Tea lovers have long been frustrated by the tendency of liquid jets to adhere to solid surfaces – the so-called teapot effect that makes the last vestiges of every pour drip down the spout. By investigating the effect with vertical rods, researchers found that, at low enough flow rates, a liquid jet is able to adhere completely, forming a liquid helix that coils around the rod. The authors were also able to construct a mathematical model to capture the behavior. They concluded that both the wettability of a surface and the curvature of the solid are critical to determining whether or not a liquid jet will stick. (Image and research credit: E. Jambon-Puillet et al.; via APS Physics; submitted by Kam-Yung Soh)
Year: 2019
Inside an Evaporating Drop
The evaporation of a simple droplet holds far more complexity than one would expect. If you look closely at the edge of the drop, there’s a tiny, beautiful display at work. It begins with small variations in surface tension at the contact line where solid, liquid, and gas meet. These could be caused by local temperature or concentration differences; either way, the gradient in surface tension creates a flow. It starts out as a series of microjets spaced evenly around the contact line (left).
As the microjets strengthen, they merge into larger and larger vortical structures (right). This kind of feature – large structures emerging from smaller ones – is known as an inverse cascade. Fluid dynamicists have traditionally studied the classic (turbulent) energy cascade, where kinetic energy moves from large scales into smaller ones, but researchers are beginning to recognize more situations where the inverse cascade occurs, such as in the storms of Jupiter. (Image and research credit: A. Ghasemi et al., source)
How Rain Can Spread Pathogens
Rainfall can help spread pathogens from an infected plant to healthy ones. This transfer can happen both through droplets and by dry-dispersal of pathogen spores (top). When a raindrop hits a leaf, its initial spread triggers a vortex ring of air that can lift thousands of dry spores into a swirling trajectory (bottom). That boost in height carries spores beyond the slower wind speeds of the plant’s boundary layer and into faster air streams that disperse it toward healthy plants. (Image and research credit: S. Kim et al.)
Fighting Resonance
Resonance is a funny creature, as Dianna discovered when she tried to sing a rising scale through a tube. At certain notes, everyone who attempted to do it had their voices crack. Tracking down the source of the mystery means digging into what exactly resonance is and what the differences are between driving a system just before, at, and after resonance. Check out the video for the full acoustic story. (Video credit: Physics Girl)
Plant Week: Bunchberry Dogwood
The bunchberry dogwood, unlike its taller relatives, is a low-lying subshrub that spreads along the ground. But it sports some of the fastest action of any plant, requiring 10,000 frames per second to capture! When young buds form in the bunchberry flower, their four petals are fused, completely hiding the stamens. As the plant matures, the pollen-carrying stamens grow faster than the petals, causing them to peek out the sides of the bud. But the petals stay attached at the tip, holding the stamens in while pressure inside the stamens creates a store of elastic energy.
When disturbed, the petals break loose and the stamens spring up and out. The anthers at their tips hold the pollen in place until the stamen reaches its maximum vertical velocity, at which point the anthers swing out to release the pollen upward. In essence, the flower works in the same manner as a trebuchet, flinging pollen with an acceleration 2,400 times greater than gravity. That’s enough to coat pollen onto nearby insects and to launch the remainder high enough for the wind to catch it. (Image and research credit: D. Whitaker et al., source; via Science News; submitted by Kam-Yung Soh)
And with that, FYFD’s Plant Week is a wrap! Missed one of the previous posts? You can catch up with them here.
Plant Week: Citrus Jets
Bartenders and citrus lovers the world over are familiar with the mist of oil that bursts from a bent citrus peel. These microjets are about the width of a human hair, but they can spray at nearly 30 m/s in some citrus species. That’s an acceleration g-force of more 5,100, comparable to a bullet fired from a gun!
The key to the jets is the structure of the fruit’s peel. Citrus fruits have a relatively thick, soft inner material, known as the albedo, which houses the oil reservoirs. The thin, stiff outer layer of the peel, called the flavedo or zest, covers that. When the peel is bent, the albedo compresses, increasing the pressure inside the oil reservoirs up to an additional atmosphere’s worth. Meanwhile, the flavedo is stretched. When that outer layer fails, it releases the oil pressure and a jet spurts out. For more on this work, including some awesome high-speed videos, check out my interview (starting at 2:59) with one of the authors in the video below. (Image and research credit: N. Smith et al.; video credit: N. Sharp and T. Crawford)
FYFD is celebrating Plant Week all this week. Check out our previous posts on how moisture lets horsetail plant spores walk and jump, the incredible aerodynamics of dandelion seeds, and the ultra-fast suction bladderworts use to hunt.
Plant Week: Jumping Spores
You might think that plants are pretty stationary, but they have evolved a myriad of ways of moving, especially when it comes to spreading their seeds and spores. Shown above is the spore of the horsetail plant, a spherical pod with four, ribbon-like elators that are moisture-sensitive. When exposed to water, the elators curl around the spore, but as they dry out, they unfurl (top). Repeated cycles of this allows the spores to “walk” short distances (middle). And, if the elators deploy quickly, the spore can even “jump” (bottom). Researchers recorded jumps high enough for the spores to catch a breeze and disperse further. For similar moisture-driven plant action, check out this seed that buries itself! (Image and research credit: P. Marmonttant et al., source; via Science News; submitted by Kam-Yung Soh)
We’re celebrating botanically-based physics all this week with Plant Week. Check out our previous posts on the ultra-fast suction of carnivorous bladderworts and the incredible flight of dandelion seeds.
Plant Week: Bladderworts
Carnivorous plants live in nutrient-poor environments, where clever techniques are necessary to keep their prey from getting away. The aquatic bladderwort family nabs their prey through ultra-fast suction. This starts with a slow phase (top) in which water is pumped out of the trap. Because the internal pressure is lower than the external hydrostatic pressure, this compresses the walls of the trap, and it leaves the trap’s door narrowly balanced on the edge of stability. A slight perturbation to the trigger hairs around the door will cause it to buckle.
That’s when things get fast. As the door buckles and the trap expands to its original volume, water gets sucked in, pulling whatever prey was nearby with it. The door reseals as the pressure inside and outside the trap equalizes, and, in only a couple milliseconds total, the bladderwort has its snack. It secretes digestive enzymes to break down what it’s caught, and over many hours, it pumps out the trap to reset it. (Image and research credit: O. Vincent et al.; submitted by David B.)
All this week, FYFD is celebrating Plant Week. Check out our previous post on how dandelion seeds fly tens of kilometers.
Plant Week: Dandelions in Flight
To kick off Plant Week here on FYFD, we’re taking a closer look at that ubiquitous flower: the dandelion. Love ‘em or hate ‘em, these little guys manage to get just about everywhere, thanks in part to their amazing ability to stay windborne for up to 150 km! To do that, the dandelion uses a bristly umbrella of tiny filaments, known as a pappus, that can generate more than four times the drag per area of a solid disk. Its porosity – all that empty space between the filaments – is also key to its stability; it helps create and stabilize a separated vortex ring that the seed uses to stay aloft. Check out the full video below! (Image and video credit: N. Sharp)
Plant Week: Introduction
Spring has sprung! The trees have leaves, the flowers are in bloom, and snow is (almost) a distant memory.* And here at FYFD, we’re getting ready to kick off a full week of celebrating the intersection of fluid dynamics and plants.
To get you into the mood, here’s a look at some previous plant-filled posts:
– How trees use negative pressure to hydrate
– The catapulting seeds of the hairyflower wild petunia
– Seeds that self-dig
– How desert moss drinks from the air
– The swimming of zoosporesStay tuned all next week for lots more plant physics!
*Confession: it’s still snowing at my house as I type this. But the trees do have leaves and there are flowers blooming. Poor things. – Nicole
(Original image: Pixabay)
