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

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

“The idea came up just where you think it came up,” Zhao Pan of Waterloo said New Scientist. “I think most of us were a bit careless in our post and looked down to find we were wearing stained trousers. “No one likes to pee everywhere, so why don’t we just create a urinal where splashing is extremely unlikely?” His graduate, Kaveeshan Thurairajah, present the results of this research during the American Physical Society (APS) last week Fluid Dynamics Meeting in Indianapolis.

This is not the first time scientists have tried to tackle this problem. Pan is a former graduate of Thad Truscott, a mechanical engineer who founded the so-calledSplash Lab” at Utah State University. In 2013, the Splash Lab (then at Brigham Young University) offered some helpful tips on how men can avoid splattering urine on their khaki pants while relieving themselves in restrooms. “Sitting on the toilet is the best technique because there is less distance for the pee to travel on its journey to the bowl,” I wrote earlier in Gizmodo. “If you choose the classic standing technique, scientists advise standing as close to the urinal as possible and trying to direct the stream at a downward angle towards the back of the urinal.”

For those lacking optimal anti-splash technique, another of Truscott’s graduate students, Randy Hurd, presented optimal design for splash-free urinal insert 2015 APS Meeting on Fluid Dynamics. There are three main types of inserts. One uses an absorbent cloth to minimize splashing; another uses a honeycomb structure – a raised layer (held by small pillars) with holes – so that urine droplets pass through but no splashes come out; and a third type involving an array of pillars. However, the absorbent tissues cannot absorb the liquid fast enough and soon become saturated, while the honeycomb structure and ordered columns prevent the gradual formation of pools of urine.

Hurd and Truscott’s insert design draws inspiration from a type of super-absorbent moss (Syntrichia caninervis), which thrives in very dry climates and is therefore very good at collecting and storing as much water as possible. And they found that the man-made material called “VantaBlack” mimicked the absorbent properties of moss. They copied the structure of this material for their urinal insert and found that it successfully blocked the leakage of urine droplets – effectively acting as a “urinal black hole”.

Nor were the ladies left out of this scientific (ahem) pissing contest. Women also suffer from urine spillage, especially when they have to pee in a cup for medical purposes. In 2018, Splash Lab conducted a series of experiments involving a model of an anatomically correct female urethra. (They used a soft polymer to model the labia.) The inspired results (patented) “Orchid” design, a funnel attachment for urine cups that reduce spillage. The research could lead to devices that allow women to pee standing up, which would benefit women in the military or women academics working in the field.

According to Pan, the key to optimal splash-free urinal design is the angle at which the pee stream hits the porcelain surface; take a small enough angle and there will be no backsplash. Instead, you get a smooth flow over the surface, preventing droplets from flying off. (And yes, there is a critical threshold at which the stream of urine goes from splashing to flowing, because phase transitions are everywhere—even in our public toilets.) It turns out that dogs have already figured out the optimal angle while lifting their legs to pee , and when Pan et al. modeled this on a computer, they determined the optimal angle for humans to be 30 degrees.

Pan and his team also conducted a series of experiments with dyed liquids sprayed in jets at varying speeds into a range of artificial urinal designs (see photo above) made of dense, epoxy-coated foam – including the standard commercial form and similar to urinal to the one Marcel Duchamp used in his famous (and controversial) 1917 art installation.”Fountain.” All produce varying degrees of backspray, which the scientists wipe up with paper towels. They weigh the wet towels and compare them to the weight of the paper towels when dry to quantify the amount of spray. The heavier the wet towels, the more – the rear part is big.

The next step was to come up with a design that would offer this optimal urine stream angle for men at a wide range of heights. Instead of the usual shallow box shaped like a rectangle, they landed on the curved structure of the nautilus shell. They repeated the simulated urine stream experiments with the prototypes, and there it is! They didn’t notice a single drop splashing back. In comparison, other urinal designs produce up to 50 times more kickback. There was one circular design with a hole shaped like a triangle that performed even better than the Nauti-loo in experiments, but Pan et al. rejected it because it would not work over a wide range of altitudes.