the droplets, bouncing can be used to produce droplets of a desired size, as shown above. Because the droplets are less viscous, they deform more than the pool does – behaving somewhat like a bouncy ball hitting a rigid wall. In this system, large droplets are unstable and will undergo partial coalescence until they are small enough to bounce stably. The size of stable drops is determined by the frequency and acceleration of the bouncing bath; by tuning these parameters, researchers can select what size droplets they want to end up with. (Research credit: T. Gilet et al.; images and submission by N. Vandewalle)

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    Bouncing Droplets

    Droplets bouncing on a pool form a beautiful and fascinating system, as recently featured by Physics Girl, Veritasium, and Smarter Every Day. The Lutetium Project – a consortium of French physics, graphic design, and music students – have their own take on the subject with beautiful short videos constructed from experimental research footage. With simple text explanations and lovely original music, they combine science, art, and outreach brilliantly. Also check out their quantum walker video and be sure to subscribe to their channel (in English or French) for more!  (Video credit: The Lutetium Project; submitted by @g_durey)

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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)