  We study the leakage of air in syringes with Teflon coated rubber stopper and
glass barrel. The leakrate depends on the interfacial surface roughness, the
viscoelastic properties of the rubber and on the elastoplastic properties of
the Teflon coating. The measured leakage rates are compared to the predictions
of a simple theory for gas flow, which takes into account both the diffusive
and ballistic air flow, and the elastoplastic multiscale contact mechanics
which determines the probability distribution of interfacial separations. The
theory shows that the interfacial air flow (leakage) channels are so narrow
that the gas flow is mainly ballistic (the so called Knudsen limit). The
implications for container closure integrity is discussed.

Soft Condensed Matter