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This may be an all-time stupid question, but do Bernoulli grippers work in a vacuum? It’s pretty obvious suction cups won’t work.

Bernoulli grippers use compressed air to create a suction cup-like effect without actually touching the object they hold. They are widely uses in the electronics industry for manipulating delicate objects (such as silicon wafers) without touching or contaminating them. In theory, they could pick up up a JWST mirror by the optical side without actually touching it. https://en.wikipedia.org/wiki/Bernoulli_grip

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If they did work in a vacuum, they would be useful for grappling and positioning during EVA.

Intuition tells me they shouldn’t work in a vacuum, but intuition also tells me they shouldn’t work at all.

Woody
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    To first order at least no, I think. Some pressure on one side, no pressure on the other. Every atom that hits a perfectly flat workpiece gives some momentum away from the gripper. But if your workpiece was porous with sculpted holes, focusing rebounding atoms normal to the surface upon exit (closer to straight out), you could produce some reduction in repulsion, but not sure it would be enough to produce attraction though. i – uhoh Jan 21 '22 at 09:22
  • There’s also a somewhat related phenomenon discussed In https://space.stackexchange.com/a/50550/12102 Maybe you can add a laser to your device and make a very very weak photophoretic gripper. – uhoh Jan 21 '22 at 09:25

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Well, no. A Bernoulli gripper is at the end of the day just a suction cup that uses an different mechanism to achieve the sucking-out. Namely, it uses inertia of the air streaming out sideways: the air stream is fast in the cup and slows down as it escapes (because it's a diverging flow; the air has more and more space as it flows outwards). That slowing must correspond to a braking force, i.e. the pressure on the inside must be lower than the pressure on the outside – the suction AKA Bernoulli effect.

But in vacuum, it clearly doesn't work this way – you can't have less than zero pressure. Instead, what happens then is that the air rapidly thins out and cools down in the process. The cooling down reduces the speed of sound, so that the air stream quickly becomes supersonic† and thus choked. That's where the divergent geometry has the opposite effect, i.e. the pressure drops as the flow widens out. It's the same effect that also happens in the diverging part of a de Laval rocket nozzle (just at much lower temperature⁄pressure⁄density), and like with a rocket nozzle it results in the object being pushed away from the Bernoulli gripper.

†I'm not sure if it actually goes supersonic or first drops below the boiling point, i.e. condenses to liquid nitrogen.

leftaroundabout
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