FYFD

Tag: electrohydrodynamics

  • Charged Drops Don’t Splash

    Charged Drops Don’t Splash

    When a droplet falls on a surface, it spreads itself horizontally into a thin lamella. Sometimes — depending on factors like viscosity, impact speed, and air pressure — that drop splashes, breaking up along its edge into myriad smaller droplets. But a new study finds that a small electrical charge is enough to suppress a drop’s splash, as seen below.

    Video showing three different droplets, each with a different electrical charge, impacting an insulated surface. From left to right, the charges are: 0.0 nC, 0.08 nC, and 0.1 nC. The uncharged drop splashes, the low charge drop splashes less, and the final charged droplet spreads without splashing.

    The drop’s electrical charge builds up along the drop’s surface, providing an attraction that acts somewhat like surface tension. As a result, charged drops don’t lift off the surface as much and they spread less overall; both factors inhibit splashing.* The effect could increase our control of droplets in ink jet printing, allowing for higher resolution printing. (Image and research credit: F. Yu et al.; via APS News)

    *Note that this only works for non-conductive surfaces. If the surface is electrically conductive, the charge simply dissipates, allowing the splash to occur as normal.

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    Salt Affects Particle Spreading

    Microplastics are proliferating in our oceans (and everywhere else). This video takes a look at how salt and salinity gradients could affect the way plastics move. The researchers begin with a liquid bath sandwiched between a bed of magnets and electrodes. Using Lorentz forcing, they create an essentially 2D flow field that is ordered or chaotic, depending on the magnets’ configuration. Although it’s driven very differently, the flow field resembles the way the upper layer of the ocean moves and mixes.

    The researchers then introduce colloids (particles that act as an analog for microplastics) and a bit of salt. Depending on the salinity gradient in the bath, the colloids can be attracted to one another or repelled. As the team shows, the resulting spread of colloids depends strongly on these salinity conditions, suggesting that microplastics, too, could see stronger dispersion or trapping depending on salinity changes. (Video and image credit: M. Alipour et al.)

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    Contactless Bending

    Using electromagnetism, researchers are bending and shaping soft liquid wires even against gravity. The team used galinstan — an alloy of gallium, indium, and tin that remains liquid at room temperature. On its own, galinstan has a high surface tension and forms droplets. But with a voltage applied, that surface tension is suppressed, making the liquid form a long, thin, still-liquid wire. Adding a magnetic field allowed the researchers to manipulate the falling stream of liquid, even levitating loops of the metal against the force of gravity! (Image, video, and research credit: Y. He et al.; via Cosmos; submitted by Kam-Yung Soh)

  • Liquid Bridges

    Liquid Bridges

    In 1893, Baron Armstrong demonstrated a peculiar phenomenon — a liquid bridge of water suspended between two beakers with a strong electric charge between them (Image 1). More than a century later, the details of the mechanism remain challenging to pin down thanks to the setup’s combination of electohydrodynamics, heat transfer (Image 2), evaporation, and chemistry (the electrodes can split water).

    Researchers have pinned down a few details, though, like that the break-up of the liquid bridge (Image 3) depends on its effective length and that the effective length grows as applied voltage increases. Researchers also found that inducing an external flow can extend the bridge’s lifetime, though it does not affect the length at which it breaks up. Interestingly, the phenomenon is not limited to water (and its odd chemistry); ethanol and glycerol have been used for liquid bridges, too! (Image and research credit: X. Pan et al.)

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    Morphing Particle Rafts

    A layer of tiny glass beads sitting atop a pool of castor oil becomes a morphing surface in this video. Applying an electric field creates enough electrostatic force to draw the interface upward against the power of both gravity and surface tension. Moving the electric field — either by shifting the electrode or simply moving a finger over the surface — is enough to pull columns of fluid along! I could imagine this making some very cool human-machine interfaces one day. (Image and video credit: K. Sun et al.)

  • Driven From Equilibrium

    Driven From Equilibrium

    With the right application of force, liquids can take on shapes that defy our intuition. Here researchers sandwiched two immiscible oils between glass slides and applied an electric field. Because the two oils have different electrical responses, charges build along the interface between them. These charges lead to non-trivial electrohydrodynamic flows and a multitude of bizarre shapes. They observed polygonal droplets, streaming droplet lattices, and spinning filaments among others. As long as the electric field remains on, the wild behaviors continue; once the field is turned off, the oils relax back to typical, rounded drops. (Image, video, and research credit: G. Raju et al.; via Physics World)

  • Artificial Microswimmers

    Artificial Microswimmers

    In a 1959 lecture entitled “There’s Plenty of Room at the Bottom”, Richard Feynman challenged scientists to create a tiny motor capable of propelling itself. Although artificial microswimmers took several more decades to develop, there are now a dozen or so successful designs being researched. The one shown above swims with no moving parts at all.

    These microswimmers are simple cylindrical rods, only a few microns long, made of platinum (Pt) on one side and gold (Au) on the other. They swim in a solution of hydrogen peroxide, which reacts with the two metals to generate a positively-charged liquid at the platinum end and a negatively-charged one at the gold end. This electric field, combined with the overall negative charge of the rod, causes the microswimmer to move in the direction of its platinum end. 

    Depending on the hydrogen peroxide concentration, the microswimmers can move as quickly as 100 body lengths per second, and they’re capable of hauling cargo particles with them. One planned application for artificial microswimmers is drug delivery, though this particular variety is not well-suited to that since the salty environment of a human body disrupts the mechanism behind its motion. (Image credits: swimmers – M. Ward, source; diagram – J. Moran and J. Posner; see also Physics Today)

  • Pyrocumulus on the Horizon

    The Cranston wildfire in California is intense enough that it’s creating its own weather. This timelapse video shows the formation and growth of a pyrocumulus cloud, also associated with volcanoes, over the wildfire. In both instances, the extreme heat causes a massive column of hot, turbulent air to rise. Because ash and smoke are carried upward as well, there are many places for any moisture in the atmosphere to nucleate, forming the cloud we see. In timelapse, the roiling nature of the air’s motion is especially apparent. This turbulence can be dangerous, as it may contribute to high winds and even lightning, both of which can spread the fire further. (Video credit: J. Morris; via James H.)

  • 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)

  • The Rose-Window Instability

    The Rose-Window Instability

    This polygonal pattern is known as the rose-window instability. It’s formed between two electrodes – one a needle-like point, the other flat – separated by a layer of oil. The pointed electrode’s voltage ionizes the air nearby, creating a stream of ions that travel toward the flat electrode below. Oil is a poor conductor, however, so the ions build up on its surface until they’re concentrated enough to form a dimple that lets them reach the lower electrode. At higher voltages, the electrical forces driving the ions and the gravitational force trying to flatten the oil reach a balance in the form of the polygonal cell pattern seen above. Smaller cells form near the needle electrode, where the electrical field is strongest and the temperature is highest, as revealed in thermal and schlieren imaging (lower images) that shows a warm stream of gas impacting there. 

    As a final note, I’ll add that the latest in this research comes from a paper by a Pakastani teenager. It’s never too early to start contributing to research! (Image and research credit: M. Niazi; via NYTimes; submitted by Kam-Yung Soh)