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If a sword were to be swung horizontally, through the torso of a human being, how fast would it need to be to not have blood on it after the swing?

Assume the sword and its user are together able to cut through all of the proper bodily organs and bones in a single strike. Assume a well-forged steel blade.

Friendlysociopath
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7 Answers7

85

Speed Not Required

Give your sword a hydrophobic coating and it will never get any blood on it. The coating repels liquids, causing them to bead up like water on a lotus leaf and just run off. Bonus it stays nice and clean.

Edit:

A blade made of steel does not mean that It cannot have a coating that makes it repel blood. Furthermore, the surface geometry of the blade may be altered to give the steel hydrophobic proeprties. This laser can do it to a variety of metals.

Joe Kissling
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    coating are generally very sensitive to scratches... cutting through a bone is going to give some scratch, or not? – L.Dutch Apr 09 '17 at 07:15
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    @L.Dutch I don't know how far it was pursued, but I heard several years ago about work being done on an ultra-hydrophobic surface which was also self-healing, based on a design inspired by the cellular structure of pitcher plants. So a durable hydrophobic coating should be possible, or at least plausible. – Dave Sherohman Apr 09 '17 at 08:15
  • @L.Dutch there are options for surface coatings that are very durable, certain types of diamond coatings for example are suitable for this task. – Joe Kissling Apr 09 '17 at 13:57
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    There's also a technique that laser etches a surface to make it hydrophobic. – T.J.L. Apr 09 '17 at 16:01
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    It's a good answer, but it won't work with blood. Sure, blood is ~80% water so one would expect it to be hydrophilic, but simplifying whole blood to water is a mistake. The remaining part is mainly proteins and lipids, both which will adhere to hydrophobic surfaces so, unfortunately, the blood will still adhere to the blade even though most of it will pour off easier by the coating. – Mrkvička Apr 09 '17 at 18:40
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    @Mrkvička Hydrophobic coatings do work on blood, some surgical tools are specifically coated in such materials. – Joe Kissling Apr 09 '17 at 18:50
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    It's the best option, but blood still adheres. I have done research at one of the major companies for dialysis and it's a big issue with how to clean dialyzers and tubings for clinics which wishes to save money by re-using equipment. Yes, the best way is to make it hydrophobic, but it's impossible to make a surface where no parts of the blood adheres. – Mrkvička Apr 09 '17 at 19:08
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    Blood is tricky, I did some work for a company that was looking into using coatings for cannulas. I'm not saying you are wrong, but this sure looks like it does a fantastic job. – Joe Kissling Apr 09 '17 at 19:15
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    SLIPS sure is amazing, but it's not a 100% guarantee. I'm highly doubtful that it would last for any longer period on a blade used for battle, but I might be wrong (after all, it only needs to withstand one full battle, then one can re-coat it of needed). I know for sure, though, that it will not be used for multiple use equipment in medical context; no one will be willing to pay for re-coating. – Mrkvička Apr 09 '17 at 19:57
  • In any case, there are durable hydrophobic coatings and there are coatings that do a good job repelling blood. It's not unreasonable for there to be coating/surface treatment that does both, if there is not one already. – Joe Kissling Apr 09 '17 at 20:41
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    @T.J.L. I'm skeptical about how well a protection dependent on a very precise surface layout would remain functional. Swords generally only had relatively blunt edges because a razor sharp one would curl over the first time it impacted bone/armor/another blade/etc and become even less capable of cutting. I'd think a surface etching would be even more vulnerable to disruption as the blade got banged around. – Dan Is Fiddling By Firelight Apr 10 '17 at 17:11
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    @Dan Neely "Perfectly", "forever", and "without maintenance" were not stipulated requirements. – T.J.L. Apr 10 '17 at 17:47
  • @T.J.L.if it fails on the first serious blow; which I'm concerned that it might then what's the point. Totally blood repellent until the first time you make a major cut at which point the repellent stops working isn't of much use. – Dan Is Fiddling By Firelight Apr 10 '17 at 19:14
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    @DanNeely The structures produced by laser etching, especially those made of steel, are not as fragile as you may think. Nor are they actually all that precise, it's basically just roughing the surface up. – Joe Kissling Apr 10 '17 at 19:23
  • Hydrophobic coating does not keep blood out. Try it out yourself and see. – The Square-Cube Law Sep 11 '18 at 17:47
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Not only is speed not required as per Joe Kissling's answer but speed is not able to keep it clean, either. The sword will be pushing through blood as it cuts, it's impossible for blood not to get on the sword.

Thus keeping it clean means either a material that blood will not adhere to, or a speed in which the air movement will scour the blade free of blood.

Consider airplane wings--even going hundreds of miles per hour is not enough to sweep ice from the wings despite a smooth surface. Ice doesn't bond all that tightly. The airline industry combats it by spraying on a coating (note that the coating also doesn't get blown off) that lasts long enough for the plane to get above the threat zone.

While this doesn't give us an exact answer it shows that we need a speed far beyond what muscles can deliver. My gut says the required speed is supersonic (which means a nasty sonic boom from the sword) but I do not know.

Loren Pechtel
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    Related question: how fast must I move my sandwich thru this stream of filth to avoid getting any filth on it. There is no speed at which I will still be interested in eating that sandwich. – Willk Apr 09 '17 at 05:07
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    You cannot clean surfaces with speed because directly at the surface boundary every molecule clings to the surface molecules due to internal adhesive forces. Combined with the fact that nearly every fluid has internal viscosity it leads to the typical speed profile of every fluid standing still at the surface and gradually increasing speed with increasing distance. Every smooth surface will have a very, very thin water layer on it which is extremely tenacious. For ultra-high vacuum we must get rid of this layer, meaning heating the surface up to 300 (!) °C. – Thorsten S. Apr 09 '17 at 10:32
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    "speed is not able to keep it clean" - it is, if high enough. At very high speeds, the blood will boil away, at even higher speeds, the steel of the sword itself will boil away. And close enough to light speed, the very fabric of reality around your sword will boil away :) – vsz Apr 09 '17 at 10:53
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    As @vsz says just check https://what-if.xkcd.com/1/ – DRF Apr 09 '17 at 11:54
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    @vsz Actually, at any speed the fabric of reality is still pretty much intact. It's accelerating / decelerating that you do that might stress it somewhat, especially if you're massive. See gravitational waves. – wizzwizz4 Apr 09 '17 at 18:24
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    @vsz Yeah, I was really asking for this type of correction.... – Thorsten S. Apr 09 '17 at 22:52
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    @ThorstenS.While there is such a speed profile if the sword is going fast enough the outer layer gets stripped off, now the profile is steeper, the next bit gets stripped and so on. Since matter isn't infinitely divisible you will be able to clean it with enough speed. (Whether the sword can survive the necessary speed is another matter...) – Loren Pechtel Apr 10 '17 at 00:59
  • This makes me wonder if by that logic there will always be blood on a bullet that passed through a body. – LukStorms Apr 11 '17 at 12:55
  • @LukStorms Yes, although it might not be enough for DNA, especially since the bullet was awfully hot when it hit. – Loren Pechtel Apr 12 '17 at 00:54
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It's the stopping speed you're after. Say the swing speed is a bit better than a golf club's speed swung by a pro-golfer. We're looking at 100 mph or 44 m/s.

If your hero/villain brings the sword to a standstill after the cut in half a second, the g-force will be ~9 Gees. I'd say it's enough to propel the blood off the sword (testing required).

isaacg
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anon
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    Unlikely. Blood is a liquid with adhesion. Consider that golf club. You hit the ball it dewy grass. The ball flies away. The bigger drops of water fly away. But the club head is still wet. I sincerely doubt that any breaks applied by human muscle will get the sword clean. – SRM Apr 09 '17 at 14:22
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    That is not too far off, actually. I learnt some japanese samurai sword fighting back in martial arts days (Iaido). All of the katas end with "shaking off" motion, that is basically a fast swing to the side/down rapidly stopped. Since we didn't train with any living humans, I can't say if it is really effective at throwing off the blood, but Iaido is not exactly known for having any useless moves. – Tom Apr 09 '17 at 16:33
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    @Tom it would remove the bulk of the blood, but it wouldn't clean the blade. I have been working with blood and I know from experience that the only two ways to get it off a surface is 1) wiping/scraping (enough with a damp cloth if it's still fresh blood) or 2) rinsing it with (a lot) of detergent and water. The Iaido moves are either to remove the bulk or simply a ritual move, a swordsman would still have to clean the blade after battle. – Mrkvička Apr 09 '17 at 19:00
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    @Tom To further iterate on Mrkvička's comment, it is useful for getting the bulk of the blood off, but there would still be some blood left. It was primarily used to keep a cutting edge relatively clean to finish the battle easier, but it never replaced a wipe-down with a fine cloth. (If you want to be showy about that, then put the cloth in your elbow or shoulder, and run the sword through it. Still badass, but practical, too!) – JessLovely Apr 09 '17 at 19:23
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    Both answers are probably right. Just to complete the picture: In Iaido this move is done immediately before sheathing the blade. That means it would be sufficiently clean to be put away. The Samurai would certainly clean it properly once safely home. Example, here's Mae, the first Iaido kata: https://www.youtube.com/watch?v=sDfcu9BP5fI – Tom Apr 10 '17 at 04:59
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To elaborate on anon's answer, there is a motion called Chiburi, excercised by the samurai. In movies it is a quick flick of the sword that removes blood, but in reality the sword still requires extensive wiping with a piece of cloth.

T.E.D.
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Daniel Vestøl
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    There are a number of comments on anon's answer that cover this. Mrkvička said, it would remove the bulk of the blood, but it wouldn't clean the blade. I have been working with blood and I know from experience that the only two ways to get it off a surface is 1) wiping/scraping (enough with a damp cloth if it's still fresh blood) or 2) rinsing it with (a lot) of detergent and water. The Iaido moves are either to remove the bulk or simply a ritual move, a swordsman would still have to clean the blade after battle. – Draco18s no longer trusts SE Apr 11 '17 at 12:33
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Okay I tried to relate kinetic energy of the sword with the temperature needed to boil blood (which would mean that the blood boils off the sword as it's cutting through the victim, and hence no blood on the sword). I don't know if I did this correctly, as I couldn't find a nice way to relate kinetic energy and temperature, but I found that the sword would have to be going 409 m/s (or 914 mph).

Edit: Here are my equations used...

$<KE> = <\frac{1}{2}mv^2> = \frac{3}{2}kT$

Which is average kinetic energy equals the average of one half the mass of an iron atom times its velocity squared. Then that equals three halves of the Boltzmann constant times temperature. I used:

  • $m$ = 9.27E-26 kg (atomic mass of iron (since swords are mostly iron))
  • $k$ = 1.38E-23 J/K (Boltzmann constant)
  • $T$ = 374 K (temperature at which water boils plus a little bit to account for the salt in blood which raises its boiling point to about 374 K)

Plug everything in and solve for $v$. That yields $v$ = 409 m/s. To see what that compares with in terms of the sword as a whole's kinetic energy, I used the kinetic energy equation again, only this time using this velocity times the sword's mass ($m$ = 1.4 kg, mass of a katana). I got 117,000 Joules of energy. Which feels kind of low to me - that's like a car hitting you on the road - but maybe I'm not relating these equations correctly.

magnetar
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    Can you add your equations to your answer? – Joe Kissling Apr 11 '17 at 05:12
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    sounds like you would turn the sword into a light saber :p – Sarfaraaz Apr 11 '17 at 08:05
  • @JoeKissling Yes I'll find the paper I wrote it on and add equations later tonight. – magnetar Apr 11 '17 at 22:13
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    This is a good attempt at using math, but you aren't comparing these things correctly. The KE that you can relate to temperature is the root mean square KE of the particle itself in the stationary frame of its surroundings. That is, for a pool of blood at rest, your individual water molecules in the blood need a RMS velocity of 409 m/s in order to evaporate. However, for a swinging sword, the air in contact with the blood would also be accelerated to near his speed due to the mechanics of the laminar flow of air around the blade. Thus, the velocity of the blood does't help you. – kingledion Apr 19 '17 at 23:47
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The speed of a sword can, in fact, affect how much blood is left on it. But for a sword to have no blood on it whatsoever, the sword would have to be faster than the speed of the blood dripping onto it while it's in the human body. the only speed at which this is possible is the speed of sound. 334 meters per second. If the sword breaks the sound barrier 1 millimeter before hitting the target it would rip the body and the sword would only continue behind the massive force of a broken sound barrier. The sword would simply act as a device to break the sound barrier and the use the explosion to cut the target in half. If any human could achieve this they may as well use something else made of steel like a crowbar or a steel pipe. So if you want to split someone in half and not get the tool you use dirty. Acquire superhuman strength and break the sound barrier next to your victim. (i think I made a new superhero ).

Caleb reyes
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OK, a theory that might actually be close:

Assumption: All the blood is equivalent to a mass of water.

Further assumption: If struck above speed of sound in water (which is ... high!), anything striking it will not be good at selectively displacing part of it to make way, and try to accelerate the whole mass, which will appear close to one solid object, which gets effectively shattered into two (heavy and hard to accelerate) or more (flying off at an angle) pieces in case you manage to cut through it.

Problem: It is unlikely that a metal edge will stay intact after that kind of impact.

Since the displacement needed for a blade to pass is very small compared to eg an unlucky person hitting a body of water flat, a lower speed impact is likely to displace some of the liquid as a liquid and accelerate it together with the cutting edge, staying on it.

rackandboneman
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