Splash Scientists Want to Make a Better Urinal

Yes, Oxford engineers study splashing, and yes, their research will be applied to urinals.

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Nov 12 2016, 2:00pm

Image: David Hunter/Flickr

Even if you're not peeing in a golden toilet in front of thousands of viewers, a visit to the urinal is a high-stakes game. Avoiding the dreaded urinal splash-back effect is basically an art form and a surprising number of physicists have dedicated their lives to perfecting the art of not splashing pee all over yourself. Take the Splash Lab at Brigham Young University for instance. Here, scientists have previously 3D printed a male urethra, attached it to a small pressurized hose, and used it to spray water at a urinal to figure out how best to avoid splash-back.

But according to new research from Oxford University, not peeing all over yourself may have less to do with the angle and proximity of the stream, and more to do with the material getting splashed.

Video from the Brigham Young splash experiment.

As detailed in a new study in Physical Review of Letters, the researchers found that coating a surface in a thin layer of silicone could limit splashing and help contain unhygienic fluids.

"We realized that no one had actually studied systematically what happens when droplets hit soft substrates," said Oxford professor of engineering Alfonso Castrejón-Pita. "Silicone—the material often used in bathroom sealants—is very useful, as it can be made to have different levels of stiffness, ranging from a material comparable to jelly to something with a consistency more like that of a pencil rubber."

According to the study, previous research has studied splashing in a vacuum and splashing against a thin, elastic membrane, both of which limit splash-back. But no research had been done on simple coatings which might achieve the same effect.

To test their hypothesis, the researchers used a high speed camera recording at 100,000 frames per second (about 4000 times faster than your smartphone camera) to record the effects of liquid hitting a silicone coating. When the researchers combined this footage with computer simulations, they found that tiny deformations in the silicone substrate occur within the first 30 microseconds after the fluid hits, which is just enough to almost entirely suppress a splash-back effect.

No splash back against a silicone substrate. Image: Castrejón-Pita, et al.

"What is most surprising is that you need about 70 percent more energy to get a drop to splash off these soft materials when compared with hard materials," said Castrejón-Pita. "If you think of a drop falling from a certain height, we need double the height to make it splash in the softest surfaces."

The researchers think their work will have applications beyond the bathroom. Anywhere where fluid containment is paramount, such as a kitchen or hospital, could benefit from suppressing the splash-back effect of dangerous or unhygienic liquids. This is a big enough problem that in places like the UK the government has launched a campaign to stop people from washing raw chicken before cooking because of splashing.

"The softer you make a material, the stickier and weaker it often becomes—two things which aren't ideal for making useful, long-term coatings," said Castrejón-Pita. "The main challenge of this work is how to overcome that. Luckily, recent work has started to develop new materials that can be soft, strong and non-sticky—like tough hydrogels—so there are certainly a lot of approaches to be explored."