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  • Swimming in Complex Fluids

    Swimming in Complex Fluids

    Bacteria like E. coli swim using flagella, helical filaments attached to biological motors on their bodies. By rotating the flagella, the bacterium generates thrust that propels it forward. Oddly, though, researchers observed decades ago that bacteria actually travel faster through complex fluids — like those with polymers or particles in them — than they do through simple fluids like water. A new study using colloids — small particles suspended in a liquid — shows why.

    The researchers compared bacteria swimming through polymer-filled fluids and colloidal fluids and found strong overlap both qualitatively and quantitatively. They observed, for example, that bacteria swim in straighter lines — they wobble less — in complex fluids. The reason, according to the authors, is the hydrodynamic influence of the added materials. Essentially, when a bacterium swims near a colloid or piece of polymer, the particle exerts a torque on the microswimmer that reduces its wobble and enhances its speed. (Image credit: Cheng Research Group; research credit: S. Kamdar et al.; via Physics World)

  • Mapping Yellowstone Underground

    Mapping Yellowstone Underground

    Yellowstone National Park is filled with geysers, hot springs, and mudpots — all geophysical features driven by the underground movement of water heated by the underlying volcano. But what does that underground plumbing look like? To find out, a team of researchers flew a 25-m diameter electromagnetic loop over portions of the park; they used the electromagnetic feedback induced in the loop to roughly map the subsurface features of the park.

    To their surprise, they found that deep hydrothermal vents in Yellowstone lie in discrete locations; previously, geologists assumed the vents were more widespread. With a better sense of what lies beneath, park officials will be able to build new infrastructure in areas better protected from one of the park’s biggest hazards: hydrothermal explosions caused by a buildup of pressure underground. (Image credits: top – I. Shturma, map – C. Finn et al.; research credit: C. Finn et al.; via Physics World)

    Editor’s Note: This article was written and scheduled prior to the historic flooding in Yellowstone in June 2022.

    Geophysical map of Yellowstone's Upper Geyser Basin, including Old Faithful.
    Geophysical map of Yellowstone’s Upper Geyser Basin, including Old Faithful.
  • Rip Currents

    Rip Currents

    Rip currents — also known as rips — are a threat to beachgoers around the world, and, unfortunately, they’re often underestimated or misunderstood. As waves crash on the shore, water must find a path back out to sea, often through deeper channels that provide a break between the waves. These flow paths are rip currents, and they can form, shift, and intensify with little warning.

    Over the years, researchers have found that efforts to educate beachgoers through signs, flags, and other methods once at the beach have done little to help visitors understand, avoid, or escape rips. Instead, it’s better to educate people long before the water is in sight. Since no one method is guaranteed success for escaping a rip, it’s better to learn to recognize and avoid these dangerous areas. Check out the video below for advice on spotting rips, and here’s a video showing rips from a surfer’s perspective, as well as one using dye flow visualization to mark a rip. Be safe and smart out there! (Image credit: P. Auitpol; video credit: Surf Life Saving Australia; via Hakai Magazine; submitted by Kam-Yung Soh)

  • A Forest of Ferrofluids

    A Forest of Ferrofluids

    Ferrofluids are made up of ferrous nanoparticles suspended in a carrier fluid like an oil. Under magnetic fields, they take on an array of shapes — from pointed spikes to elaborate labyrinths — depending on the field strength and what fluids they’re surrounded by. This photographic series by Linden Gledhill captures some of that fantastic variety, with ferrofluids that look like cells and nebulas in addition to mazes and tridents. See more of Gledhill’s work at his website and in previous posts. (Image credit: L. Gledhill)

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    Stably Jammed

    Granular materials like sand, gravel, and medications can become a rigid mass when squeezed or sheared. Even with a relatively loose packing, these materials can jam together to act like a solid if the contacts between grains no longer allow particles to shift or rotate. In this video, researchers explore how stable these jammed states are by repeatedly shearing the mixture and observing how it changes.

    Most of the videos are set up as a triptych, where all three panels show the same material. On the left, you see a simple view showing the position of each particle. In the middle, the disks are viewed through polarized filters, so that the material looks brightest where it is stressed. This view lets us see the force chains that run through the material. On the right, UV-sensitive ink on each marker glows to show any rotation particles experience.

    In the first sample, repeated shearing slowly unjams the mixture and allows it to shift and flow once more. We see this from the decreasing brightness in the middle panel. The slow fade to black means that the force chain network has disappeared entirely. In contrast, the second sample ultimately reaches an “ultra-stable” jammed state, in which further shear cycles cause no change to the network. Once again, this is easiest to observe in the middle image, where the bright force network stops changing after 2,000 cycles or so. (Image and video credit: Y. Zhao et al., research pre-print)

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    How Wells and Aquifers Work

    When rain falls, some of that water turns into run-off in storm systems but much of it seeps into the ground. What happens to that water? In most places, it joins the local aquifer, infusing the spaces between soil particles underground. In this video, Grady takes us through some of the interactions between surface water, aquifers, and the wells we use to access water underground. He’s even built some great demonstrations to show how aquifers and surface water like rivers pass water back and forth. (Image and video credit: Practical Engineering)

  • Surf’s Up

    Surf’s Up

    Surfers flock to northern Peru to enjoy what’s been called the world’s longest wave. These waves are generated by storms thousands of miles away in the Pacific and Southern Oceans. In the open water between, the waves sort themselves into groups of similar wavelength and speed. With the deep water off Peru, the large swells continue to travel together until close to the shore. Surfers also benefit from the tendency for incoming waves to arrive nearly parallel to the coastline, creating long shoreline stretches for breaking. Where many famous wave breaks can be ridden for seconds, surfers can ride these for minutes! (Image credit: L. Dauphin; via NASA Earth Observatory)

  • When Seeing a Flow Changes It

    When Seeing a Flow Changes It

    Adding dye to a flow is a common technique for visualization. After all, many flows in fluids like air and water are invisible to our bare eyes. But for some classes of flows — especially those driven by variations in surface tension — adding dye can have unforeseen effects. A recent study shows how true this is for bursting Marangoni droplets, where evaporation and alcohol concentration can pull a water-alcohol droplet apart.

    Composite series of photos showing the effect of increased dye concentration on Marangoni bursting.
    As more dye is added to the experiment, the daughter droplets grow larger and more ligaments form. In the first three images, a dashed black line has been added to show the location of the droplet rim.

    Without dye, it’s nearly impossible to see the phenomenon since the refractive indices of the two component liquids are so close. But the researchers found that, as they added more methyl blue dye, it did more than increase the contrast in the flow. It changed the flow, making the droplets larger and creating ligaments between them. They believe that the dye’s own surface tension creates local gradients that alter the flow. It’s a reminder that experimentalists have to be careful to consider how our efforts to measure and observe a flow can change it. (Image credit: top – The Lutetium Project, bottom – C. Seyfert and A. Marin with modification; research credit: C. Seyfert and A. Marin)

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    Recreating Flocks

    Birds, fish, and other creatures form amazing, undulating swarms of individuals. How these collectives comes together and move continues to fascinate scientists. Here, researchers look at simple particles with two “instructions,” if you will. One causes the particle to self-navigate toward a target; the other causes short-range repulsion if the particle gets too close to another one. With only these two simple guidelines, a flock of these particles forms complex, ever-changing flows! (Image and video credit: M. Casiulis and D. Levine)

  • The Odd Air Bubbles

    The Odd Air Bubbles

    Though it looks like a strange underwater panorama, this image by photographer Marek Miś actually captures air bubbles trapped beneath the slip cover of a microscope slide smeared with drying callus remover. According to Miś, “Callus remover is one of my favourite agents for taking micrographs. It can create unusual crystalline forms. This time I found on the slide these interesting air bubbles before the callus remover started to crystallise.” I confess that I wouldn’t have thought to use callus remover for art!

    This image earned 3rd place in the Micro category of the Close-Up Photographer of the Year awards. See more winners here, and find more from Miś on the web and Instagram. (Image credit: M. Miś)

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