Drip some ethanol on a hot surface, and you’d expect it to spread into a thin layer and evaporate. But that doesn’t always happen, and a recent study looks at why.
Ethanol is what’s known as a volatile liquid, meaning that it evaporates easily at room temperatures, well below its boiling point. When dropped on a uniformly heated surface above 45 degrees Celsius, the drop contracted into a hemisphere and then began to wander randomly across the surface. Researchers trained an infrared camera on the drop from below (above image), and found an unsteady, roiling motion inside the drop. These asymmetric flows, they concluded, drive the drop’s erratic self-propulsion. They suspect the mechanism may explain why some ink droplets wind up in the wrong place on a page during ink-jet printing. (Image and research credit: P. Kant et al.; via APS Physics)
Earth, Saturn, and Jupiter have auroras at their poles, generated by the interaction of their global magnetic fields with the solar wind. Mars has no global magnetic field, only remnants of one frozen into areas of its crust; yet it, too, has auroras. Mars’s auroras are rarer and discrete. They occur most often over the southern hemisphere, and researchers now think they know why.
Four billion years ago, we think Mars had a global magnetic field, much like Earth does. But somehow the planet lost that field. The traces that remain are caught in the minerals of its crust, much like the ancient magnetic fields recorded in areas of the Earth’s sea floor. These magnetized regions of Mars’s crust, shown above as contours in pink and blue, are where the discrete auroras occur.
Using data from NASA’s MAVEN spacecraft, which orbits Mars, the team discovered a pattern. They found that auroras occur most often when the magnetic lines of the incoming solar wind run antiparallel to the magnetic field lines of the crust. This suggests that the auroras happen as a result of magnetic reconnection, a process where antiparallel magnetic field lines rearrange themselves, releasing energy as a result. Reconnection events provide an opportunity for electrons from the solar wind to accelerate into Mars’s atmosphere, exciting molecules there and generating the auroras. So far we’ve only caught the auroras in UV light, but hopefully one day we’ll see them in visible light as well. (Image credit: R. Lillis et al.; research credit: C. Bowers et al. and B. Johnston et al.; via APS Physics)
Working with living creatures can’t always reveal their mechanics. That’s one reason engineers like building biorobots. Here, researchers built 1-guilla, an eel-like swimmer, and studied how its body motions affected its swimming. Eels are anguilliform swimmers that use a traveling wave moving along their body from head to tail for propulsion. In the video (and paper), they break down the robot’s motion step by step — looking at amplitude, wavelength, and tail angle — to find the optimal values for maximizing speed and, separately, efficiency in swimming. (Video and image credit: A. Anastasiadis et al.; research credit: A. Anastasiadis et al.)
In graduate school, my advisor introduced us to a particularly vexing fluid dynamical thought experiment known as the Feynman sprinkler. After observing an S-shaped sprinkler that rotated when water shot out its arms, physicist Richard Feynman wondered what would happen if the device were placed in a tank of water with the flow reversed. If the sprinkler was sucking in water, would it rotate and, if so, in what direction?
This seemingly simple question has confounded physicists ever since, in part because you can make believable arguments for multiple different results. Attempts to build the apparatus experimentally produced differing results, too — often due to variables that don’t appear in the thought experiment, like friction in the sprinkler’s bearing. But, at long last, a group posits they have the final answer to the problem.
They cleverly built their sprinkler so that it floats in its tank, with the addition or removal of water from the sprinkler controlled by a second siphon-connected tank. With no solid-solid contacts, the sprinkler can rotate with very little friction.
Flow visualization of the sprinkler in reverse (suction) mode. For image clarity, the device is held in place to prevent spinning. Notice how the jets coming into the hub glance off one another and form counter-rotating vortex pairs at an angle. This asymmetry is the source of the sprinkler’s rotation when allowed to move.
The team found that sucking water into the sprinkler does, indeed, reverse the sprinkler’s rotation, but it’s not a simple reversal of the forward sprinkler’s flow. To see why, check out the video above, which visualizes flow inside the sprinkler during suction. For clarity, the device is held fixed in place during flow visualization. Notice that the two arms of the sprinkler sit directly opposite one another in the hub. Thus, you’d expect their two jets to collide and form counter-rotating vortices along a vertical axis. But the vortex pairs are offset from the centerline.
This asymmetry takes place because the velocity profiles of flow across the hub inlets are skewed. Instead of the largest velocity occurring on the centerline of the inlet, each occurs slightly to one side. So when the jets collide, they do so off-center and impart a torque to the sprinkler. The reason for the skewed profiles at the inlets lies further upstream in the curved arms of the sprinkler. Centrifugal force from turning the corner leaves a mark on the flow, leading, ultimately, to the skewed velocity profiles, offset jets, and spinning sprinkler. (Image and research credit: K. Wang et al.; via APS Physics)
In their natural habitat, fire ants experience frequent floods and so developed the ability to form rafts. Entire colonies will float out a flood in a two-ant-thick raft anchored to whatever vegetation they can find. Because ants in the upper layer of the raft are constantly milling about, the rafts have some ability to “self-heal” as they’re stretched.
Pulling slowly gives the ants time to “heal” their stretching raft.
In these experiments, researchers slowly (above) and quickly (below) stretched ant rafts to see how they responded. Given a slow enough stretch, the ants were able to adjust and keep the raft together until it doubled in length. In contrast, a faster stretching rate overwhelmed the raft by the time it was 30% longer. (Image credit: top – Wikimedia Commons, others – C. Chen et al.; research credit: C. Chen et al.; via APS Physics)
Pulling quickly breaks an ant raft because the ants cannot react fast enough to heal the raft.
The thin shells of bubbles interact with light in fascinating ways; that is, of course, the source of their brilliant colors. In this recent study, researchers discovered that bubbles can serve as tunable lasers. A laser has three major components: an energy source, an optical resonator, and a gain medium that amplifies light in the resonator. For bubble lasers, an external pump laser provides energy and the bubble’s thin shell acts as a resonator. Fluorescent dye in the bubble serves as the gain medium.
Once formed, the bubble lasers are incredibly sensitive to electric fields and pressure changes, making them excellent sensors. For added stability, the team is using smectic liquid crystal bubbles, which, unlike soap bubbles, don’t evaporate easily. (Video, image, and research credit: Z. Korenjak and M. Humar; via APS Physics)
When snowflakes and volcanic ash fall, they tumble. Historically, it’s been too hard to observe this behavior first hand — the particles are too small to easily follow with a camera — so scientists instead looked at larger particles falling through water. That change preserves important characteristics of the physics, but it misses out on one key feature: in air, the density of the falling particle is much higher than air’s.
A football-shaped particle wobbles around its stable orientation as it falls through air.
To account for that, researchers built a special apparatus that drops particles one-at-a-time through the field of view of four high-speed cameras. This setup gave them a narrow 1-mm band where they could track a falling particle’s orientation — provided the particle fell through the band, which happened about 20% of the time. Their results show that particles in air tumble and oscillate back and forth around their stable orientation more than in water experiments. This difference affects how quickly particles settle, which, in turn, affects how much they tend to clump and grow. (Image credit: snow – A. Burden, experiment – T. Bhowmick et al.; research credit: T. Bhowmick et al.; via APS Physics)
Grinding coffee beans builds up electrical charge as the beans fracture into smaller and smaller pieces. The polarity of the charge depends on the bean’s moisture content; lighter roasts tend toward a positive charge, and darker roasts skew negative. The finer the grind, the stronger the electrical charge and the greater the problem of clumping grains becomes. Adding a few drops of water to the beans before grinding, researchers found, drastically reduces the electrical charge and clumping. This, the team reports, would let espresso lovers brew a stronger cup with less material. A well-compacted bed of unclumped grains has less void space, which slows down water’s percolation and increases the amount of coffee the water can extract. The authors encourage readers to try adding water in their own home brews, but they caution that coffee mass and grind setting should also be variables in the experiment. (Image credit: N. Van; research credit: J. Harper et al.; via APS Physics)
Older models of Mars assumed a liquid metal core beneath a solid mantle of silicates, but recent studies indicate that structure is missing at least one layer. Using data from the InSight lander’s seismometer, two teams independently calculated that a liquid silicate layer must surround the planet’s core. In September 2021, three meteorite pieces impacted Mars far from the InSight lander’s position. Since the Mars Reconnaissance Orbiter could exactly pinpoint the impact location, researchers were able to calculate just how long it took seismic waves from the impact to reach the lander.
Like on Earth, Mars has two varieties of seismic wave: transverse S-waves that only travel through solids and longitudinal P-waves that travel through both liquid and solid layers. S-waves reflect off any liquid-solid boundary, following a different path to a seismometer than P-waves that refract across the boundary and travel through liquid. For more of the story behind this discovery, check out this article at Physics Today. (Image credit: Mars – NASA/JPL-Caltech/University of Arizona, illustration – J. Sieben/J. Keisling; research credit: H. Samuel et al. and A. Khan et al.; via Physics Today)
An illustration of Mars’ interior and the paths followed by seismic waves before InSight picked them up.
Inspired by a muddy hike with a dog, today’s study looks at how fur in a flow can shed dirt and debris. Researchers placed beaver, coyote, and synthetic hairs in a flow chamber with a slurry of titanium dioxide particles in water. After 24 hours, they counted the particles stuck on each hair. The more flexible a hair, the cleaner it stayed. Long hairs collected fewer particles per unit surface area than short ones, thanks to their larger deflection in the flow. The effect, they discovered, is a bit like when paint or glue dries on your hand. The more you move and flex your skin, the harder it is for crusty material to stick. This self-cleaning with flex and flow occurs in nature, too: the only furry mammal with consistently dirty fur is the notoriously inactive sloth. (Image credit: T. Umphreys; research credit: M. Krsmanovic et al.; via APS Physics)
