PMD Physics
Surface Tension

Surface tension creates a pressure differential across a curved interface that is defined as

The sum of the reciprocal principal radii of curvature can be positive or negative. A positive value indicates a lower pressure in the liquid.

All PMD components except troughs make use of this relationship between pressure and mean Gaussian radius. Vanes and galleries use this pressure to drive liquid uphill against the hydrostatic and flow losses. Sponges and traps use this pressure to retain propellant in an adverse acceleration field. Porous elements use this pressure to keep gas from flowing through them while allowing liquid passage.

Solving this equation coupled with the boundary conditions (generally fully wetting), hydrostatics and fluid dynamics is what PMD physics is all about.



With the exception of troughs, all Propellant Management Device (PMD) components use surface tension to control and/or deliver liquid. Surface tension is a force created by the asymmetry of the molecules at the surface of liquids. This force is quite small in comparison to the force of gravity here on earth but in space it is often the dominant force present and will dictate liquid motion and fluid equilibrium in tanks.