droplet impacts

If you dropped a water balloon on a bed of nails, you’d expect it to burst spectacularly. And you’d be right – some of the time. Under the right conditions, though, you’d see what a high-speed camera caught in the animation above: a pancake-shaped bounce with nary a leak. Physically, this is a scaled-up version of what happens to a water droplet when it hits a superhydrophobic surface. 

Water repellent superhydrophobic surfaces are covered in microscale roughness, much like a bed of tiny nails. When the balloon (or droplet) hits, it deforms into the gaps between posts. In the case of the water balloon, its rubbery exterior pulls back against that deformation. (For the droplet, the same effect is provided by surface tension.) That tension pulls the deformed parts of the balloon back up, causing the whole balloon to rebound off the nails in a pancake-like shape. For more, check out this video on the student balloon project or the original water droplet research. (Image credits: T. Hecksher et al., Y. Liu et al.; via The New York Times; submitted by Justin B.)

aquilaswing  asked:

(as much as i like droplets and want to see it finished, i understand that you have moved on to other things that you're more passionate about and your happiness is more important than a story, especially since you're doing all the work of writing it. honestly i'm just glad that you plan to finish droplets at all so i don't mind waiting for it.)((also do you think you'll ever finish on the diamond mountain? no pressure, just asking))

i think it’s been important for me to establish ground rules for how much of an impact droplets has on my life, and since i’ve become more firm about that, i’ve felt a lot better with droplets and with myself. i know people really love the story, and i am forever foreveeeer grateful and humbled by that, but i also really love not feeling so much pressure on my shoulders from the fandom to update and/or cater for specific content they want to see. it’s been a bit of a relief to have some of the heat die down, and i think that will lend itself to quality material when i eventually get the rest of droplets finished :^) it’s taken a while, but i think i’m getting back on track with writing because i love to write and because i have stories to tell. it means a lot to me that people have been (mainly) understanding about this :^) 

as for OTDM, i’m really annoyed with myself about that one because ??? i have had chapter 2 nearly finished for a whole year now ??? i literally have 17,000 words ready to go, just with a few gaps here and there. but again, life escaped me early last year and suddenly got super hectic, so OTDM has to be pushed back a lot … i hope to finish that up too, as it had /a lot/ of reception for only one chapter! at the moment of course, i am working hard on another lover, so hopefully once that’s through (i don’t expect it to exceed 7 chapters), i’ll get back to OTDM as my new secondary project to droplets :^D

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A drop of water that impacts a flat post will form a liquid sheet that eventually breaks apart into droplets when surface tension can no longer hold the water together against the power of momentum flinging the water outward. But what happens if that initial drop of water is filled with particles? Initially, the particle-laden drop’s impact is similar to the water’s – it strikes the post and expands radially in a sheet that is uniformly filled with particles. But then the particles begin to cluster due to capillary attraction, which causes particles at a fluid interface to clump up. You’ve seen the same effect in a bowl of Cheerios, when the floating O’s start to group up in little rafts. The clumping creates holes in the sheet which rapidly expand until the liquid breaks apart into many particle-filled droplets. To see more great high-speed footage and comparisons, check out the full video.  (Image credit and submission: A. Sauret et al., source)

The simple coalescence of a drop with a pool is more complicated than the human eye can capture. Fortunately, we have high-speed cameras. Here a droplet coalesces by what is known as the coalescence cascade. Because it has been dropped with very little momentum, the droplet will initially bounce, then seem to settle like a bead on the surface. A tiny film of air separates the drop and the pool at this point. When that air drains away, the drop contacts the pool and part–but not all!–of it coalesces. Surface tension snaps the remainder into a smaller droplet which follows the same pattern: bounce, settle, drain, partially coalesce. This continues until the remaining droplet is so small that it can be coalesced completely. (Image credit: Laboratory of Porous Media and Thermophysical Properties, source video)

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I often receive questions about how fluids react to extremely hard and fast impacts. Some people wonder if there’s a regime where a fluid like water will react like a solid. In reality, nature works the opposite way. Striking a solid hard enough and fast enough makes it behave like a fluid. The video above shows a simulated impact of a 500-km asteroid in the Pacific Ocean. (Be sure to watch with captions on.) The impact rips 10 km off the crust of the Earth and sends a hypersonic shock wave of destruction around the entire Earth. There’s a strong resemblance in the asteroid impact to droplet impacts and splashes. Much of this has to do with the energy of impact. The asteroid’s kinetic (and, indeed, potential) energy prior to impact is enormous, and conservation of energy means that energy has to go somewhere. It’s that energy that vaporizes the oceans and fluidizes part of the Earth’s surface. That kinetic energy rips the orderly structure of solids apart and turns it effectively into a granular fluid. (Video credit: Discovery Channel; via J. Hertzberg)

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When a droplet impacts a pool at low speed, a layer of air trapped beneath the droplet can often prevent it from immediately coalescing into the pool. As that air layer drains away, surface tension pulls some of the droplet’s mass into the pool while a smaller droplet is ejected. When it bounces off the surface of the water, the process is repeated and the droplet grows smaller and smaller until surface tension is able to completely absorb it into the pool. This process is called the coalescence cascade.

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This high-speed video captures the impact of liquid droplets onto a granular surface. While there is some similarity to liquid-solid and liquid-liquid impacts, the permeability of the granular surface helps to “freeze” the splash rather quickly. Energy is dissipated in the initial impact, causing a splash of grains.  Then the surface tension, viscosity and inertia of the droplet compete in causing the deformations seen in the video. The deformation appears strongly dependent on the kinetic energy with which the droplet hits the surface (i.e. proportional to the height from which it is dropped). (Video credit: G. Delan et al)

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Sadly, this video shows no droplet impacts on a heart-shaped post, but maybe you can imagine what it would look like after seeing other geometrical shapes. Happy Valentine’s Day, guys!