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  • Settling in Straws

    Settling in Straws

    At some point in your life, you’ve probably stuck your finger over the end of a straw and used it to pick up the liquid you’re drinking. If you lift the straw so that the end is still in your drink and remove your finger from the top, the liquid level in the straw will drop, then bounce up and down a couple times before it settles. This is what we see happen in the series of snapshots in the top image. Eventually, the liquid level settles at its equilibrium position, marked by the red arrow at the far right.

    The liquid has to bounce before settling because capillary forces and the liquid’s inertia are battling it out moment by moment. Just how long the rebound takes depends on the initial height of the fluid and the depth the straw is immersed at, but it doesn’t depend on the fluid’s viscosity. Lower viscosity fluids do sometimes have a neat jet (bottom image) that forms at the immersed end of the straw, though. (Image and research credit: J. Marston et al.)

  • How Mantas Filter But Never Clog

    How Mantas Filter But Never Clog

    Manta rays spend much of their time leisurely cruising through the water with their meter-wide mouths open. As they swim, they filter plankton, which makes up most of their diet, from the water. And they do so without ever clogging. 

    The inside of the manta’s mouth is lined with gill rakers (upper right), a series of comb-like teeth. When flow hits the leading edge of these (bottom), it creates a vortex that accelerates any particles caught in the flow. They essentially ricochet along the top of the gill rakers, getting led straight into the manta’s digestive system – while excess water gets deflected between the gill rakers and back out the manta’s gills. To drive this, all the manta has to do is swim; with the right flow speed, the shape of the gill rakers handles all the filtration with no additional effort. (Image credit: manta ray – G. Flood; gill rakers – M. Paig-Tran; flow vis – R. Divi et al., source; research credit: M. Paig-Tran et al.; via The Atlantic; submitted by Kam-Yung Soh)

  • Using Sound to Print

    Using Sound to Print

    Inkjet printing and other methods for directing and depositing tiny droplets rely on the force of gravity to overcome the internal forces that hold a liquid together. But that requires using a liquid with finely tuned surface tension and viscosity properties. If your fluid is too viscous, gravity simply cannot provide consistent, small droplets. So researchers are turning instead to sound waves. 

    Using an acoustic resonator, scientists are able to generate forces up to 100 times stronger than gravity, allowing them to precisely and repeatably form and deposit micro- and nano-sized droplets of a variety of liquids. In the images above, they’re printing tiny drops of honey, some of which they’ve placed on an Oreo cookie for scale. The researchers hope the technique will be especially useful in pharmaceutical manufacturing, where it could precisely dispense even highly viscous and non-Newtonian fluids. (Image and research credit: D. Foresti et al.; via Smithsonian Mag; submitted by Kam-Yung Soh)

  • The Challenges of Blowing Bubbles

    The Challenges of Blowing Bubbles

    Although every child has experience blowing soap bubbles with a wand, only in recent years have scientists dedicated study to this problem. It turns out to be a remarkably complex one, with subtleties that can depend on the size of the wand relative to the jet a bubble-blower makes as well as the speed at which the air impacts the film. A recent study found that, at low or
    moderate speeds, the film takes on a stable, curved shape (top image), but once you increase to a critical speed, the film will overinflate and burst. The key to forming a bubble, the authors suggest, is hitting that critical speed only briefly; if you slow down before the film ruptures, then the bubble has a chance to disconnect and form a sphere without breaking. 

    The work also suggests there are two reliable methods for bubble making in this way. One is to impulsively move the wand through the background fluid, as shown in the lower animation. The other is the one familiar to children: blow a jet just fast enough to overinflate the film, then let up so the bubble forms without breaking. (Image and research credit: L. Ganedi et al.; via Ars Technica; submitted by Kam-Yung Soh)

  • Breaking Up Drops

    Breaking Up Drops

    Lots of applications – from rocket engines to ink jet printing – require breaking large droplets into smaller ones, so there are many methods to do this. Some techniques rely on fluid instabilities, others use ultrasonic vibration. But one of the most effective methods may also be the simplest: placing a mesh between large drops and their target.

    That’s the idea at the heart of this new study, which uses a wire mesh to break large droplets into a spray of finer ones 1000 times smaller. The target application is agricultural spraying, and the researchers argue that their method would allow farmers to treat their crops effectively with fewer chemicals and less run-off. Drops impacting the mesh form a narrow cone over the plant, and the smaller, slower droplets are better at sticking to the plant instead of bouncing away. They’re also less likely to injure crops, since they don’t disturb the leaves the way larger drops do. (Image and research credit: D. Soto et al.; via MIT News; submitted by Omar M.)

  • Antibubbles

    Antibubbles

    Antibubbles are peculiar and ephemeral creations. A bubble typically encloses a gas within a thin layer of fluid. As the name suggests, an antibubble does the opposite: it’s a thin film of gas enclosing a liquid droplet within a larger background liquid. That thin gas film makes antibubbles extremely delicate. Disturb it at all – as the thinning jet at the top of the animation above does – and that film will break apart, much like a soap bubble. To see more antibubble action, check out some of our previous entries, including antibubbles in a vortex and a simple way to create antibubbles.  (Image credit: C. Kalelkar and S. Phansalkar, source)

  • The Driver of Hydraulic Jumps

    The Driver of Hydraulic Jumps

    You’ve seen it a million times. When you turn on your kitchen faucet, the falling water forms a distinctive ring – known as a hydraulic jump – in the bottom of your sink. First described by Leonardo da Vinci, this phenomenon has been studied for centuries, and, for nearly all of that time, scientists assumed that gravity played a major role, even in kitchen-sink-sized hydraulic jumps. But that’s not the case.

    A newly published study shows that gravity can’t be a major player in setting the radius of these small-scale hydraulic jumps because they form the same whether the jet impinges from above, below, or sideways. Instead, the researchers found that surface tension and viscosity are the parameters that determine the jump’s formation. It’s not every day that you get to overturn a centuries-old theory in physics! (Image credit: J. Kilfiger; research credit: R. Bhagat et al.; via Silicon Republic; submitted by Patrick D.)

  • 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.)

  • A Burst of Microdroplets

    A Burst of Microdroplets

    If you hold a bubbly beverage like champagne or soda near your face, you’ll feel a light mist of tiny, nearly invisible droplets.These droplets form when bubbles reach the surface and pop, generating a tiny jet that ejects an even tinier droplet, as shown in the animation above. This process is remarkably common; its occurrence in the ocean results in billions of tons of sea salt entering our atmosphere each year. Since these tiny microdroplets stay aloft for far longer than their larger brethren, understanding how they form and just how small they can be is vital for understanding their impact on climate, pathogen spreading, and other topics. A new study suggests that the minimum size for an ejected droplet is just 1% of the size of the bubble that births it. (Image and research credit: C. F. Brasz et al., source)

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    The Kaye Effect

    Allow a stream of shampoo to fall into a pile and you’ll catch a glimpse of the bizarre Kaye effect. A jet of shampoo will briefly rise up before becoming chaotic and falling. The key to this behavior is the shear-thinning of the shampoo. When the shampoo is just sitting on a surface, it’s quite viscous, but slide your hand across it, and the shampoo will become much less resistant to flowing.

    When the jet of falling shampoo hits the pile, it creates a little dimple. Sometimes the incoming jet hits that dimple and slips along it, thanks to a sudden decrease in viscosity. That can send an outgoing jet of shampoo riding off the dimple like a ramp. As the dimple deepens, the outgoing streamer rises up until it hits the incoming jet and becomes unstable. The shampoo streamer collapses, only to be restarted when a new dimple forms. (Image and video credit: S. Mould; h/t to Guillaume D.)

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