Friday, October 13, 2017

Ask a Physicist: The Blood of a Starship

Recently, a reader by the name of Robert wrote in with a fun question that has an even more fun answer:

I'm trying to find a material that acts like a liquid under high pressures, but also acts as a solid at low pressures. I'm trying to design a kind of fictional armor for my spacecraft, I want something that will fill holes produced by impact and weapons fire. The only problem is: I don't know if something like that can exist.
Thanks for writing in! What you're describing is a non-Newtonian fluid; something that has different viscous properties depending on the circumstances. The most popular and well-known non-Newtonian fluid is "oobleck", the corn starch/water mixture that makes for popular home experiments.

Oobleck is "shear-thickening", though, which means that it acts like a liquid when it's handled gently, but firms up to act like a solid under impact—meaning you can run across it if you're moving fast, but will sink into sticky slime if you stand still.

This interesting combination of properties is has already led to shear-thickening non-Newtonian fluids being developed as a kind of armor, although in a different way than you've suggested—"liquid body armor" will hopefully give wearers flexibility and range of motion in a way that normal kevlar just can't, while still hardening up to disperse the force of any projectile impact.

A fluid like you're describing, though, has the opposite properties—flowing under pressure and solidifying without it, so we'd describe such a fluid as "shear-thinning". Coincidentally, there are several well-known shear-thinning fluids as well, and one of them is pumping through your body right now!

That's right, blood has the very properties you've described, though not quite to the extreme extent you might be imagining. For instance, it's not really the lack of pressure that causes blood to coagulate and scab up when it finds itself outside your body; that has more to do with platelet activation and fibrin formation. But when your circulatory system is working properly (i.e. with no leaks in the line), blood's shear-thinning properties make it ideal for what it does—you want a fluid that's going to flow most easily under extreme pressures, like when it's being pumped out of the heart. If it were shear-thickening instead, and became more viscous when your heart tried to squeeze it out, that'd be a perfect recipe for a quick heart attack.

But even though it's not the shear-thinning-ness of blood that makes it coagulate, this is still a great jumping-off point for your story; biology inspires some of the most elegant and successful designs in engineering. Consider that we've had a few billion years of natural selection encouraging us to develop a very efficient way of plugging holes in high-pressure systems, and suddenly it becomes the most natural idea in the world to give your ship a circulatory system, a fractal network of high-pressure veins and arteries that perfuse its surface. Fill it with a bio-inspired but heavily engineered shear-thinning fluid, maybe throw in some platelet-style nano-balloons; thanks to the ideal gas law, a rapid drop in pressure will cause a fixed amount of gas in an elastic container to expand enormously, so your ship's "blood" could rapidly form a foam-like scab. As in animals, this system wouldn't work too well for large-scale breaches; you'll need the ability to seal off an entire area of the ship if the hull's integrity has been compromised, but that ought to be a standard feature on any interstellar-class spaceship.

A rendered concept image of a theoretically "warp-capable" ship, albeit one that relies on exotic matter that's never been observed.
Image Credit: Mark Rademaker and Dr. Harold White, via Gizmodo.
All in all, this question—and its answer—scream good sci-fi; a plot device like your self-healing spacecraft becomes an opportunity for character development and world-building. Early on in the story, maybe your protagonist quietly contemplates the almost-living nature of the ship itself, revealing the protective systems that will come into play when the action picks up and holes start appearing. Maybe the blood that fills its veins isn't just bio-inspired, but biologically produced by genetically modified bone marrow cells, further blurring the line between organism and ecosystem. An extraordinary amount of thought and ingenuity goes into the design of something like a spaceship, so finding subtle ways to show off that you've put thought into yours is an essential part of pulling readers in and making the universe you create feel real. There are endless opportunities here, not only to explore whatever corner of the infinite universe you imagine, but also to turn an eye back to Earth and ask the questions that you can only ask from such a remote perspective.

I can't wait to see what you come up with.

Stephen Skolnick

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