Free Falling powder in a vacuum

[ravindrar]

I made experiment on "Free Fall". I made a conical hopper of 15 liter volume and a vacuum chamber of , installed hopper upon vacuum chamber of 65 liter volume. Fixed a 2.5" gate valve to discharge powder in vacuum chamber. Connected both, hopper and vacuum chamber to vacuum pump (63 cum/ hr) and evacuated air to till pressure was <-759 mm HG, from both hopper and chamber. By opening the valve of the hopper dropped the powder in vacuum chamber.

As per Newtonian physics, free fall is any motion of a body where gravity is the only acceleration acting upon it. In the context of general relativity, where gravitation is reduced to a space-time curvature, a body in free fall has no force acting on it.

But the powder did not fall straight instead it scattered all over the vacuum chamber.

 

[ianchristie]

This is not my area of expertise, but two possibilities spring to mind immediately.

1. No matter how good your vacuum pump is and even if you leave it to run for a long enough time to compensate for out-gassing from the chamber walls, there will always be a residual level of gas in the chamber. Perhaps this is sufficient to scatter the powder as it falls.

2. Is there any possibility of a static charge on the powder and/or the chamber?

Your describe a pressure of about 1 Torr. At this pressure the molecular density if of the order of 10^18 molecules per cc. By the standards of vacuum physics this is still only a medium vacuum.

 

[sophiecentaur]

If the particle didn't fall vertically then I'd suspect some electrostatic effect. Powders are devils for picking up charge as they jostle together. Equal and opposite charges but different masses could, I suspect, cause an apparent spreading of the cloud of particles.

 

[Nik_2213]

I'd go with 'static'. IIRC, 'fly' lunar dust proved a serious issue for the Apollo crews, spoiling sample box seals, abrading space-suits, getting into moving joints and totally contaminating LEM interior, causing breathing difficulties.

 

[DBO]

I agree that you need a better vacuum. Charging of the dust particles is definitely an issue and you might consider using this to sort particles by size. You could even put in a UV lamp to increase the particle charging.

 

[MikeP63]

Even with a perfect vacuum, I would suspect that the conical hopper is mostly to blame. The clump of powder will behave like a fluid as it falls. The particles near the edge of the hopper will attain an horizontal velocity component which will induce turbulence into the general mass. Even with a cylindrical hopper, the shear turbulence induced by the walls of the hopper would probably cause some scattering.

 

[ravindrar]

This is not my area of expertise, but two possibilities spring to mind immediately.

1. No matter how good your vacuum pump is and even if you leave it to run for a long enough time to compensate for out-gassing from the chamber walls, there will always be a residual level of gas in the chamber. Perhaps this is sufficient to scatter the powder as it falls.

2. Is there any possibility of a static charge on the powder and/or the chamber?

Your describe a pressure of about 1 Torr. At this pressure the molecular density if of the order of 10^18 molecules per cc. By the standards of vacuum physics this is still only a medium vacuum.

Thank you. I agree with you. Particularly when molecular density is 10^18. But have you seen Brian Cox's video. A large feather is falling straight though the surface area of area is very large.

If the particle didn't fall vertically then I'd suspect some electrostatic effect. Powders are devils for picking up charge as they jostle together. Equal and opposite charges but different masses could, I suspect, cause an apparent spreading of the cloud of particles.

I agree that you need a better vacuum. Charging of the dust particles is definitely an issue and you might consider using this to sort particles by size. You could even put in a UV lamp to increase the particle charging.

Thank you, I can not sort particles. I will try UV as suggested. However I am also looking for any other solution to fill the powder without scattering.

 

[ravindrar]

Even with a perfect vacuum, I would suspect that the conical hopper is mostly to blame. The clump of powder will behave like a fluid as it falls. The particles near the edge of the hopper will attain an horizontal velocity component which will induce turbulence into the general mass. Even with a cylindrical hopper, the shear turbulence induced by the walls of the hopper would probably cause some scattering.

Thanks for suggestion. Will think over it