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Knowing what this world is about

August 9, 2025 at 3:59 PM by Dr. Drang

This Scientific American article (web | Apple News) about new hydrogel adhesives is interesting, but I have one small nit I need to pick with it.

Here’s the paragraph that brought me up short:

After synthesizing the ultrasticky gels, the scientists took two of them into the field to test their real-world capabilities. The researchers used one gel to seal a hole at the base of a three-meter-long pipe that was filled with tap water to simulate a high-pressure water leak. And they used the other to affix a rubber duck onto a rock to see how well the technology fared in seawater. One day these gels could help researchers develop artificial skin or repair underwater and offshore structures.

Emphasis added to the problem sentence. The pressure at the bottom of a three-meter pipe isn’t high, at least not by most standards. And it’s easy to calculate.

The pipe is oriented vertically and open to the atmosphere at its top. (If this weren’t the case, the length of the pipe wouldn’t matter—also, the video in the article briefly shows both a tall vertical tube and someone patching a hole near its base.) So the water pressure at the bottom, p, is

p=ρgh

where ρ is the density of water, g is the acceleration due to gravity, and h is the height of the water column. Plugging in the values for all these, we get

p=(1000kgm3)(9.81ms2)(3m)=29,430Pa

where 1Pa=1N/m2. Converting from pascals into US Customary units for people like me yields 4.27 psi.

Is this a high pressure? I wouldn’t say so. If my garden hose had a leak, a patch would have to withstand a pressure an order of magnitude higher than this, and most people wouldn’t characterize residential water pressure as particularly high.1

This doesn’t mean the adhesive isn’t impressive—it certainly is. It’s the SciAm writer and editor I’m picking a nit with, not the researchers.

By the way, I knew the pressure at the bottom of the pipe was going to be less than 5 psi before doing any calculations. At some point in my life, I read an article—probably by Isaac Asimov—about how one of the early uses of steam engines was to power pumps to pull water out of mines. These were vacuum pumps installed at the surface, and it was soon learned that no matter how powerful the steam engine was, or how good the seals were in the valves and connections, water could not be pulled up more than about thirty feet. That’s because the pressure of a thirty-foot column of water was roughly equal to atmospheric pressure, which in turn is about 14.7 psi.

Since three meters is about one-third of thirty feet, the pressure at the bottom of the pipe was going to be something less than 5 psi. It’s funny how some things stick with you.

  1. There’s also the question of whether the water pressure is the only thing that matters when it comes to sealing a leak. It isn’t. You also have to account for the area of the hole, the area of the patch that’s sticking to the pipe, and the strength of the hydrogel patch material itself. But here we’re only questioning the “high-pressure” part of the article. ↩

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