When we drink our morning coffee, we are using a trough. Unlike the other PMD devices, a trough does not rely on surface tension for liquid retention (though surface tension may refill a refillable trough). Troughs use the hydrostatics present in an acceleration field to control propellant. The trough shown to the right is a "tire-shaped" device which can retain propellant in any lateral acceleration field. Note that the volume of propellant retained is small compared to the total trough volume - making troughs relatively heavy and expensive. This type of trough is refillable as a large section of the housing is open (the inboard side).
Refillable troughs provide some of the same capabilities as sponges but at higher accelerations. However, they are not as mass or cost efficient as sponges and thus are suitable only when the acceleration is sufficient high to preclude the use of a sponge.
A different kind of trough is created with a tortuous tubular entrance as shown below. A tubular trough is not refillable but can be constructed to retain all the trough volume in any attitude. This makes this type of trough useful in very high g environments with accelerations in a multitude of directions such as horizontal handling (required for Proton and Sea Launch), spinning launch (Like Delta 2), and reentry (the Space Shuttle).