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Search results for: “water droplet”

  • Exploding a Drop

    Exploding a Drop

    Leidenfrost drops levitate over a hot substrate on a thin layer of their own vapor, constantly replenished as the drop evaporates. For the most part, previous studies have focused on pure droplets, but a new one looks at what happens when you add surfactants – and the results are, well, explosive.

    Surfactants are a type of chemical that like to gather at the surface of a drop, and, unlike water, they’re nonvolatile – they don’t evaporate easily. So as the Leidenfrost drop evaporates and shrinks, the surface of the drop becomes more and more crowded with surfactant molecules. Eventually, they form an elastic shell around the remaining water, making evaporation more difficult.

    Inside the droplet, the temperature continues to rise, eventually reaching a point where bubbles of vapor can nucleate inside. When that happens, the bubbles expand almost instantaneously and the internal pressure spike bursts the shell, causing the entire droplet to explode. (Image and research credit: F. Moreau et al.)

  • Avoiding Ice

    Avoiding Ice

    Keeping ice from forming on a surface is a major engineering challenge. Typically, there’s no controlling certain factors – like the size and impact speed of droplets – so engineers try to tame ice by changing the surface. This can be through chemicals – as with deicing fluids used on aircraft – or by tuning the surface itself.

    One way to do this is by making the surface superhydrophobic – or extremely water repellent. These surfaces are rough on a nanoscale level, but they’re delicate, and once ice gets a grip on them, it’s even harder to remove. In a recent study, however, researchers used particles with both hydrophobic and hydrophilic – water-attracting – properties to create a superior ice-resistant surface. The combination of hydrophobic and hydrophilic aspects to the particles made supercooled droplets break up on contact with the surface. This made the drops smaller and decreased their contact time, making it harder for them to stick and freeze. (Image credit: Pixabay; research credit: M. Schwarzer et al.; via Chembites; submitted by Kam-Yung Soh)

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    Sonic Tractor Beam

    Acoustic levitation uses the radiation forces generated by sound waves to trap small, lightweight particles at the nodes of standing waves. We’ve seen this a number of times previously, both with solid objects and liquid droplets. What makes this example particularly impressive, though, is that these researchers use an array of speakers to manipulate multiple objects at once. Check out the video above for a whole series of clips from the research. (Video credit: Science; research credit: A. Marzo and B. Drinkwater)

  • Waves

    Waves

    Photographer Ray Collins is known for his striking portraits of waves, some of which I’ve featured on previous occasions. Collins is colorblind, so he focuses heavily on shape and texture in the wave, which produces some stunningly dramatic views of moving water frozen in time. There’s great power and beauty in breaking waves, and researchers are still actively learning just how significant they are to our planet’s cycles. 

    Note the spray blurring the edges of every wave here; these are some of the largest droplets the wave will make. As it crashes forward, the wave traps pockets of air, and, as those bubbles burst, they will create a spray of tinier droplets that carry moisture and salt into the atmosphere to seed clouds and, eventually, rain.

    Collins’ work reminds us both of the ocean’s power and its fragility as it undergoes rapid changes due to humanity’s influence. For more photos as well as a great interview with Collins, check out My Modern Met. (Image credit: R. Collins; via My Modern Met and James H.)

  • Entrained

    Entrained

    When an object hits water whether it draws air in with it depends on its shape, impact speed, and surface characteristics. In this experiment, though, there’s a bit of a twist. Here the sphere is passing through an interface with surfactants added. On the left, the sphere passes through smoothly without entraining air or creating a cavity. On the right, the same sphere impacts at the same speed but this time the interface is covered in a layer of bubbles. As a result, the sphere pulls a large air cavity into the water with it. Why the big difference?

    As the sphere passes through the bubbles, they burst, spraying the sphere with droplets. On impact, those droplets disrupt the layer of water traveling up the sides of the sphere, causing it to pull away from the surface and form a splash. Instead of smoothly coating the sphere in water, air can now stick to the sphere and get pulled in with it. (Image and research credit: N. Speirs et al., source)

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    A Musical Splatter

    High-speed video is wonderful for appreciating fluid motion in ways we can’t on our own. In this video from Warped Perception, we see what happens when a vibrating tuning fork is lowered into water. The tines of the tuning fork create a spray of tiny droplets, reminiscent of what happens in ultrasonic atomization or when blowing through an immersed straw. The ejected droplets fall slowly back onto the disturbed surface; many of them bounce rather than coalescing. This is because the surface’s vibration pushes the drops aloft again before the air layer separating the drop from the surface has the time to drain away. (Video credit: Warped Perception)

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    “Float”

    In “Float” artist Susi Sie uses water and oil to create a whimsical landscape of bubbles and droplets. Coalescence is a major player in the action, though Sie uses some clever time manipulations to make her bubbles and droplets multiply as well. Watching coalescence in reverse feels like seeing mitosis happen before your eyes. (Video and image credit: S. Sie)

  • Using Paper to Avoid Splashback

    Using Paper to Avoid Splashback

    Daily life and countless pool parties have taught us all that objects falling into water create a splash. Sometimes that splash is undesirable, and while there are many ways to tune a splash – by adding surfactants or changing the fluid’s viscosity – there’s a relatively common one that’s escaped scientific study until now. Researchers looked at how splashes change when you add a thin, penetrable fabric – commonly known as toilet paper – to the water surface. 

    Now, the common assumption is that adding a sheet of toilet paper can prevent splashback, but the story is not quite that simple. On the left, you see a splash generated without toilet paper. Because the ball is hydrophilic (water-loving), it does not pull any air into a cavity as it passes. There’s a nice axisymmetric Worthington jet formed, and it doesn’t splash very high, although some of the satellite droplets go quite a bit higher.

    On the right, we see a splash with a single sheet of toilet paper. In this case, the impact of the sphere penetrates the paper, and the way the paper deforms causes air to get sucked down into a cavity behind the ball. That creates a wider, amorphous jet that rebounds higher than the jet in clean water, though it does not shed satellite drops. 

    The researchers found that single and even double sheets of toilet paper can actually increase the height of the splash jet if the object penetrates them. The hole the object makes actually helps focus the jet. Adding a couple more layers, though, can eliminate splashing completely. (Image and research credit: D. Watson et al.)

  • Coalescence

    Coalescence

    Simple acts like the coalescence of two droplets sitting on a surface can be beautiful and complex. As the droplets come together, they form a thin neck between them, and the curvature of that surface causes capillary forces that drive fluid into the neck. For two dissimilar droplets, like the ones above, there can be additional forces. Here, the upper drop is pure water, but the lower one has added surfactants, which reduce its surface tension. That difference in surface tension creates a Marangoni flow that tends to pull fluid away from the neck. The result is that full coalescence takes longer. Depending on other factors in this tug-of-war between capillary action and Marangoni flow, the process of coalescence can look very different. In this example, there’s a fingering instability that occurs as the neck spreads. Change the circumstances slightly and the drops may chase each other instead of merging or will merge with a perfectly smooth contact front. (Image and research credit: M. Bruning et al.)

  • A Star Drop

    A Star Drop

    There are many ways to make a droplet oscillate in a star-shape – like vibrating its surface or using acoustic waves to excite it – but these methods involve externally forcing the droplet’s oscillation. Leidenfrost drops – liquids levitating on a film of their own vapor caused by the extremely hot surface below – turn themselves into stars. It all starts with the constant evaporation driven by the heat below. This creates a thin, fast-moving layer of vapor flowing beneath the drop. That vapor shears the drop, causing capillary waves – essentially ripples – that travel through the drop in a characteristic way. Those ripples in turn cause pressure oscillations in the vapor layer, alternately squeezing and releasing it. Feedback from the vapor layer then drives the droplet into star-shaped oscillations. Under the right conditions, water drops can form stars with as many as 13 points! (Image and research credit: X. Ma and J. Burton, source)

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