Marsh grass shrimp, despite their small size, are zippy swimmers. They move using a series of closely-spaced legs that stroke asynchronously. Researchers found that the flexibility and stiffness of the legs are critical for the shrimp’s efficiency. During the power stroke, the shrimp’s leg is held stiff, maximizing the force it’s able to transfer to the water. But during the forward-moving recovery stroke, the shrimp bends its legs almost horizontal and presses both legs in the pair together tightly. This action minimizes the area of the leg pair and reduces the drag they cause as they move into position for the next stroke. (Image, video, and research credit: N. Tack et al.; via Ars Technica; submitted by Kam-Yung Soh)
Tag: swimming
Swimming Together
Scientists have long pondered the possibilities of hydrodynamic benefits to the ways fish school. But most analyses of schooling have assumed a fixed spacing that’s far more orderly than what we observe in nature. In this experiment, researchers instead used a pair of robotic swimmers (essentially hydrofoils) to explore a range of swimming formations. What they found was a map of places where a second swimmer could easily “lock in” to a position relative to the leader and have their positioning stabilized by interactions with the leader’s wake (lower image). Interestingly, the beneficial regions extend much further downstream for fish positioned diagonally to the leader than they do for one directly following. With such a wide range of easily-stabilized following positions, it’s no wonder that schools of fish are amorphous instead of strictly crystalline! (Image credit: top – S. Pena Lambarri, map – J. Newbolt et al.; research credit: J. Newbolt et al.)
The shaded areas of this map represent areas where a second swimmer can passively “lock-in” relative to the leader’s position, shown in gray. This data is based on tests with robotic swimmers. 
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)
Swimming in Line
When swimming in open waters, it pays to keep your ducks (or your goslings!) in a row. A recent study examined the waves generated behind adult water fowl and found that babies following directly behind them benefit from their wake. In the right spot behind its mother, a duckling sees 158% less wave-drag than it would when swimming solo. That’s such a large reduction that the duckling actually gets pulled along! And the advantage doesn’t just help one duckling; a properly-placed duckling passes the benefit on to its siblings as well. So any duckling that stays in line has a much easier time keeping up, but those who slip out of the ideal spot will have a much tougher time. (Image credit: D. Spohr; research credit: Z. Yuan et al.; via Science News; submitted by Kam-Yung Soh)
Benefits of Schooling
Though fluid dynamicists have long theorized about the hydrodynamic benefits of fish swimming in schools, nailing down the actual physics has been quite difficult. Fish rarely swim exactly as an experimenter would like, and measuring quantities like swimming efficiency in a living fish is tough to do. In the numerical realm, it’s tough to simulate multiple fish swimming at realistic conditions. So some teams have turned to biomimetic robotic platforms to study schooling, as in this new research.
Once you’ve built a robotic fish that swims in a realistic way, that fish will have no problem swimming the same experimental patterns over and over. In this work, the researchers compared their robots swimming solo and swimming with a partner. In the partnered studies, they looked at fish swimming in phase — with their undulations matching one another — and out of phase — where the fish move opposite one another. They found that having a nearby partner improved the speed and efficiency for both fish, regardless of phase. But they also found a peculiar exception.
If one fish modifies their tailbeat frequency relative to their partner, they can slightly increase their power efficiency. But if they do so, it costs their partner more energy. That implies that fish could employ competitive dynamics, but, of course, it doesn’t tell us that they do! (Image and research credit: L. Li et al.; submitted by Kam-Yung Soh)
Jellyfish Make Their Own Walls
When we walk, the ground’s resistance helps propel us. Similarly, flying or swimming near a surface is easier due to ground effect. Most of the time swimmers don’t get that extra help, but a new study shows that jellyfish create their own walls to get that boost.
Of course, these walls aren’t literal, but fluid dynamically speaking, they are equivalent. Over the course of its stroke, the jellyfish creates two vortices, each with opposite rotation. One of these, the stopping vortex, lingers beneath the jellyfish until the next stroke’s starting vortex collides with it. When two vortices of equal strength and opposite rotation meet, the flow between them stagnates — it comes to halt — just as if a wall were there.
In fact, mathematically, this is how scientists represent a wall: as the stagnation line between a real vortex and a virtual one of equal strength and opposite rotation. It just turns out that jellyfish use the same trick to make virtual walls they can push off! (Image and research credit: B. Gemmell et al.; via NYTimes; submitted by Kam-Yung Soh)
Recreating Infinity
In the ocean, tiny organisms can migrate hundreds of meters through the water column. Recreating and tracking those journeys in a lab is quite a challenge, but it’s one the researchers behind the Gravity Machine have conquered. This apparatus uses a wheel to essentially give micro-organisms an infinite water column to traverse while keeping them fixed in the lab microscope’s field of view.
With the device, researchers can watch organisms switch naturally between rising, sinking, and feeding behaviors as they would in the wild. The group is working to make it so that anyone with a microscope can recreate their set-up for observations. (Image, video, and research credit: D. Krishnamurthy et al.; see also Gravity Machine; submitted by Kam-Yung Soh)
Artificial Microswimmers
Tiny organisms swim through a world much more viscous than ours. To do so, they swim asymmetrically, often using wave-like motions of tiny, hair-like cilia along their bodies. Mimicking this behavior in artificial swimmers is tough; how would you actuate so many micro-appendages? A new study offers a different method: inducing cilia-like waves using magnetic fields.
The researchers’ microswimmers are actually arrays of ferromagnetic particles. The Cheerios effect helps draw the particles together, while magnetic repulsion pushes them apart. Together, these forces help the particles assemble into crystal-like arrays.
To make the particles swim, the researchers shift the magnetic field. All of the outer particles of the array behave like individual cilia. As the magnetic field moves, the cilia-particles move in waves, much like their natural counterparts. Using this technique, the researchers were able to demonstrate both rotational and straight-line (translational) swimming. (Image, research, and submission credit: Y. Collard et al.)
Audubon Photography Awards
Several of this year’s Audubon-Photography-Award-winning photos feature birds interacting with fluids. The Grand Prize Winner, by Joanna Lentini, features a diving double-crested cormorant. Like many other species, these cormorants launch themselves into shallow waters from above and endure some incredible forces to do so. They’re no slackers underwater, either; when I encountered a flightless cormorant while snorkeling in the Galapagos, it outswam me in an instant.
The other prize winners above are a little more splashy. The American dipper’s splash curtain comes from sticking its head underwater in search of prey. The Anna’s hummingbird seen in the last image is playing in the spray of a fountain and showing off its aerial agility while doing so! (Image credits: cormorant – J. Lentini, dipper – M. Fuller-Morris, hummingbird – B. Ghosh; via DPReview; submitted by Kam-Yung Soh)
Steering as a Boxfish
Coral reefs are full of odd-looking denizens, but one of the funniest-looking ones must be the boxfish. This family of fish lives up to its name; their bodies feature an angular, bony carapace that helps protect them. But you don’t have to be a fluid dynamicist to wonder how in the world they swim with that kind of shape.
There’s actually disagreement in scientific circles as to whether the basic shape of a boxfish is stabilizing or destabilizing, in other words, whether the fish’s body shape will try to automatically turn or roll when flow moves past. A new study focuses instead on the role the fish’s tail fin serves. Through experiments (on a fish model) and simulations, the researchers showed that boxfish rely on their tail fins both as rudders and course-stabilizers.
Living around coral reefs means that boxfish need to be highly maneuverable, and this research indicates that the fish’s body shape, combined with the stabilizing power of its tail, are key to its ability to quickly and easily turn in any direction. (Image credits: boxfish – D. Seddon, simulation – P. Boute et al.; research credit: P. Boute et al.; via NYTimes; submitted by Kam-Yung Soh)
