# New Liquid Body Armor- How the heck does this work? **URL:** \ **Category:** Factual Questions\ **Created:** [April 23, 2004, 7:32pm UTC](https://boards.straightdope.com/t/new-liquid-body-armor-how-the-heck-does-this-work/241694 "2004-04-23T19:32:13Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\ **Post date:** [April 23, 2004, 7:32pm UTC](https://boards.straightdope.com/t/new-liquid-body-armor-how-the-heck-does-this-work/241694/1 "2004-04-23T19:32:13Z")
Re this new liquid body armor, the explanation given in the news article was that “it transitions” to rigid state upon impact. Why? How? [Army Scientists, Engineers develop Liquid Body Armor](http://www.military.com/NewsContent/0,13319,usa3_042104.00.html?ESRC=dod.nl) > [@](#): > > ABERDEEN PROVING GROUND, Md. – Liquid armor for Kevlar vests is one of the newest technologies being developed at the U.S. Army Research Laboratory to save Soldiers’ lives. > > This type of body armor is light and flexible, which allows soldiers to be more mobile and won’t hinder an individual from running or aiming his or her weapon. > > The key component of liquid armor is a shear thickening fluid. STF is composed of hard particles suspended in a liquid. The liquid, polyethylene glycol, is non-toxic, and can withstand a wide range of temperatures. Hard, nano-particles of silica are the other components of STF. This combination of flowable and hard components results in a material with unusual properties. > > “During normal handling, the STF is very deformable and flows like a liquid. However, once a bullet or frag hits the vest, it transitions to a rigid material, which prevents the projectile from penetrating the Soldier’s body,” said Dr. Eric Wetzel, a mechanical engineer from the Weapons and Materials Research Directorate who heads the project team. > > To make liquid armor, STF is soaked into all layers of the Kevlar vest. The Kevlar fabric holds the STF in place, and also helps to stop the bullet. The saturated fabric can be soaked, draped, and sewn just like any other fabric. ---
**Author:** ![Ross](https://avatars.discourse-cdn.com/v4/letter/r/e274bd/32.png) [@Ross](https://boards.straightdope.com/u/Ross)\ **Post date:** [April 23, 2004, 9:38pm UTC](https://boards.straightdope.com/t/new-liquid-body-armor-how-the-heck-does-this-work/241694/2 "2004-04-23T21:38:58Z")
Sounds great. Here, you test it first. Gimme that rifle. Stand back a little. Okay, relax… ---
**Author:** ![Balance](https://avatars.discourse-cdn.com/v4/letter/b/ccd318/32.png) [@Balance](https://boards.straightdope.com/u/Balance)\ **Post date:** [April 23, 2004, 9:51pm UTC](https://boards.straightdope.com/t/new-liquid-body-armor-how-the-heck-does-this-work/241694/3 "2004-04-23T21:51:30Z")
It sounds like [ooblick](http://www.catb.org/~esr/jargon/html/O/ooblick.html), a popular “non-Newtonian liquid” or dilatant compound, which act like a solid under pressure during very short timeframes. Ooblick relies on its colloidal properties to resist sudden deformation. As I understand it, you can treat the particles in a colloidal suspension as a series of plates. The interparticle forces slow propagation of a shockwave through the material. Here’s a [painfully detailed examination](http://www.princeton.edu/~hehmeyer/docs/Pastes.pdf) of flow in a colloidal material. The short version is that the particles just can’t move out of the way quickly enough when hit hard and suddenly. Another material that shows similar non-Newtonian behavior is ThinkGeek’s [Smart Mass](http://www.thinkgeek.com/cubegoodies/toys/5ac8/detail/#shatter) putty, but the explanation for its properties doesn’t seem applicable here. ---
**Author:** ![Harmonix](https://avatars.discourse-cdn.com/v4/letter/h/3ec8ea/32.png) [@Harmonix](https://boards.straightdope.com/u/Harmonix)\ **Post date:** [April 23, 2004, 11:40pm UTC](https://boards.straightdope.com/t/new-liquid-body-armor-how-the-heck-does-this-work/241694/4 "2004-04-23T23:40:54Z")
:eek: !!! I had this idea years ago back when I first discovered ooblick. I always wondered if something similar could work as a liquid body armor. My question however is… because the bullet causes the body armour to become rigid – doesn’t that mean theres no deforming of the body armour? Meaning force is transfered to the person instantaneously rather then over the small amount of time that kevlar deforms? ---
**Author:** ![Balance](https://avatars.discourse-cdn.com/v4/letter/b/ccd318/32.png) [@Balance](https://boards.straightdope.com/u/Balance)\ **Post date:** [April 24, 2004, 12:29am UTC](https://boards.straightdope.com/t/new-liquid-body-armor-how-the-heck-does-this-work/241694/5 "2004-04-24T00:29:25Z")
> [@Harmonix](#): > > :eek: !!! > > I had this idea years ago back when I first discovered ooblick. I always wondered if something similar could work as a liquid body armor. > > My question however is… because the bullet causes the body armour to become rigid – doesn’t that mean theres no deforming of the body armour? Meaning force is transfered to the person instantaneously rather then over the small amount of time that kevlar deforms? Traditional ooblick isn’t a stable enough suspension for armor, I don’t think (yes, I considered it, too)–the particles settle out too easily. Besides, it decays. I don’t think I’d care to wear soggy, rotting cornbread. I suspect that the layered nature of both the suit and the suspension result in the force of the impact being redirected laterally through the suspension medium between the layers. The full force is still transferred to the target, but it would be spread over a larger area and probably a longer period. The key advantage this approach seems to offer is that it’s more flexible than traditional armor under normal, not-currently-being-shot conditions. ---
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [April 24, 2004, 3:07am UTC](https://boards.straightdope.com/t/new-liquid-body-armor-how-the-heck-does-this-work/241694/6 "2004-04-24T03:07:20Z")
> [@astro](#): > > Re this new liquid body armor, the explanation given in the news article was that “it transitions” to rigid state upon impact. Why? How? I can’t explain the “why” or “how” in any detail, but I can show you how to see that it happens. Go to a toy store and buy an egg of Silly Putty. Roll it into a slightly oblong shape, grasp it between your hands, and slowly pull them apart. It’ll stretch out like pulling taffy. Now roll it back into an oblong shape and grasp it between your hands. Yank it sharply, and it’ll “break” apart, leaving a clean flat surface. Basically, when the force is applied quickly (say from a bullet impact), the “liquid” will behave more like a solid. When force is applied slowly (like walking around) it’ll behave more like a liquid. ---
**Author:** ![Balance](https://avatars.discourse-cdn.com/v4/letter/b/ccd318/32.png) [@Balance](https://boards.straightdope.com/u/Balance)\ **Post date:** [April 24, 2004, 5:29am UTC](https://boards.straightdope.com/t/new-liquid-body-armor-how-the-heck-does-this-work/241694/7 "2004-04-24T05:29:43Z")
Yeah, the Smart Mass stuff I linked to above is basically souped-up Silly Putty. The non-Newtonian characteristics of the putty results from the fact that it consists of dynamically crosslinked polymer chains. The elasticity comes from polymeric relaxation (the tendency of stretched or compressed chains to relax into a rest state through thermal movement). The crosslinks between chains stabilize it into a semisolid form. If you apply force on a much shorter timescale than the relaxation period, the material acts as a solid. On a longer timescale, the crosslinks break and reform to accomadate the changes in the chain. At any rate, that’s how I understand it. IANASPC (I Am Not A Silly Putty Chemist). This is different from the material described in the article about the armor. ---
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [April 24, 2004, 5:35am UTC](https://boards.straightdope.com/t/new-liquid-body-armor-how-the-heck-does-this-work/241694/8 "2004-04-24T05:35:28Z")
> [@Balance](#): > > At any rate, that’s how I understand it. IANASPC (I Am Not A Silly Putty Chemist). This is different from the material described in the article about the armor. Yeah, reading it again it does seem that they mean more something that could sit in a jar, but when a high enough power flux hits it (say, a large amount of kinetic energy added in a very short time, like a bullet) it solidifies. Weird. ---
**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\ **Post date:** [April 24, 2004, 6:00am UTC](https://boards.straightdope.com/t/new-liquid-body-armor-how-the-heck-does-this-work/241694/9 "2004-04-24T06:00:47Z")
> [@Mathochist](#): > > Yeah, reading it again it does seem that they mean more something that could sit in a jar, but when a high enough power flux hits it (say, a large amount of kinetic energy added in a very short time, like a bullet) it solidifies. Weird. [Like this?](http://www.sunbelt-software.com/stu/putty/) ---
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [April 24, 2004, 6:17am UTC](https://boards.straightdope.com/t/new-liquid-body-armor-how-the-heck-does-this-work/241694/10 "2004-04-24T06:17:18Z")
> [@astro](#): > > [Like this?](http://www.sunbelt-software.com/stu/putty/) Evidently not quite. That stuff is effectively generic Silly Putty. Now I know why my friend has been trying to get people to go in on an order. It’s something like $8/lb in 50lb increments, with a minimum of 100lb to an order. I mean, can’t you come up with 20 people to each pay $40 for _five pounds of Silly Putty_? Expecially if you can throw it off a building… ---
**Author:** ![Cisco](https://avatars.discourse-cdn.com/v4/letter/c/ccd318/32.png) [@Cisco](https://boards.straightdope.com/u/Cisco)\ **Post date:** [April 24, 2004, 6:32am UTC](https://boards.straightdope.com/t/new-liquid-body-armor-how-the-heck-does-this-work/241694/11 "2004-04-24T06:32:51Z")
Is this the same stuff Stretch Armstrong was made out of? I remember you could moosh him slowly with no problem but if you punched him it would actually hurt your fist. ---
**Author:** ![Niggle](https://avatars.discourse-cdn.com/v4/letter/n/e9bcb4/32.png) [@Niggle](https://boards.straightdope.com/u/Niggle)\ **Post date:** [April 24, 2004, 6:43am UTC](https://boards.straightdope.com/t/new-liquid-body-armor-how-the-heck-does-this-work/241694/12 "2004-04-24T06:43:02Z")
That’s amusing… A dude I knew got together with a bunch of people from work and did exactally that… They tossed something liek 5lb balls of silly putty off the 8th floor of EA’s QA department building in vancouver. Ha! It bounces pretty well, apparently. -n > [@Mathochist](#): > > Evidently not quite. That stuff is effectively generic Silly Putty. > > Now I know why my friend has been trying to get people to go in on an order. It’s something like $8/lb in 50lb increments, with a minimum of 100lb to an order. I mean, can’t you come up with 20 people to each pay $40 for _five pounds of Silly Putty_? Expecially if you can throw it off a building… ---
**Author:** ![Larry\_Mudd](https://avatars.discourse-cdn.com/v4/letter/l/f14d63/32.png) [@Larry\_Mudd](https://boards.straightdope.com/u/Larry_Mudd)\ **Post date:** [April 24, 2004, 8:08am UTC](https://boards.straightdope.com/t/new-liquid-body-armor-how-the-heck-does-this-work/241694/13 "2004-04-24T08:08:11Z")
> [@Cisco](#): > > Is this the same stuff Stretch Armstrong was made out of? I remember you could moosh him slowly with no problem but if you punched him it would actually hurt your fist. That’s only because you were eight years old. 😃 (What I remember about Stretch is that if you left him in an oven and your mom came along later to preheat it, Stretch ended up evenly distributed over the bottom of the oven like so much crispy Polydent and your dad made you keenly regret making him buy a new appliance.) ---
**Author:** ![Bob55](https://avatars.discourse-cdn.com/v4/letter/b/e9c0ed/32.png) [@Bob55](https://boards.straightdope.com/u/Bob55)\ **Post date:** [April 24, 2004, 10:42pm UTC](https://boards.straightdope.com/t/new-liquid-body-armor-how-the-heck-does-this-work/241694/14 "2004-04-24T22:42:13Z")
They mention that polyethylene glycol (PEG) is the the key ingredient. I’m not sure how this would harden, I use it in a 50-100% solution and it never hardens on me. But the higher molecular weight PEGs, like 20,000 kDa and above, are very, very thick, and it takes almost a full minute to pipet a milliliter of this stuff. Possibly they’re using very, very high molecular weight PEGs, like 100,000 kDa or greater? That would be some pretty thick stuff. ---
**Author:** ![Bob55](https://avatars.discourse-cdn.com/v4/letter/b/e9c0ed/32.png) [@Bob55](https://boards.straightdope.com/u/Bob55)\ **Post date:** [April 24, 2004, 10:44pm UTC](https://boards.straightdope.com/t/new-liquid-body-armor-how-the-heck-does-this-work/241694/15 "2004-04-24T22:44:54Z")
I meant 20,000 Daltons, or 20 kDa (not 20,000 kDa). ---
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [April 24, 2004, 10:47pm UTC](https://boards.straightdope.com/t/new-liquid-body-armor-how-the-heck-does-this-work/241694/16 "2004-04-24T22:47:41Z")
> [@Bob55](#): > > I meant 20,000 Daltons, or 20 kDa (not 20,000 kDa). Note that they also mention it’s got “nano-particles” of silica in it. The PEG seems to be a suspension medium for the silica, which is the hard part. ---
**Author:** ![Balance](https://avatars.discourse-cdn.com/v4/letter/b/ccd318/32.png) [@Balance](https://boards.straightdope.com/u/Balance)\ **Post date:** [April 25, 2004, 12:28am UTC](https://boards.straightdope.com/t/new-liquid-body-armor-how-the-heck-does-this-work/241694/17 "2004-04-25T00:28:57Z")
Right, the silica is analogous to the cornstarch in ooblick. The PEG by itself wouldn’t harden under impact. I suspect it was chosen because its viscosity lends itself well to maintaining a colloidal suspension. ---
**Author:** ![Girl\_Next\_Door](https://avatars.discourse-cdn.com/v4/letter/g/977dab/32.png) [@Girl\_Next\_Door](https://boards.straightdope.com/u/Girl_Next_Door)\ **Post date:** [April 25, 2004, 7:59am UTC](https://boards.straightdope.com/t/new-liquid-body-armor-how-the-heck-does-this-work/241694/18 "2004-04-25T07:59:34Z")
IIRC, Stretch Armstrong was filled with corn syrup. ---
**Author:** ![nocturnal\_tick](https://avatars.discourse-cdn.com/v4/letter/n/3d9bf3/32.png) [@nocturnal\_tick](https://boards.straightdope.com/u/nocturnal_tick)\ **Post date:** [April 25, 2004, 1:34pm UTC](https://boards.straightdope.com/t/new-liquid-body-armor-how-the-heck-does-this-work/241694/19 "2004-04-25T13:34:35Z")
> [@Girl Next Door](#): > > IIRC, Stretch Armstrong was filled with corn syrup. If this is true it may function as a non-Newtonian liquid. The only non-Newtonian liquid I have ever heard of is custard. Yes, custard. I have seen tons of people make up a batch and walk on top of it. In fact on a TV program they filled a swimming pool full of it and as long as the guy kept moving it was all dandy. When he stopped he sunk like in quicksand and it took a few guys to get him out! > [@Harmonix](#): > > My question however is… because the bullet causes the body armour to become rigid – doesn’t that mean theres no deforming of the body armour? Meaning force is transfered to the person instantaneously rather then over the small amount of time that kevlar deforms? If the armor behaves like the custard it would be like shooting into jelly. While the substance will attempt to prevent plastic deformation it will still behave elastically which would remove some of the initial force of impact. ---
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [April 25, 2004, 6:32pm UTC](https://boards.straightdope.com/t/new-liquid-body-armor-how-the-heck-does-this-work/241694/20 "2004-04-25T18:32:56Z")
> [@nocturnal\_tick](#): > > The only non-Newtonian liquid I have ever heard of is custard. You’ve been reading this thread and not heard us talking about Silly Putty or “ooblick”? [Next page](https://boards.straightdope.com/t/new-liquid-body-armor-how-the-heck-does-this-work/241694.md?page=2)