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Search results for: “droplet”

  • Moving Droplets

    Moving Droplets

    Microfluidic devices – such as those used by individuals with diabetes to monitor their blood glucose levels – are all about transport. Typically, these devices use some kind of externally applied force, like a temperature gradient or electrical field, to force liquids through the device’s narrow channels. But a new study describes a way to move droplets without an external force.

    The researchers built their devices using two slips of glass, coated with an oil-attracting, water-repellent mixture. They attached the glass slips with a narrow spacer at one end, leaving the other end free. This made a narrow, but slightly flexible gap. When the scientists placed an oil drop inside the closed end, it spread on the glass, pulling the two sides closer to one another. Water drops, on the other hand, tried to force the walls apart, in an effort to minimize contact. Both sets of drops, interestingly, moved toward the open end of the device.

    The researchers found that the shapes assumed by the droplets create an internal pressure gradient, which, in both cases, slowly moves the drops. They call this method bendotaxis, a type of self-propulsion driven by the drops’ ability to bend the material they’re touching. It’s not a fast way to transport fluids – the drops moved only a few micrometers per second – but it may be useful for applications like drug deliveries where the liquid needs to be administered slowly over a longer period. (Image credit: TesaPhotography; research credit: A. Bradley et al.; via APS Physics; submitted by Kam-Yung Soh)

  • What Drives Droplets

    What Drives Droplets

    There’s been a lot of interest recently in what goes on inside droplets made up of more than one fluid as they evaporate. This can be entertaining with liquids like whiskey or ouzo, but it has practical applications in ink-jet printing and manufacturing as well. And a new experiment suggests that we’ve been fundamentally wrong about what drives the flow inside these drops.

    As these drops evaporate, a donut-shaped recirculating vortex forms inside them, as seem in the cutaway views above. Conventional wisdom says that vortex is driven by surface tension. Evaporation of components like alcohol is more efficient at the edges of the drop, and as the alcohol evaporates, it creates a higher surface tension at the drop’s edge than at its peak. Marangoni forces then pull fluid down toward the edges, creating the vortex. That explanation is  consistent with observations of a sessile drop sitting on top of a surface (left side of images).

    But those observations are also consistent with another explanation: evaporating ethanol makes the local density higher, so alcohol-rich parts of the drop rise toward the peak while alcohol-poor regions sink. This difference in density would also create a flow pattern consistent with observations. So which is the real driver, surface tension or gravity?

    To find out, researchers flipped the drop upside-down (right side of images). When hanging, the preferred flow direction due to surface tension doesn’t change; flow should still go from the deepest point on the drop toward the edge. But gravity is swapped; alcohol-rich areas should be found near the edge and attachment points of the drop because buoyancy drives them there. And that is exactly what’s observed. The flow direction inside the hanging droplet is consistent with the direction prescribed by buoyancy-driven flow, thereby upending conventional wisdom. It turns out that gravity, not surface tension, is the major driver of internal flow in these multi-component droplets! (Image and research credit: A. Edwards et al.; via APS Physics; submitted by Kam-Yung Soh)

  • Growing Droplets

    Growing Droplets

    The moisture in clouds eventually condenses into droplets that grow into raindrops and fall. Some steps in this process are well understood, but others are not. In particular, scientists have struggled with the problem of how droplets grow from about 30 microns to 80 microns, where they’re big enough to start falling and merging.

    Laboratory experiments and numerical simulations (below) have shown that turbulence can help drive small water drops together. When droplets are tiny and light, they simply follow the air flow. But when they’re a little heavier, turbulent eddies (seen in orange below) act like miniature centrifuges, flinging larger water droplets (shown in cyan below) out into clusters, where they’re more likely to collide with one another.

    Although this effect has been seen in experiments and simulation, it’s been difficult to capture in clouds themselves. But a new set of test flights (above) confirms that this mechanism is present in the wild as well! (Image credit: UCAR/NCAR Earth Observing Laboratory, P. Ireland et al., source; research credits: M. Larsen et al., P. Ireland et al.; via APS Physics; submitted by Kam-Yung Soh)

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  • 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)

  • Manipulating Droplets Remotely

    Manipulating Droplets Remotely

    Using acoustic levitation and an array of carefully-placed speakers, researchers can manipulate droplets without touching them. This lets scientists study the physics of droplet coalescence (top) without interference from solid surfaces, but it also provides opportunities for mixing two different substances in the final droplet. 

    On the bottom left, we see a droplet formed from the coalescence of a dyed droplet (visible as gray) and an undyed droplet. The swirling and mixing in the levitating droplet is fairly slow. By contrast, the droplet on the right is vibrated by manipulating the sound waves holding it aloft. This mixes the droplet quite efficiently, allowing it to reach a uniform state more than six times faster than the other droplet. (Image and research credit: A. Watanabe et al., source)

  • Spinning Droplet Galaxies

    Spinning Droplet Galaxies

    Water flung from a spinning tennis ball takes on a shape reminiscent of a spiral galaxy. As it detaches, water leaves the surface with both the tangential velocity of the spinning ball and a radial velocity due to the centrifugal force flinging it. The continued spin of the ball makes the thin ligaments of water still attached to it spiral and stretch. Eventually, surface tension can no longer hold the water together against the centrifugal forces, and the ligaments split into a spray of droplets. (Image credit: W. Derryberry and K. Tierney)

  • Giving Droplets a Kick

    Giving Droplets a Kick

    Giving droplets a kick by accelerating the surface they sit on creates elaborate shapes as the drops respond. As the surface accelerates upward, the droplet flattens into a pancake. When the plate slows down, the droplet continues rising, stretching into a cone as its rim flies upward and its lower surface adheres to the surface. The rim retracts with a constant acceleration while the drop detaches with a constant velocity. That velocity depends on how well it adheres to the surface. The interplay between those two variables determines how conical or cylindrical the drop appears. See more in the full video below. (Image and video credit: P. Chantelot et al.)

  • Nestling Droplets

    Nestling Droplets

    Pay attention after a rainfall, and you may notice beads of water gathering in the corners of a spider’s web or along the leaves of a cypress tree (bottom right). Look closely and you’ll notice that the largest droplets don’t form along a straight fiber. Instead they nestle into the corners of a bent fiber (top image). Researchers recently characterized this corner mechanism and found that the angle at which the largest droplets form is about 36 degrees. This angle provides the optimal conditions for capillary action and surface tension to hold large drops in place. At smaller angles, a growing droplet’s weight pulls it down until the thin film holding the droplet near the top ruptures and the droplet falls. At larger angles, a heavy droplet will slowly detach from one side of its fiber and shift toward the other side until its weight is too great for the wetted length of fiber to hold. Then it detaches completely and falls. (Research and image credit: Z. Pan et al.; via T. Truscott)

  • Wrapping Droplets

    Wrapping Droplets

    Future efforts for targeted drug delivery may require encapsulating droplets before transporting them to their final location. One method for encapsulation is wrapping a drop in a thin, solid sheet. Previously, we saw that drops can wrap themselves with a little outside assistance, but here the drops achieve that same feat on their own, using the energy of droplet impact to wrap liquids. 

    Here’s how it works: float a thin sheet on a bath of a liquid like water, then let an oil drop fall into the bath. Its impact deforms the air-water interface and, with a sufficiently energetic impact, causes the oil droplet to pinch off. The flexible sheet wraps around the droplet, and the encapsulated droplet sinks due to gravity. The shape of the final drop depends on the sheet’s initial geometry. The researchers have successfully used circular, triangular, and cross-shaped sheets to wrap droplets. Check out the original paper or the video below for more. (Image and research credit: D. Kumar et al.; video credit: Science)

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    Water Walking, Exploding Droplets, and Colliding Vortices

    Every year I look forward to the APS DFD conference in November. It brings thousands of researchers together to share the latest in fluid dynamics. So much goes on in those three days that it’s impossible to capture, but last year I teamed up with Tom Crawford and the Journal of Fluid Mechanics to attempt just that. We interviewed 50 researchers on their projects, and we’ll be bringing you their work, in their words, each month leading up to the 2018 APS DFD meeting.

    This first video focuses on some of the awesome entries to the 2017 Gallery of Fluid Motion. Watch to learn about oil droplets that go flying everywhere when you’re cooking, balls that walk on water, the water music of Vanuatu and more! To see the videos we discuss and all the other entries, go to gfm.aps.org. (Video credit: N. Sharp and T. Crawford)

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