Skipping a stone on water requires a flat, disk-like stone thrown at a shallow angle, but elastic spheres are remarkable skippers, too, even at higher impact angles. Researchers at the Splash Lab have just published their work on why these balls skip so well. As seen in the top animation, the elastic spheres deform on impact, flattening to a more disk-like shape that rides at an angle of attack relative to the air-water interface. Both features are important to the spheres’ enhanced skipping. By flattening, the sphere comes into greater contact with the water and by orienting at a larger angle of attack, the sphere increases the vertical component of force the water generates on the sphere. It’s this vertical force that lifts the sphere up and lets it keep bouncing.  

Because the ball is soft, it keeps deforming after its impact and bounce (see top animation). For some skips, the timescale of the sphere’s elastic waves is smaller than the length of time the sphere is in contact with the water. When this is the case, the sphere’s elastic waves will affect the impact cavity in the water, forming what the researchers call a matryoshka cavity, after the Russian nesting dolls. An example is shown in the second animation. For more, check out the USU press releasethe original paper, or the award-winning video they made a few years ago.  (Image credits: J. Belden et al./The Splash Lab)

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Fluids Round-Up

Time for another fluids round-up! Here’s some of the best fluid dynamics from around the web:

- Band Ok Go filmed their latest music video in microgravity, complete with floating, splattering fluids. Here they describe how they did it. Rhett Allain also provides a write-up on the physics.

- Scientists are trying to measure the impact of airliners’ contrails on climate change. (pdf; via @KyungMSong)

- Researchers observing the strange moving hills on Pluto suspect they may, in fact, be icebergs.

- The best angle for skipping a rock is 20-degrees. Related: elastic spheres skip well even at higher angles. (via @JenLucPiquant)

- Fluid dynamics and acoustics have some fascinating overlaps. Be sure to check out “The World Through Sound” series at Acoustics Today, written by Andrew “Pi” Pyzdek, who also writes one of my favorite science blogs

- Over at the Toast, Mallory Ortberg explores the poetry of the Beaufort wind scale.

- Could dark matter be a superfluid? (via @JenLucPiquant)

- Understanding the physics of the perfect pancake is helping doctors treat glaucoma. (submitted by Maria-Isabel)

- Van Gogh’s “Starry Night” shows swirling skies, but just how turbulent are they? (submitted by @NathanMechEng)

- The physics (and fluid dynamics!) of throwing a football - what’s the best angle for a maximum distance throw? (submitted by @rjallain)

(Video credit: Ok Go)

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What the signs remind me of
  • Aries:telling stories with an excitement filled voice, putting your hair up in a ponytail, getting mad after losing a game
  • Taurus:tapping your foot while listening to a song, putting a blanket over a sleeping person
  • Gemini:being the best hider while playing hide and seek, smiling at strangers
  • Cancer:sleeping with a stuffed toy, making friendship bracelettes, ice skating
  • Leo:old coffee shops, dancing around in your room, kissing the mirror
  • Virgo:sending your text to the wrong person, staring out of an open car window, to do lists
  • Libra:making shadow figures on the walls, turning a page in a book
  • Scorpio:glaring at annoying people, switching off the light, old photo albums
  • Sagittarius:pillow fights, not paying attention, biting your lip
  • Capricorn:rolling your eyes, stroking someones hair, scented candles
  • Aquarius:laughing untill you cry, doing a handstand and falling over, skipping stones
  • Pisces:spacing out, having weird dreams, making silly faces