Year: 2020

Synchronizing Microfluidic Drops
In nature, synchronization occurs when oscillators interact. A group of metronomes shifting to tick in unison is a classic example. Here, the system is a microfluidic T-junction and the oscillators are the liquid interfaces along the narrower inlet channels. Systems like this one have long been used to create alternating droplets (Image 1), corresponding to out-of-phase synchronization. But a new paper shows that the same system can perform in-phase synchronization (Image 2), too, generating droplets at the same time.
For any synchronization to occur, the main channel must be narrow enough for the two side channels to influence one another. Once that’s the case, the out-of-phase synchronization happens at a relatively high flow rate, and lowering the flow rate causes the system to transition to in-phase synchronization. (Image and research credit: E. Um et al.; submitted by Joonwoo J.)
November 19, 2020
Dead Water
In the days before motorized propulsion, sailors would sometimes find themselves slowed nearly to a stop by what they called ‘dead water‘. As discovered in laboratory experiments over a century ago by Vagn Walfrid Ekman, the dead water phenomenon occurs where a layer of fresh water exists over saltier water. The ship’s motion generates internal waves in the salty layer, which in turn causes substantial additional drag on the boat. In a related phenomenon, named for Ekman, the internal waves generated by a boat’s initial acceleration cause its speed to fluctuate.
While these phenomena have little effect on today’s shipping, they can be relevant for swimmers in areas like harbors and fjords where fresh water meets the sea. And their effects were undoubtedly substantial for much of history. There is even speculation that dead water might have caused the defeat of Mark Antony and Cleopatra’s superior navy at the hands of Octavian’s smaller ships in the Battle of Actium. (Image credit: M. Blum; research credit: J. Fourdrinoy et al.; via Hakai Magazine; submitted by Kam-Yung Soh)
November 18, 2020
Hydrodynamic Bearings
If you twirl a glass syringe, it spins quite nicely, lubricated on a micron-thin layer of air. This is an example of a hydrodynamic bearing, a device where the viscosity of a fluid and relative motion of two closely-spaced surfaces provides the cushion necessary to keep the surfaces separate. In this video, Steve Mould explains the phenomenon in more detail and shares some awesome examples of this hydrodynamic levitation in action. (Image and video credit: S. Mould; submitted by clogwog)
November 17, 2020
Collecting Animal Tears
Like humans, most vertebrates rely on tear films to keep their eyes moist and protected from the environment. But compared to humans, some animals’ tears have superior staying power. The caiman, for example, can go up to 2 hours between blinks without their eyes drying out; in contrast, humans have to blink about 15 times per minute – and sometimes even that isn’t enough to keep our eyes moist!
Researchers are collecting animal tears and studying their composition to better understand how their tears protect vision. Subtle changes in chemical make-up can lead to large variations in performance; just look at the many dried tear patterns in Image 2. Scientists hope that understanding other species’ tears will help us develop better treatments for our own vision problems. As someone who struggles with dry eyes at times, I’d be happy for some caiman-tear-inspired eye drops! (Image credit: A. Oriá; research credit: A. Raposo et al.; via NYTimes; submitted by Kam-Yung Soh)
November 16, 2020
Coalescing Drops
This year’s Nikon Small World in Motion competition was won by fluid dynamics! The first place video shows droplets on a superhydrophobic surface coalescing. The droplets are a mixture of water and ethanol. Their initial merger creates a ripple of waves that’s followed by a ghostly vortex ring that jets into the interior. Previous research on coalescence during impact shows jets driven by surface tension but the jet here doesn’t appear to be confined to the surface. (Image and video credit: K. Rabbi and X. Yan; via Nature; submitted by Kam-Yung Soh)
November 13, 2020
Colorful Kelvin-Helmholtz Clouds
Like breaking waves at the beach, these wavy clouds curl but only for a moment. The photo was captured near sunset on a late August evening in Arlington, MA. This short-lived cloud shape forms due to the Kelvin-Helmholtz instability, which is driven by shear forces between two layers of air moving at different speeds. The situation is a common one in the atmosphere, where air layers at altitude move in different directions and at different speeds. Most of the time we cannot see the curls that form between these air layers because of air’s transparency. But occasionally the mismatch happens right at a cloud layer and the condensation of the cloud gets pulled into these distinctive curls. (Image credit: B. Bray; submitted by Mark S.)
November 12, 2020
Making a Miniature River
Despite wide differences in ecology and geology, rivers around the world share certain fundamental features. Physicists study these characteristics by creating small-scale rivers in the laboratory, like the experiment featured in this Lutetium Project video. Within these systems, scientists can carefully control variables and discover useful patterns, like the two parameters needed to describe the shape of a river’s profile! (Image and video credit: The Lutetium Project)
November 11, 2020
Understanding Stars’ Seismology
Our understanding of Earth’s interior is based mostly on observations of seismic waves, which travel differently through our rocky crust and the molten core. Scientists similarly use seismic waves in stars to determine their interiors. But the pressure and temperature conditions in stars are far beyond anything we have here on Earth, which makes predicting how waves will travel in such exotic material difficult.
To better understand these extreme temperatures and pressures, scientists are using Lawrence Livermore’s National Ignition Facility (NIF) to mimic conditions similar to the outer envelope of a white dwarf star, like the one shown in the center of the image above. NIF’s laser array – shown as the blue lines in the artist’s conception above – can generate spherical shock waves that, as they converge on a solid sample, create pressures as high as 450 Mbar, more than 400 million times sea level atmospheric pressure here on Earth. Although the shock wave takes only 9 ns to travel across the sample, it’s enough to give researchers a glimpse into star-like conditions. (Image credit: NASA/ESA/C. O’Dell/D. Thompson, Lawrence Livermore National Laboratory; via Physics Today)
November 10, 2020
Spinning Bubbles
Fluid dynamics is largely about figuring out the relationship between forces. For a soap bubble sitting still, that’s primarily the effect of gravity, which makes the fluid in the soap film drain downward, and surface tension, which tries to maintain a spherical shape for the bubble.
Once you start spinning the bubble, though, there are new forces that come into play. One is the centrifugal force caused by the rotation, and another is the drag force between the rotating soap bubble and the air inside and outside of it. The addition of these forces drastically changes the bubble’s shape. It becomes wobbly and flattens out. Watch the contact line where the bubble meets the surface and you’ll also see it creeping outward toward the edge of the platform. (Image credit: C. Kalelkar and S. Paul, source)
November 9, 2020
Storm Eyes and Mushrooms in a Drop
In industry, drying droplets often have many components: a liquid solvent, solid nanoparticles, and dissolved polymers. The concentration of that last component — the polymers — can have a big effect on the way the droplet dries, as seen in the video above.
Without polymers, the droplet dries similarly to a coffee ring stain. But at moderate concentration, we see something very different. The droplet forms an eye in the middle, similar to a hurricane’s, and the edges of the droplet sprout mushroom-shaped plumes that grow and merge with one another along the edge. With even larger polymer concentrations, the mushrooms sweep their way inward, leaving a feathery stain behind. (Video, image, and research credit: J. Zhao et al.)
November 6, 2020