FYFD

Tag: surface tension

  • Dip Coating

    Dip Coating

    Imagine dipping a rod into a liquid mixture filled with particles. When you pull the rod out, do particles stick to it? The answer depends on the relative importance of two sets of forces: the viscous drag as you lift the rod and adhesive power of surface tension. Scientists express this as a dimensionless ratio known as the capillary number.

    When the capillary number is small, viscous drag dominates, and any particles that try to stick to the rod get pulled away (upper left). But as you increase the capillary number, surface tension helps particles clump together and stick to the rod (lower left and right). If the surface tension forces are strong enough – meaning that the capillary number is high –  you can actually get multiple layers of particles adhering to the dipped surface. (Image and research credit: E. Dressaire et al.)

  • Finding New Shapes in Foam

    Finding New Shapes in Foam

    In the summer of 2018, a group of researchers announced they’d discovered a new geometrical shape, the scutoid. They found the scutoid, a sort of twisted prism, in the shape of epithelial cells packed between curved surfaces. Having heard of this new geometry, a different group of physicists wondered if they could find scutoids elsewhere, specifically, in the cells of a foam. As shown in the picture above, they did.

    To visualize a scutoid, first image a prism. Take two polygons with an equal number of sides and connect them. But if you imagine packing such prisms between two curved surfaces, you’ll quickly see that it won’t work. They just don’t fit together. Instead, one face may adopt, say, six sides, while the other takes on five. To join those two end faces, one of the sides will have to have a Y-shaped junction and a triangular face. This is a scutoid.

    You can see two such shapes in the image above. In the left bubble, the far side forms a pentagon, while the near face is a hexagon. On the right, the bubble has six faces in the background and eight in the foreground. And between them, you can just see the triangular face that connects the two scutoids.

    It’s not only exciting to find scutoids in a new, non-biological medium; it suggests a physical mechanism behind their formation. Foams are a well-known example of energy minimization. The fact that scutoids are found in a curved foam suggests that the shape itself is connected to energy minimization, something that could help us understand how biological scutoids grow and form. (Image and research credit: A. Mughal et al.; via Physics World; submitted by Kam-Yung Soh)

  • Water-Walking Geckos

    Water-Walking Geckos

    Many animals can run on water. The tiniest creatures, like water striders, use surface tension to keep themselves atop the water.  Larger creatures like the basilisk lizard or the grebe slap the water’s surface to generate a vertical impulse that keeps them aloft. Geckos, it turns out, can run on water, too, but they’re too big to stay up with surface tension and too small to support their weight by slapping. So they’ve developed their own method.

    As you see in the top image, geckos use the slapping method for part of their support. Their slaps generate a little less than half of the force needed to keep them out of the water. 

    Surface tension is an important component, too. Geckos are extremely water repellent, which helps boost the lift they get from surface tension. In the bottom image, you see a gecko attempting to run on soapy water, which has a lower surface tension. The gecko is mostly submerged and more swimming than running – a clear demonstration that surface tension is important to its water-walking.

    Finally, the gecko undulates its body as it runs, much the way an alligator swims. The researchers suspect this helps the gecko generate forward thrust. Altogether, it creates a water-walking gait that, for now, is unique among observed mechanisms. (Image and research credit: J. Nirody et al.; via Ars Technica; 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)

  • Featured Video Play Icon

    “If I Say”

    The new Mumford & Sons single “If I Say” features a fluid-dynamical music video. It’s full of dendritic fingers and flowing colors – likely from combinations of inks, paints, and other fluids. Although the fingers are reminiscent of the viscosity-dependent Saffman-Taylor instability, these appear to be driven by variations in surface tension between the different fluids. That’s a major feature throughout the video; although some of the flow is caused by the syringes depositing fluids, much of it seems to be a Marangoni effect, where flow moves away from areas of low surface tension to ones with higher surface tension. (Video credit: Mumford & Sons; filmed by P. Hofstede; via Katie M.)

  • 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)

  • Soap Film Filter

    Soap Film Filter

    Inspired by the self-healing properties of soap films, scientists have created a liquid filter capable of trapping small particles while allowing larger ones to pass through. Instead of filtering particles by size, as conventional filters do, this liquid membrane filters particles by kinetic energy; only large, fast-moving objects  pass through while slower and smaller ones get trapped. The membrane is a mixture of deionized water and sodium dodecyl sulfate, which allows researchers to finely tune the membrane’s surface tension and, therefore, how the filter behaves. Unlike soap films, the membrane is quite long-lived and robust. The team poked one for more than 3 hours without rupturing it.

    The researchers envision some pretty neat applications for these membranes, including a surgical membrane that would keep out dust and bacteria while doctors work or a membrane in a waterless toilet that could trap odors inside. (Image and research credit: B. Stogin et al.; video credit: Science; submitted by Kam-Yung Soh)

  • 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.)

  • Convection Without Heat

    Convection Without Heat

    We typically think of convection in terms of temperature differences, but the real driver is density. In the animations above, cream sitting atop a liqueur is undergoing solutal convection – no temperature difference needed! The alcohol in the liqueur mixes with the cream to form a lighter mixture that rises to the surface. The lower surface tension of the alcohol is also good at breaking up the cream, forming little cells. As the alcohol in those cells evaporates, the cream gets heavier and sinks down into the liqueur, where it can pick up more alcohol, rise back to the surface, and begin the cycle again. (Image credit: J. Monahan et al., source)