I'm writing a story, and 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.

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.

Recently, a reader by the name of Robert wrote in to ask a fun question, with an even more fun answer. Robert,First off, thanks for writing in—I love questions like this. It turns out that what you're describing is pretty common; it's called a non-Newtonian fluid: something that has different viscous properties depending on the circumstances. The most popular and well-known non-Newtonian fluid is probably "oobleck", the corn starch/water mixture that makes for popular home experiments , but they can be found everywhere.Oobleck is "shear-thickening", which means that it acts like a liquid when it's handled gently, but firms up to act like a solid under impact—so you can run across it if you're moving fast, but you'll sink into sticky slime if you stand still.This interesting combination of properties is has already led to shear-thickening non-Newtonian fluids being investigated as a kind of experimental 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 can't, while still hardening up to disperse the force of any projectile impact.You're looking for something with the opposite properties, though—flowing under pressure and solidifying without it—so we'd describe this kind a fluid as "shear-thinning". Ketchup is one example (hence its tendency to glop out of a glass bottle all at once, after obstinately refusing to pour) but it turns out that there's an even more common one, and it's a perfect model for your spaceship's protective system: blood.As you might have gathered, non-Newtonian fluid behavior occurs in nature all the time, and it just so happens that blood has precisely the properties you're looking for—though not quite to the extreme extent you might be imagining. 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 the formation of long, fibrous proteins that get activated by injury. 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. A fluid that flows more easily under extreme pressures, like when it's being pumped out of the heart, lets it put less strain on the rest of your blood vessels. If it were shear-thickening instead, and becameviscous 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 properties of blood that make 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-ish network of high-pressure veins and arteries that perfuse its surface. Fill it with a bio-inspired but heavily engineered shear-thinning fluid, and when a micrometeorite pierces the hull, the rapid drop in pressure could thicken your ship's outflowing "blood" to a gelatinous paste, or a quick-hardening cement.You could enhance the system, giving it artificial platelet-like features, by throwing in micro-balloons full of gas. 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, plugging up the hole. 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 (like a tourniquet, in the event you lose an arm all at once) but that ought to be a standard feature on any interstellar-class spaceship.All in all, this questiongood 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, subtly revealing the protective systems that will come into play later on, when the action picks up and holes start appearing—this kind of exposition is important, so you don't have to take the reader out of the action to explain how the passive defenses work. Maybe the blood that fills your ship's veins isn't just bio-inspired, but biologicallyby genetically engineered 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 intois an essential part of pulling readers in, 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.