A Penetrable Fabric, Like Toilet Paper, Affects a Projectile’s Big Splash

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Researchers discover that they can fine-tune the size of a splash through the number of layers of toilet paper they place on the water’s surface.

From the Journal: 

WASHINGTON, D.C., August 14, 2018 -- Experts in fluid dynamics and kids jumping into a pool both know that an object falling into a liquid makes a splash. A new study finds that a single layer of a penetrable fabric – in this case, toilet paper – causes a wettable ball to make an especially tall splash, but additional layers can stop the splash entirely.

In a new study published in Physics of Fluids, from AIP Publishing, researchers at the University of Central Florida investigated the effects of a thin layer of unwoven fabric on the splashing dynamics of falling balls. Their findings show that adding layers can modify the size of the splash.

The initial inspiration for the study came from “the perennial concern over adequate dryness when visiting the restroom,” said Daren Watson, an author on the paper. In other words, the use of a preemptive layer of toilet paper to prevent splash-back. But, the phenomenon they discovered could be useful in manufacturing processes where splashing hot or corrosive liquids could pose a hazard.    

Physicists have studied most aspects of the fluid dynamics of an object hitting a liquid, but no one had tried adding a penetrable layer to the surface. “One of the motivations for this project was the fact that when people investigate splash heights, they modify the fluid conditions, or the impactor shape and dynamics,” Watson said. “We thought, how can we tune these splashes without altering the fluid itself?”

The researchers used a high-speed camera to record balls of different sizes dropping off a platform into an aquarium layered with up to four squares of toilet paper. “What we saw was a bit surprising,” said Andrew Dickerson, another author on the paper. “With meager layers of penetrable fabric, you actually can see greater splash heights. You need to really load up the surface with paper to see the benefit of splash reduction.” 

When the ball penetrates a single layer of fabric, it causes flow separation of the water, so air becomes sucked into a subsurface bubble or “cavity” that forms behind the ball. The surfacing of this cavity yields a higher jet than the water alone. The cavity also creates extra drag on the ball, even at low speeds. Usually, this type of cavity only forms with balls made from materials that repel water.

When the ball hits three or four layers, however, it cannot puncture the fabric, which squelches the splash. Going forward, the group plans to test other types of fabric with different properties. They tried paper towels, but found them to be too thick for the balls to penetrate.

The researchers caution that the current study focused only on perfectly round spheres. Results for other shapes may vary, especially with different entry angles and speeds.

Overall, the findings show that “we can use something as simple and commonplace as toilet paper to tune the height of splashing objects,” Dickerson said. But be careful: “A little bit of fabric might actually be worse than no fabric at all.”


For More Information:
Rhys Leahy
media [at] aip.org

Article title: 
Daren Watson, Jeremy Stephen and Andrew K. Dickerson
Author affiliations: 
University of Central Florida
About the journal: 

Physics of Fluids

Physics of Fluids is devoted to the publication of original theoretical, computational, and experimental contributions to the dynamics of gases, liquids, and complex or multiphase fluids.