What is the best design for a splash-free urinal? Physics now has the answer

Enlarge / Can you spot the urinal design with the optimal splash reduction angle? It’s the second from the right.

Mia Shi/University of Waterloo

Scientists at the University of Waterloo have determined the optimal design for a splash-free urinal: a tall, slender porcelain structure with curves reminiscent of a nautilus shell, playfully dubbed the “Nauti-loo.” This is good news for men tired of having urine splatter all over their pants and shoes, and for the poor souls who have to regularly clean up all the splatter. Bonus: It’s a pretty aesthetically pleasing design, giving this public restroom workhorse a touch of class.

“The idea was born exactly where you think it was born,” Zhao Pan from Waterloo told New Scientist. “I think most of us were a bit inattentive at our post and looked down to find we were wearing speckled pants. No one likes peeing everywhere, so why not just create a urinal where splashing is extremely unlikely? His graduate student, Kaveeshan Thurairajah, presented the results of this research at last week’s American Physical Society (APS) meeting on fluid dynamics in Indianapolis.

This is not the first time that scientists have tried to solve this problem. Pan is a former graduate student of Tadd Truscott, a mechanical engineer who founded the so-called “Splash Lab” at Utah State University. In 2013, the Splash Lab (then at Brigham Young University) offered some practical advice on how men could avoid staining their khaki pants with splashes of urine while relieving themselves in the bathroom. “Sitting on the toilet is the best technique because there’s less distance for pee to travel to the bowl,” I previously wrote to Gizmodo. “If you opt for the classic standing technique, scientists advise standing as close to the urinal as possible and trying to direct the spray at a downward angle towards the back of the urinal.”

For those lacking an optimal splash-proof technique, another of Truscott’s graduate students, Randy Hurd, presented an optimal design for a splash-free urinal insert at the 2015 APS Fluid Dynamics meeting. three basic types of inserts. One uses an absorbent cloth to minimize splashing; another uses a honeycomb structure – a raised layer (held up by small pillars) with holes – so that urine droplets pass through but splatter does not come out; and a third type featuring a set of pillars. However, absorbent fabrics cannot absorb liquid quickly enough and quickly become saturated, while honeycomb and network pillar structures do not prevent the gradual formation of urine puddles.

In 2013, the Splash Lab demonstrated that urine splashes could be reduced by aiming for a vertical surface, getting closer to the urinal, and decreasing the angle of impact.

Hurd and Truscott’s insert design was inspired by a type of super absorbent foam (Syntrichia caninervis) which thrives in very dry climates and is therefore very efficient at collecting and storing as much water as possible. And they discovered that the artificial material called “VantaBlack” mimicked the absorbent properties of foam. They copied the structure of this material for their urinal insert and found that it successfully prevented urine droplets from escaping, thus acting as a “urinal black hole”.

The ladies haven’t been left out of this scientific pissing contest either (ahem). Women also suffer from urine leakage, especially when they have to urinate into a cup for medical testing. In 2018, the Splash Lab conducted a series of experiments involving an anatomically correct female urethra model. (They used a flexible polymer to model the lips.) The results inspired the (patented) design of the “Orchid,” a funnel-shaped attachment for urine cups that reduces spillage. The research could lead to devices that allow women to pee standing up, which would be a boon for women in the military or academics working in the field.

According to Pan, the key to optimal splash-free urinal design is the angle at which the pee jet hits the porcelain surface; get a small enough angle and there won’t be any splashing. Instead, you get an even flow over the surface, preventing droplets from flying away. (And yes, there is a critical threshold at which the stream of urine changes from splashing to steady flow, because phase transitions are everywhere, even in our public restrooms.) It turns out dogs have already found the optimal angle when they raise their legs to pee, and when Pan et al. modeled this on a computer, they set the optimal angle for humans at 30 degrees.

Enlarge / ‘La Fontaine’ by Marcel Duchamp photographed by Alfred Stieglitz at Art Gallery 291 after the 1917 Society of Independent Artists exhibition.

Pan and his team also conducted a series of experiments with dyed fluids sprayed in jets at varying speeds into a range of faux urinals (see top photo) made of epoxy-coated dense foam, including the standard commercial form and a similar urinal. to the one Marcel Duchamp used in his famous (and controversial) 1917 art installation “La Fontaine”. All produced splashes of varying degrees, which the scientists wiped away with paper towels. They weighed the wet towels and compared that to the weight of the paper towels when dry to quantify the amount of splatter. The heavier the wet towels, the larger the credenza.

The next step was to find a design that would provide that optimal urine flow angle for men over a wide range of heights. Instead of the usual shallow rectangle-shaped box, they landed on the curved structure of the nautilus shell. They repeated the simulated urine flow experiments with the prototypes, and There you go! They did not observe a single splashing droplet. In comparison, other urinal models produced up to 50 times more splashbacks. It had a round design with a triangle shaped opening that worked even better than the Nauti-loo in experiments, but Pan et al. rejected it because it wouldn’t work over a wide range of heights.

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