Weaponising Implosion instead of Explosion

Question

Explosions are really messy - they cause debris to be spread over a wide area. They may also cause collateral damage. I want to use implosion instead.

I wish to use a force-field sphere that contains a vacuum. The sphere is used as an invisible booby-trap or mine.

I imagine that such a sphere 'bursting' in mid air would sound very similar to a balloon being burst except that the air would rush inwards.

Question

How big should my vacuum-filled sphere be in order to kill a single human?

Assumption - The person walks into the force-field thus setting it off.

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As requested, here is my attempt at illustrating the idea.

Imagine that the rubber of the balloon is an invisible force-field. The air inside the balloon is the vacuum. The whole thing is invisible to the man (unless there is smoke billowing around outside that shows its shape). As soon as the man touches the 'balloon' by placing his hands on it or walking into it, it will activate itself. It does not wait until the man is inside the force field - it is triggered by touch. Therefore the man will presumably be pushed violently into the empty sphere by the air pressure from behind him. This might break his neck, damage his lungs, etc. There could even be a metal pole in the middle that he can see, but he won't be expecting to be thrown violently against it.

Answer 1

You might be able to kill a human, but probably not in the way you expect.

Intense and rapid changes of pressure can cause Cavitation in liquids.

Cavitation is a phenomenon in which rapid changes of pressure in a liquid lead to the formation of small vapor-filled cavities, in places where the pressure is relatively low.

When subjected to higher pressure, these cavities, called "bubbles" or "voids", collapse and can generate an intense shock wave.

Walking into a bubble of vacuum would form air bubbles in our blood, similar to decompression sickness. Depending on how many and how deep in the body you're able to create those bubbles, the victim would suffer from thromboses, stroke, paralysis, suffocation or almost instant heart failure.

Since the vacuum force field is deactivated upon contact and the pressure is equalized almost instantly, the bursting cavitation bubbles might be able to damage some fragile blood vessels as well, causing internal bleeding.

Realistically, though, the vacuum would only be able to cause damage in the skin, ears and maybe the eyes of the victim. So gruesomely popped eyeballs and eardrums and death by thrombosis is your best bet.

As to the required size of the vacuum bubble, I would go with twice the size of your victim. That way the strongest effect of the vacuum hits the most vulnerable part of the body (the head) and it has enough volume to hopefully have any effect at all. If it manages to form bubbles in the blood, the distance to the brain (where blockages of blood vessels have the most dire effects) is the shortest.

Answer 2

It won't kill the human.

Air will rush in fast to the vacuum, and that will cause a wind, but probably not a shockwave strong enough to harm a human.

Explosions kill by throwing shrapnel around which is not the case here, or by overpressure. The shockwave is damaging because you're compressing a lot of fluid into a small space - which is also why underwater explosions are much more powerful than aerial ones. But with a vacuum induced implosion, the fluid (air in this case) will exert little pressure, less than one atmosphere. It'll be more an annoyance than anything else.

The victim may be thrown in some direction, and get hurt somewhat by the fall, but unless you are throwing them off a ledge they will just stand up and pat some dust off their clothes.

If you were to generate the vacuum inside that person's cavities (such as stomach & lungs), that would be something else. The person would be instantly crushed to death.