The practice of marking territory with urine is widespread in the mammal world — but in our modern society it’s, well, let’s say frowned upon. Nonetheless, men commonly and inadvertently mark bathroom floors with unintentional urinal splash back.
Urinals are pretty big business, estimated at several hundreds of millions of dollars a year. Understandably, many producers would want to make a splash in the industry — which in this case, means not making a splash.
Urinals have some surprisingly complex physics to them. The fluid dynamics of peeing into a urinal can get very complex, since no two streams are alike. Plus, men have different heights and different bodily geometries that can also affect the stream, and subsequently, the splash. “This problem has persisted since the invention of the urinal over a century ago,” the authors of the new study write.
Mechanical engineer Zhao Pan of the University of Waterloo in Canada wanted to study this in more detail. Along with several colleagues from the same university, they carried out numerical models to see what could impact the splashback and found that a key element is the angle at which the stream comes.
“We found that when a liquid jet or droplet train impacts a rigid surface below a certain critical impinging angle, almost no splatter is generated. Thus, a surface designed to always intersect the urine stream equal to or smaller than the critical angle prevents splash back,” the study abstract reads.
They took inspiration from unlikely sources: nautilus shells and dog urination. Nautilus shells have been studied in various aspects of physics because of their intriguing geometry, and the prototypes designed by Pan and colleagues also drew from their geometry. The team carried out numerical experiments, and then of course, tested the designs experimentally (using dyed fluids).
“Our numerical and experimental validations show that our urinal designs are superior to the typical use of a popular urinal available on the market as well as scenarios where urine streams are highly unstable (e.g. urinals in ships and airplanes undergo perturbation).”
This may sound like a whimsical study, but it could have significant real life implications. Just think about all the bars and other public venues with cleaner floors — this means not only more hygienic bathrooms, but also less cleaning work.
“Our new urinal designs will keep bathrooms cleaner and reduce the labor, water, and chemicals required for periodic cleaning to promote more sustainable bathroom maintenance,” the researchers conclude.
It’s not the first time urinals came in the spotlight of physics. In previous research, a team from the University of Oxford devised a soft material that could be used as coating and reduce splashback — but the problem with that approach is that these soft materials are also weak, which makes them unreliable for long-term coating. Also, a decade ago, a team of four physicists at Brigham Young University (calling themselves “wizz-kids”) also studied the properties of toilets and urinals. They concluded men should stand as close to the urinal as possible and aim to hit the back of the urinal at a downward angle.
But as we all know, you can’t really rely on men to pee properly, so we’re probably best off with better designs that make things splash-proof. Ultimately, mixing a useful geometry with better materials seems like the way to go. Quite frankly, we can’t wait for the next-gen urinals.
The study was presented at the 75th Annual Meeting of the Division of Fluid Dynamics