Water striders are masters of life at the interface of water and air. Their spindly legs are skinnier than the capillary length of water, meaning that, at their size, surface tension is strong enough to overcome gravitational effects. Thus, their feet leave dimples on the interface, but the water itself holds them up. To keep from getting accidentally drenched (and thus weighed down), the striders are covered in tiny hairs that trap a layer of air that makes them hydrophobic or water-repellent. To get around, these masters of the interface use their middle legs in a manner similar to oars. They push against the dimple around their legs, which generates vortices under the surface and helps propel them. Even more impressive, the water strider can jump off the surface, a feat that requires remarkable adaptation in order to maximize the jump without breaking surface tension. (Video credit: Deep Look)
Tag: biology
Soaring Pelicans
Earlier this summer, I looked up on a bright, sunny day and saw a quartet of black and white figures soaring overhead. Initially, I thought it might be a formation of kites or unmanned aerial vehicles (UAVs) because I saw no flapping as the group wheeled about. With the help of the Cornell Lab of Ornithology’s awesome Merlin app, I was able to identify the soarers as American white pelicans – not a species I’d expected to find flying along the Front Range of the Rocky Mountains! (Turns out, they breed on lakes around here.)
The reason I saw so little flapping is that the birds were riding thermals. As the sun heats the ground, air near the surface warms up and begins to rise due to its buoyancy. Pelicans interested in flying between breeding and foraging grounds will start testing the thermals early in the day, as soon as they begin to form. As the heating continues, the intensity of thermals strengthens and they extend higher into the atmosphere. This is where the birds can really excel at using atmospheric energy for their flight. Pelicans will circle within a thermal until they reach roughly the middle of its height. Then they will glide, gradually losing altitude until they reach another thermal where they can climb without expending their own energy. With a 2.7 meter wingspan and a relatively low drag coefficient, the pelicans can glide and soar remarkably well. Researchers have even suggested using them as a sort of biological UAV for studying atmospheric dynamics! (Image credits: D. Henise, M. Stratmoen; research credit: H. Shannon et al., pdfs – 1, 2)
Hair in the Flow
Humans are hairy on the inside. Not in the way that we are on the outside, but in the sense that many interior surfaces of our bodies are covered in small, flexible, hair-like protrusions like the papillae on our tongues or the cilia in our intestines. Many of these fibers are immersed in fluids, raising the question of how the flow and the hairs interact. An elastic fiber immersed in a flow will bend in the direction of the flow (bottom); this helps reduce the drag and widens the channel flow goes through compared to a stiff, upright fiber.
But what happens when the fibers are all mounted at an angle? In this case, researchers found an asymmetric response. If flow moves in the direction of the fibers’ bend, the hairs don’t impend the flow at all. If flow moves against that direction, however, the hairs start to stand upright, blocking the flow channel and increasing the drag. The researchers suggest this sort of mechanism could be use in micro-hydraulic devices in the same way as a diode in a circuit – allowing flow in only one direction. For another biological example of flow control, check out how a shark’s denticles can prevent flow separation. (Image credits: hairy surface – J. Alvarado et al., flow around a hair – J. Wexler et al.; research credit: J. Alvarado et al.)
Chains of Salps
Salps are small, jellyfish-like marine invertebrates that swim by ejecting a pulsatile jet. They are unusual creatures whose lives have two major stages: one in which salps swim individually and one in which they link together and swim in large chains. In the chain, salps don’t synchronize their jetting; each salp jets with its own phase and frequency. A new study suggests that, in spite of this lack of synchronicity, the salp chain’s swimming reduces the animals’ drag. There are several factors that contribute to this result. One is that drag is generally lower on a body moving at constant speed compared to one moving in bursts. When linked together and firing randomly, all the individual jets tend to average out into one continuous swimming speed. There’s even a benefit to being out of sync: previous work showed that synchronized jets lose some of their thrust when they are too close together. Salps avoid that loss by keeping to their own beat. (Image and research credit: K. Sutherland and D. Weihs, source; via Gizmodo)
When Fire Ants are a Fluid
Substances don’t have to be a liquid or a gas to behave like a fluid. Swarms of fire ants display viscoelastic properties, meaning they can act like both a liquid and a solid. Like a spring, a ball of fire ants is elastic, bouncing back after being squished (top image). But the group can also act like a viscous liquid. A ball of ants can flow and diffuse outward (middle image). The ants are excellent at linking with one another, which allows them to survive floods by forming rafts and to escape containers by building towers.
Researchers found the key characteristic is that ants will only maintain links with nearby ants as long as they themselves experience no more than 3 times their own weight in load. In practice, the ants can easily withstand 100 times that load without injury, but that lower threshold describes the transition point between ants as a solid and ants as a fluid. If an ant in a structure is loaded with more force, she’ll let go of her neighbors and start moving around.
When they’re linked, the fire ants are close enough together to be water-repellent. Even if an ant raft gets submerged (bottom image), the space between ants is small enough that water can’t get in and the air around them can’t get out. This coats the submerged ants in their own little bubble, which the ants use to breathe while they float out a flood. For more, check out the video below and the full (fun and readable!) research paper linked in the credits. (Video and image credits: Vox/Georgia Tech; research credit: S. Phonekeo et al., pdf; submitted by Joyce S., Rebecca S., and possibly others)
ETA: Updated after senoritafish rightfully pointed out that worker ants are females, not males.
The Mantis Shrimp’s Left Hook
The mantis shrimp is a tiny, clown-colored juggernaut of underwater physics. Some species have modified claws that serve as clubs for punching their prey, and the mantis shrimp swings that club fast – its acceleration is comparable to a bullet’s! Moving that quickly in water causes a drastic drop in local pressure, low enough to form a cavitation bubble. Such low-pressure bubbles themselves are not particularly dangerous, but their collapse is incredibly violent, especially near a solid surface, like the shell of the shrimp’s prey. Collapsing cavitation bubbles can send out shock waves, shatter glass, and even generate light. In the case of the mantis shrimp, it’s more than enough to stun, if not outright kill, its prey. (Video credit: Physics Girl)
Leaping Droplets
Many fungi use coalescing water droplets to launch and spread their spores. The process is recreated in the laboratory in the animation above. Initially, there is a small spherical drop and a second, flattened drop stuck to the backside of the spore. In the animation, the large object on the right is actually both spore and droplet. The spore is spherical on one side and flattened on the other and starts out tipped up on its edge. When the spherical drop gets large enough to reach the flattened drop, they merge. This reduces the total surface area of the drop and thus releases some surface energy. It’s that surface energy that drives the spore’s jump. Even launching just a centimeter away from the host fungus is enough for a breeze to carry the spore further, allowing the fungus to reproduce. (Image and research credit: F. Liu et al., source; submitted by Kam-Yung Soh)
The Hydraulics Behind a Tuna’s Turns
Tuna are remarkably agile for their size. Many species reach lengths exceeding the height of a human adult, yet they can still make tight turns, especially when hunting. A recent study described one mechanism that aids the fish – a built-in hydraulic system for raising its second dorsal and anal fins. The tuna use fluid from their lymphatic system – which produces and transports white blood cells in both humans and tuna – to pressurize chambers at the base of some fins, causing the fin to rise. The extra support puts the fin in a hydrodynamically advantageous position and helps stabilize the fish when turning quickly, allowing them to change direction without slowing. (Video credit: Science; research credit: V. Pavlov et al.)
Hagfish Crash
Last week a flatbed truck in Oregon overturned and released 3400 kilograms of live hagfish on the highway and nearby cars. Hagfish are eel-like fish known for their impressive slime production. When threatened, the hagfish produce mucins that, when combined with water, form an extremely viscoelastic mucus. As it’s stretched, the mucus thickens and becomes more viscous. Normally, hagfish use this property to clog the gills of fish trying to eat them. The slime is weak, however, to shearing; hagfish actually tie themselves in knots to slide the slime off when there’s too much of it. The Oregon Department of Transportation managed to clear the road of mucus (and hagfish) using bulldozers and fire hoses, but it did take them several hours. For more photos and videos from the incident, check out Gizmodo and the Oregon State Police Twitter feed. (Image credit: Oregon State Police; via Gizmodo)
How Hummingbirds Drink
Hummingbirds are incredible acrobatic fliers, capable of hovering for more than 30 seconds at a time, even in windy conditions. Their feeding habits are equally impressive. Many species of hummingbirds have a forked tongue, each half of which curls over like a partial straw. As the bird extends its tongue, its beak compresses the space inside the tongue’s curls. Once in the nectar, both halves of the tongue re-expand, pulling liquid in along the full length of the tongue. For the birds, this is a much faster technique than simply sucking the nectar up like a straw. Hummingbirds can lick nectar more than ten times a second this way. For more gorgeous imagery of hummingbirds, be sure to check out National Geographic’s full feature. (Image credit: A. Varma, source; via Aarthi S.)
