FYFD

Tag: surface tension

  • Active Cheerios Self-Propel

    Active Cheerios Self-Propel

    The interface where air and water meet is a special world of surface-tension-mediated interactions. Cereal lovers are well-aware of the Cheerios effect, where lightweight O’s tend to attract one another, courtesy of their matching menisci. And those who have played with soap boats know that a gradient in surface tension causes flow. Today’s pre-print study combines these two effects to create self-propelling particle assemblies.

    The team 3D-printed particles that are a couple centimeters across and resemble a cone stuck atop a hockey puck. The lower disk area is hollow, trapping air to make the particle buoyant. The cone serves as a fuel tank, which the researchers filled with ethanol (and, in some cases, some fluorescent dye to visualize the flow). Like soap, ethanol’s lower surface tension disrupts the water’s interface and triggers a flow that pulls the particle toward areas with higher surface tension. But, unlike soap, ethanol evaporates, effectively restoring the interface’s higher surface tension over time.

    With multiple self-propelling particles on the interface, the researchers observed a rich series of interactions. Without their fuel, the Cheerios effect attracted particles to each other. But with ethanol slowly leaking out their sides, the particles repelled each other. As the ethanol ran out and evaporated, the particles would again attract. By tweaking the number and position of fuel outlets on a particle, the researchers found they could tune the particles’ attractions and motility. In addition to helping robots move and organize, their findings also make for a fun educational project. There’s a lot of room for students to play with different 3D-printed designs and fuel concentrations to make their own self-propelled particles. (Research and image credit: J. Wilt et al.; via Ars Technica)

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  • Predicting Droplet Sizes

    Predicting Droplet Sizes

    Squeeze a bottle of cleaning spray, and the nozzle transforms a liquid jet into a spray of droplets. These droplets come in many sizes, and predicting them is difficult because the droplets’ size distribution depends on the details of how their parent liquid broke up. Shown above is a simplified experimental version of this, beginning with a jet of air striking a spherical water droplet on the far left. In less than 3 milliseconds, the droplet has flattened into a pancake shape. In another 4 milliseconds, the pancake has ballooned into a shape called a bag, made up of a thin, curved water sheet surrounded by a thicker rim. A mere 10 milliseconds after the jet and drop first meet, the liquid is now a spray of smaller droplets.

    Researchers have found that the sizes of these final droplets depend on the balance between the airflow and the drop’s surface tension; these two factors determine how the drop breaks up, whether that’s rim first, bag first, or due to a collision between the bag and rim. (Image credit: I. Jackiw et al.; via APS Physics)

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    “Chemical Somnia”

    Under a macro lens, even a petri dish worth of fluids comes vividly to life. Here, artist Scott Portingale explores crystallization, Marangoni effects, and other phenomena alongside a haunting soundtrack from musician Gorkem Sen. Enjoy! (Image and video credit: S. Portingale et al.)

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    Marangoni Blossoms

    When surface tension varies along an interface, fluids move from regions of low surface tension to higher surface tension, a behavior known as the Marangoni effect. Here, a drop of (dyed) water is placed on glycerol. The two fluids are miscible, but water has much a lower viscosity and density yet a higher surface tension. The drop’s interface quickly becomes unstable; viscous fingers form along the edge as the less viscous water pushes into the more viscous glycerol. Eventually, the surface-tension-driven Marangoni flow breaks those fingers off into lip-like daughter drops. The researchers also show how the interplay between viscosity and surface tension affects the size of fingers that form by varying the water/glycerol concentration. (Image and video credit: A. Hooshanginejad et al.)

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    Tweaking Coalescence

    When a drop settles gently against a pool of the same liquid, it will coalesce. The process is not always a complete one, though; sometimes a smaller droplet breaks away and remains behind (to eventually do its own settling and coalescence). When this happens, it’s known as partial coalescence.

    Here, researchers investigate ways to tune partial coalescence, specifically to produce more than a single droplet. To do so, they add surfactants to the oil layer surrounding their water droplet. The surfactants make the rebounding column of water skinnier, which triggers the Rayleigh-Plateau instability that’s necessary to break the column into more than one droplet. (Image and video credit: T. Dong and P. Angeli)

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    Playing With Water in 2D Containers

    Once again Steve Mould is putting his prototyping skills to use to work out what goes on inside tricky containers. Here he looks at a “magic” wizard’s cup where — like the assassin’s teapot — cleverly placed holes in the side of the cup can block or allow air’s escape. In the wizard’s cup this lets the wizard refill the cup at will.

    He also takes a look at how draining works, using tracer particles and a video editing effect that “echoes” previous frames in a video. For the tracer particles, this algorithm effectively visualizes pathlines in the flow. Areas with faster-moving fluid have longer pathlines that are closer together, whereas slow-moving regions have short pathlines. (Video credit: S. Mould)

  • Making Reconfigurable Liquid Circuits

    Making Reconfigurable Liquid Circuits

    Microfluidic circuits are key to “labs on a chip” used in medical diagnostics, inkjet printing, and basic research. Typically, channels in these circuits are printed or etched onto solid surfaces, making it difficult to reconfigure them. A group in China developed an alternative design, inspired by reconfigurable toys like Lego blocks. Their set-up, shown above, uses a pillared surface immersed in oil. To create the channels, they pipette water — one droplet at a time — into the space between pillars. The combination of oil and pillars traps the drop. With multiple drops linked together, they get channels, like the ones above that mix two fluids. When the time comes to reconfigure the channels, they just pipette the water out and cut the channel with a sheet of coated paper. (Image and research credit: Y. Zeng et al.; via Physics Today)

  • Evolving Fingers

    Evolving Fingers

    If you sandwich a viscous fluid between two plates and inject a less viscous fluid, you’ll get viscous fingers that spread and split as they grow. This research poster depicts that situation with a slight twist: the viscous fluid (transparent in the image) is shear-thinning. That means its viscosity drops when it’s deformed. In this situation, the fingers formed by the injected (blue) fluid start out the way we’d expect: splitting as they grow (inner portion of the composite image). But then, the tip-splitting stops and the fingers instead elongate into spikes (middle ring). Eventually, as the outer fluid’s viscosity drops further, the fingers round out and spread without splitting (outer arc of the image). (Image credit: E. Dakov et al.; via GoSM)

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    “Color Show”

    Brightly colored paints and inks mix and flow in artist Roman De Giuli’s “Color Show.” De Giuli typically creates this fluid art in thin layers atop paper. He’s a master of the form, manipulating surface tension gradients to create streaming flows, dendritic patterns, and feathery wisps. If this kind of art is your jam, he offers an app full of live wallpapers* for Android phones. See more of his work on his website and on Instagram. (Video and image credit: R. De Giuli)
    *Not sponsored, I just like his art!

  • Dendritic Painting Physics

    Dendritic Painting Physics

    In the art of Akiko Nakayama, colors branch and split in a tree-like pattern. In studying the process, researchers found the physics intersected art, soft matter mechanics, and statistical physics. In dendritic painting, the process starts with an underlying layer of acrylic paint, diluted with water. Atop this wet layer, you place a drop of acrylic ink mixed with isopropyl alcohol.

    The combination of both layers is key. The alcohol-acrylic drop on a Newtonian substrate will show spreading, driven by Marangoni forces, but no branching. It’s the slightly shear-thinning nature of the diluted acrylic paint substrate that allows dendrites to form. As the overlying drop expands, it shears the underlayer, changing its viscosity and allowing the branches to form. You can see video of the process here. (Image credit: A. Nakayama; research credit: S. Chan and E. Fried; via Physics World)