airelectrostatics
It is known that an electrostatically charged object can repel or attract a flow of water. Can this also apply to an air flow? And if so, would it happen only due to the humidity via water molecules, or even dry air would be affected?
For example, if there is a metal grid inside a tube with flowing air, can its aerodynamic resistance be varied by applying high voltage to it?
There are two ways this can happen.
One, the water becomes slightly polarized but net neutral charge. You end up with a dipole and a weak attraction.
Two, the polarized water "breaks" so some charge is left behind. This charge can flow back through the main water pipe to ground and the container with the water becomes slightly charged. You could prove this to yourself by running the deflected water into a insulated bowl and connecting it to an electrometer. You will find the foils indicate a charge buildup in the bowl. There would be no change in the attraction because the repelled charged don't "stay behind in the tap" but flow away to ground; this was one of the things you were concerned about.
There is a related experiment in which you use a pair of crossed wires connected to two tin cans that catch the water - with the right arrangement of tins and wires you can create a high voltage generator, suggesting that explanation 2 is the right one. Google "Kelvin water dropper" for pictures and explanation - see for example http://en.m.wikipedia.org/wiki/Kelvin_water_dropper
It is a remarkably easy experiment, and it is exponential: initially it starts slowly but the force with which charge is separated increases as voltage builds up so $dQ/dt$ is linear in $V = Q/C$ leading to exponential behavior and the possibility to build up some serious sparks.
Note that this is the same physical phenomenon that leads gas (petrol) stations (in the US at least) to prohibit you from filling non-approved (read: non-conducting) gas cans at the gas station, and from filling any gas cans while they are inside a car/on a truck bed. In both those cases you run the risk of (exponential) static buildup and sparks - except this time sparks plus an explosive vapor...
The answer to your dilema is that there is no need for any counter force.
Statistically it's likely that many molecules would come into the region where the vacuum was - however no force pushes them there.
In the kinetic theory of gases, intermolecular forces and collisions are ignored, but there will be, by chance, many molecules travelling towards the vacuum region. They continue in a straight line with no force acting, as in Newtons 1st Law.
Since there were no molecules in the vacuum region to leave, it just happens by chance that a short time later, more molecules will be in the region than before and the vacuum region has been 'filled'.
Best Answer
There have been attempts to use ionized air to reduce drag on aircraft