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Search results for: “liquid jet”

  • Cavitation Through Acceleration

    Cavitation Through Acceleration

    Cavitation refers to the formation of destructive bubbles of vapor within a liquid. Traditionally, we think of it as occurring when the velocity in a flow becomes high enough for the pressure to drop below the local vapor pressure, causing bubbles to form. This is what we see around turbine blades and ship propellers.

    But cavitation also occurs in situations where the overall velocity is relatively low, provided there’s a sudden acceleration. That’s the situation we see above. The impact — either of a mallet off-screen or of the tube striking the floor — causes the liquid inside suddenly accelerate upward. Notice in the second image how the liquid interface moves upward as the first bubbles form.

    Each of these cavitation bubbles has such a low pressure that they’re basically a vacuum, and their collapse can cause shock waves that reverberate through the container, causing it to break. Check out that test tube in the last image. Notice that there’s no sign of cracking when the test tube hits the floor; in fact, the researchers demonstrate in their paper that an empty test tube dropped from the same height doesn’t break. Fractures only form after the cavitation bubbles do. (Image and research credit: Z. Pan et al.; submitted by A.J.F.)

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    Waltzing Defects

    Liquid crystals are a peculiar state of matter with both liquid and crystalline properties. In this video, a microfluidic device breaks water into droplets surrounded by a shell of liquid crystal. Because the molecular structure of the liquid crystals is helical and cannot pack neatly in a spherical shell, there are visible defects in the liquid crystal shells. Given time, those defects can merge as the liquid crystal shell thickens. (Image and video credit: The Lutetium Project)

  • Breaking Up

    Breaking Up

    The dripping of a faucet and the break-up of a jet into droplets is universal. That means that the forces – the inertia of the fluid, the capillary forces governed by surface tension, and the viscous dissipation – balance in such a way that the initial conditions of the jet – its size, speed, etc. – don’t matter to the process of break-up. 

    We’d expect that the inverse situation – the breakup of a gas into bubbles in a liquid – would be similarly universal, but it’s not. When unconfined bubbles pinch off, the way they do so is heavily influenced by initial conditions. But that changes, according to a new study, if you confine the gas to a liquid-filled tube before pinch-off. Confinement forces a different balance between viscous and capillary effects, one which effectively erases the initial conditions of the flow and restores universality to the pinch-off process. (Image and research credit: A. Pahlavan et al.; via phys.org)

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    Engineering Droplets

    A jet of falling liquid doesn’t remain a uniform cylinder; instead, it breaks into droplets. In this video, Bill Hammack explores why this is and what engineers have learned to do to control the size of the droplets formed.

    The technical name for this phenomenon is the Plateau-Rayleigh instability. It begins (like many instabilities) with a tiny perturbation, a wobble in the falling jet. This begins a game of tug of war. One of the competitors, surface tension, is trying to minimize the surface area of the liquid, which means breaking it into spherical droplets. But doing so requires forcing some of the the liquid to flow upward, against both gravity and the liquid’s inertia. The battle takes some time, but eventually surface tension wins and the jet breaks up.

    That’s not necessary a bad thing. It’s actually key to many engineering processes, like ink-jet printing and rocket combustion, as Bill explains in the full video. (Video and image credit: B. Hammack; submitted by @eclecticca)

  • Inside an Evaporating Drop

    Inside an Evaporating Drop

    The evaporation of a simple droplet holds far more complexity than one would expect. If you look closely at the edge of the drop, there’s a tiny, beautiful display at work. It begins with small variations in surface tension at the contact line where solid, liquid, and gas meet. These could be caused by local temperature or concentration differences; either way, the gradient in surface tension creates a flow. It starts out as a series of microjets spaced evenly around the contact line (left). 

    As the microjets strengthen, they merge into larger and larger vortical structures (right). This kind of feature – large structures emerging from smaller ones – is known as an inverse cascade. Fluid dynamicists have traditionally studied the classic (turbulent) energy cascade, where kinetic energy moves from large scales into smaller ones, but researchers are beginning to recognize more situations where the inverse cascade occurs, such as in the storms of Jupiter. (Image and research credit: A. Ghasemi et al., source)

  • 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)

  • 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.)

  • 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)

  • 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)

  • Rim Break-Up

    Rim Break-Up

    Splashing drops often expand into a liquid sheet and spray droplets from an unstable rim. Although this behavior is key to many natural and industrial processes, including disease transmission and printing, the physics of the rim formation and breakup has been difficult to unravel. But a new paper offers some exciting insight into this unsteady process. 

    The researchers found that if they carefully tracked the instantaneous, local acceleration and thickness of the rim, it always maintained a perfect balance between acceleration-induced forces and surface tension. That means that even though different points on the rim appear very different, there’s a universality to how they behave. They found that this rule held over a remarkably large range of situations, including across fluids of different viscosities and splashes on various surfaces. (Image and research credit: Y. Wang et al.; via MIT News; submitted by Kam-Yung Soh)

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