  Seals are extremely useful devices to prevent fluid leakage. However, the
exact mechanism of roughness induced leakage is not well understood. We present
a theory of the leak-rate of seals, which is based on percolation theory and a
recently developed contact mechanics theory. We study both static and dynamics
seals. We present molecular dynamics results which show that when two elastic
solids with randomly rough surfaces are squeezed together, as a function of
increasing magnification or decreasing squeezing pressure, a non-contact
channel will percolate when the (relative) projected contact area, A/A_0, is of
order 0.4, in accor dance with percolation theory. We suggest a simple
experiment which can be used to test the theory.
