Why Big Babies Leak, Part 2: Curvature

Real babies have it all. In part one, we saw how their greater surface area ratio makes them less leak-prone. Unfortunately, it doesn't stop there - they also have a better curvature. Real babies are pudgy. Their legs and waist are small and round. In contrast, adult babies come in a wide range of shapes and sizes. They might be bony, muscular, heavy, etc. As we will see, any shape other than small and round is more leak-prone.

To avoid delving into a detailed discussion of human anatomy, let's consider a simpler shape; a hardcover book. While any hardcover book will do, I've chosen Wilhelm Stekel's "Patterns of Psychosexual Infantilism." Like any hardcover book, it has a flat back and a groove where the back connects to a curved spine. While insightful for 1952, the differences between psychosexual infantilism and paraphilic infantilism tend to make the book a bit runny; thus the plastic pants.

These plastic pants have elastic in the hems, to prevent leaks. When the hem wraps around something, the tension in the elastic holds the hem closed. However, that something must be round.

For example, consider the section where the back cover connects to the spine of the book. This area is concave, it isn't even touching the hem. No matter how tight the elastic is, the plastic pants won't get a good seal here. Adult babies have similar concave areas next to their own spines. Many an adult baby has slept on his back, only to wake up with a wet T-shirt after his diaper leaked around the spine. (Again, sadly, diaper lovers who wet are also subject to this problem.)

The flat section of the cover is also highly prone to leaks. Even though the hem is touching the book, there isn't any force to hold the seal closed. This is because the cover is flat, not round: There is no curvature.

The only place where the hem can make a good seal is around the curved spine of the book. Here, the book is round, so the hem can force the seal closed. The more round, the better the seal. Mathematically, the seal force is inversely proportional to the radius of curvature. As a result, areas with no curvature or a negative curvature have no seal. At best, the adult baby will have a large curvature and OK seals. The real baby will have a small curvature and much more effective seals.

Here again, physics is unfair. Getting even a halfway decent seal around the legs and waist of an adult baby requires much more tension in the elastic. This is one of two reasons why baby diapers can seal well with only two tapes, while adult diapers need a gazillion. We'll get into the other reason in part #3, shear.

Last Update:24 March 2011  First:3 Dec 2006   Email BitterGrey   

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