Grains of pollen are caught amid droplets on a spider’s web in this award-winning image by John-Oliver Dum. How droplets behave on fibers has been a popular topic in recent years with research on how droplets nestle into corners, how they slide on straight or twisted wires, the patterns formed by streams of falling drops, and what happens to a droplet on a plucked string. (Image credit: J. Dum; via Ars Technica)
Tag: droplets
Bouncing Indefinitely
On the surface of a gently vibrating liquid, a droplet can bounce indefinitely without coalescing, kept aloft by an air film too small to see. As long as the droplet lifts off before the air layer drains out from under it, the droplet won’t contact the water below. Now scientists have shown that this is possible with a solid surface, too.
Using an atomically smooth mica plate, researchers were able to bounce a droplet indefinitely without wetting the surface. At higher vibration rates (below), the droplet essentially hovers in place, bouncing so quickly that we simply see its shape vibrating in response to the surface. (Image and research credit: L. Molefe et al.; via APS)
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A Drop of Algae
Spheres of a Volvox colonial algae glow green inside a droplet in this award-winning microphotograph by Jan Rosenboom. Pinned on an inclined surface, the droplet is frozen in a balance between gravity and surface tension that keeps its shape–and its contact angles–asymmetric. Droplets will also take on a shape similar to this when air is blowing past them. (Image credit: J. Rosenboom; via Ars Technica)
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Superwalking Droplets
When placed on a vibrating oil bath, droplets have many wild behaviors, some of which mirror quantum mechanics. Even big droplets — bigger than 2 millimeters in diameter — can get in on the fun. This video shows several of these “jumbo superwalkers” in action, both singly and in groups. (Video and image credit: Y. Li and R. Valani; via GFM)
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Droplets Through a Forest
When droplets flow through a forest of microfluidic posts, they can deform around the obstacle or break up into smaller droplets. Here, researchers explore the factors that control the outcome, as well as when droplets collide, coalesce, and mix. (Video and image credit: D. Meer et al.)
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“Legends of the Falls”
Strong winds blew curtains of mist across SkΓ³gafoss in this image of nesting northern fulmars by photographer Stefan Gerrits. Despite water’s high density compared to air, fine droplets are able to stay aloft for long periods, given the right breeze. Mists, fogs, and sea spray can float surprising distances; droplets exhaled from our lungs can persist even farther. (Image credit: S. Gerrits; via Colossal)
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Why Sharper Knives Mean Fewer Onion Tears
Onions are a well-known source of tears for many a cook. And while the chemical source of their power–onions release a chemical that reacts in our eyes to produce tears–has been known for years, no one has looked at the fluid dynamics in the process until now.
As seen above, a knife piercing the onion’s surface releases a mist of high-speed droplets, followed by a slower spray. Much like a citrus fruit’s microsprays, the onion’s fountain depends on both solid and fluid mechanics. As the knife presses into the onion’s stiffer outer layer, pressure builds in the softer layer underneath, which contains pores of fluid. Once the knife breaks the epidermis, that pressurized fluid sprays out.
The good news is that the team also confirmed a common culinary wisdom: using a sharper knife and a slower, gentler cut will reduce the spray and its speed, resulting in fewer tears. (Image credit: M. Stone; research credit: Z. Wu et al.)
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“Sensations”
Beautiful colors, subtle flows, and sudden fractals animate Thomas Blanchard’s “Sensations,” which, like his other short films, is entirely CGI-free. It’s a lovely exploration of droplets, liquid lenses, Marangoni effects, and fingering instabilities. (Video and image credit: T. Blanchard)
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Dancing Metal Droplets
Droplets of a gallium alloy are liquid at room temperature. When spiked with aluminum grains and immersed in a solution of NaOH, the droplets change shape and move in a random fashion. This video delves into the phenomenon, describing how a chemical reaction with the aluminum grains changes the local surface tension and creates Marangoni flows that make the droplets move. To get the droplet motion, you need to have the aluminum concentration just right. With too little, there’s not enough Marangoni flow. With too much, the hydrogen gas produced in the chemical reaction disrupts the droplet motion. (Video and image credit: N. Kim)
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“Droplet on a Plucked Wire”
What happens to a droplet hanging on a wire when the wire gets plucked? That’s the fundamental question behind this video, which shows the effects of wire speed, viscosity, and viscoelasticity on a drop’s detachment. With lovely high-speed video and close-up views, you get to appreciate even subtle differences between each drop. Capillary waves, viscoelastic waves, and Plateau-Rayleigh instabilities abound! (Video and image credit: D. Maity et al.)
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