What would happen if we use vacuum cleaner in space? What would happen if we were to use a vacuum cleaner in space?
Will the vacuum cleaner work?
Would the vacuum cleaner be able to suck anything?
 A: Vacuum cleaners work by creating a pressure difference - if there is no pressure, they do not work. Therefore, the vacuum cleaner will not work in space.
A: At ordinary earth-surface pressures, air acts like a fluid. There are circumstances where we can say that the interplanetary (or interstellar, or intergalactic) medium acts like a fluid, but not on the size scales of a household vacuum cleaner.
Air, at standard pressure, is a pretty good approximation of a “non-interacting ideal gas.” The “non-interacting” part is a little misleading: it means that, when two gas molecules interact with each other, they do so in a completely elastic billiard-ball-like collision, exchanging momentum and energy but not undergoing any chemical transformation or hiding energy in some internal degree of freedom.  The process of deriving the ideal-gas equation (in the discipline called “statistical mechanics”) assumes that the number of these collisions is very large, so that you can consider only their average properties rather than worrying about fluctuations.
That means there is a length scale associated with an ideal gas: the average distance between collisions, or the “mean free path.” If you are working in a system which is very large compared to the mean free path, the continuum approximation is good and you can use fluid dynamics. But if your system is small compared to the mean free path, an air molecule is more likely to interact with the boundary of your system than it is to bounce off of some other air molecule.  In a system where air molecules don’t bounce off of each other, you can’t rely on air pressure to steer residual molecules to your vacuum pump or cryogenic charcoal or mercury fountain or whatever system you have built to maintain your vacuum. You just have to wait for the residual air molecules to make their way to your pump randomly, thanks to interactions with the walls.
The interplanetary medium has only a few particles per cubic centimeter near Earth, so my vacuum cleaner with its centimeter-scale apertures would be working in the particle region, not in the fluid region. I might with patience be able to move some gas particles from the pump inlet to the pump outlet, but they would not generate any wind that could carry macroscopic debris, so the machine would not clean my floors.
However, space is big, and there are circumstances where it does make sense to talk about collective behavior in the interstellar medium: in nebulae, for instance.  If the mean free path in a medium were, say, an astronomical unit (so that a typical particle might travel the sun-to-earth distance without undergoing any scattering off another particle), it might still make sense to talk about fluid behaviors on length scales of light-years.
