although i didn't think of actually propelling it forward, the principle seems fairly similar. if a wall could be made that only allows particles through one side, it would make allot of switches and machine easy to use, as there would be no backwash at all.
How about a coloured element (not wall) that moves liquids it touches upwards (it would be a bit like dan balls fan) and of course it doesn't move air thus it won't make things glitch up.