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Search results for: “droplet”

  • 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)

  • 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)

  • 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)

  • A Viscous Splash

    A Viscous Splash

    The splash of a drop may be commonplace, but it is still a mesmerizing and fertile phenomenon. When it comes to splashing, scientists are still learning how to predict the outcome. Here a drop of silicon oil impacts a film of silicon oil with an even higher viscosity. The momentum of that impact creates a crater and a splash curtain that rises and expands from the initial point of impact. Because the film viscosity is higher than the drop’s, the evolution of the corona slows down. Eventually, surface tension and gravity start pulling the splash curtain back down as the crater collapses. Meanwhile at the top of the splash, capillary forces pull fluid into the rim, which becomes unstable and grows cusps that eventually eject a cloud of smaller droplets. (Image and research credit: H. Kittel et al., source)

  • Breaking With a Wave

    Breaking With a Wave

    For rocket combustion and other applications, like watering your lawn with a hose, a stream of fluid may need to be broken up into droplets. While simply spraying a liquid jet will make it break up, waving that jet back and forth will break it up faster. A recent study simulated this problem numerically to determine the exact mechanisms driving that break-up. The researchers found two major culprits.

    The first is a Kelvin-Helmholtz, or shear-based, instability. When a jet leaves the nozzle, there’s friction between it and the comparatively still air surrounding it. This creates tiny ripples in the surface that eventually grow into the distortions we can see, and it’s found in all jets, regardless of their side-to-side motion.

    The second culprit, which is only found in the oscillating jet, is a Rayleigh-Taylor instability. By moving the jet side-to-side, you’re driving the dense liquid into less dense air, which creates a different set of disturbances that also help break up the jet. The final result: swinging the jet side-to-side breaks it into smaller droplets faster. (Image and research credit: S. Schmidt et al.)

  • Collecting Fog

    Collecting Fog

    In some parts of the world, fog is a major source of freshwater, but collecting it is a challenge. Most systems use a wire mesh to capture and collect droplets, but the process is highly inefficient, pulling only 1-3% of droplets from the fog. Researchers found that this is due largely to aerodynamic effects. The presence of the wire deflects droplets around it (bottom left). To solve this, engineers introduced an electric charge into the fog. The subsequent electric field actually pulls droplets to the wires (bottom right). When applied to a mesh (top), the efficiency of fog capture improves dramatically. 

    The technique can also be used to capture water vapor that would otherwise escape from the cooling towers of power plants. The MIT researchers who developed the technique will conduct a full-scale test at the university’s power plant this fall. They hope the technique will recapture millions of gallons of water that would otherwise drift away from the plant. (Image credits: MIT News, source; image and research credits: M. Damak and K. Varanasi, source)

  • Using Embolisms to Fight Cancer

    Using Embolisms to Fight Cancer

    Blocking blood vessels by creating embolisms is, under most circumstances, very bad. But researchers are exploring ways to fight cancer by intentionally and strategically creating these blockages. In gas embolotherapy, researchers inject fluid droplets, which can carry chemotherapy drugs, into the bloodstream. Once they circulate into a cancerous tumor, they use ultrasound to vaporize the droplet and create a gas bubble. Those bubbles lodge inside the capillaries of the tumor, starving it of fresh blood and trapping the chemotherapy drugs inside. It’s a one-two punch to the cancer. Without blood flow, the cancer cells die, and, since the cancer-killing drugs get mostly trapped inside the tumor, patients may require lower dosages and endure fewer side effects. The technique is currently in animal testing, but hopefully it will be a valuable therapy for human patients in the future. (Image credit: Chemical & Engineering News; research credit: Y. Feng et al.; via AIP)

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