New parenthood got fluid dynamics experts thinking about what ends in the diaper and they headed to the zoo and lab to come up with a physics story of defecation.
The ancient Chinese practiced copromancy, the diagnosis of health based on the shape, size and texture of feces. So did the Egyptians, the Greeks and nearly every ancient culture. Even today, your doctor may ask when you last had a bowel movement and to describe it in exquisite detail.
Sure, it’s uncomfortable to talk about. But that’s where science comes in, because what we don’t like to discuss can still cause harm. Irritable bowel syndrome, inflammatory bowel disease, gastrointestinal infections and other poop-related ailments cost Americans billions of dollars annually.
But trying to stem these problems was not our main motivation for trying to figure out some of the physics of defecation. It was something else, much more sinister.
From personal observation, into the lab
When parenthood hits, it hits hard. One of us is a working dad who survived by learning a new set of skills, one of which was fecal analysis. Years of diaper changes and then potty training turned me from a poo-analysis novice to a wizened connoisseur. My life passes by in a series of images: hard feces pellets like peas to long feces like a smooth snake to a puddle of brown water.
Unlike the ancients, we didn’t believe that we could predict the future from children’s stool. But we did think it was worth trying to understand where all these shapes come from. Having a laboratory to answer questions about the everyday world is one of the distinct pleasures of being a scientist.
As fluid dynamicists, we joined forces with colorectal surgeon Daniel Chu, and two stalwart undergraduates, Candice Kaminski and Morgan LaMarca, who filmed defecation and hand-picked feces from 34 mammalian species at Zoo Atlanta in order to measure their density and viscosity.