Mercury spreading on silver films: Interface characteristics

We have studied the spreading of a liquid Hg drop on a Ag film. Geometrical characteristics of the propagating Hg spot front, its roughness and hull exponents have been investigated. The mechanism of Hg spot front formation has been analysed by optical microscopy, transmission electron microscopy, scanning electron microscopy and X-ray diffraction. We found that at short scales (up to about 20 μm) the roughness exponent of the Hg steady state interface is α ≈ 1 which is characteristic for self-similar objects. We also studied the hull exponent ν at these scales and found that ν = 1.3–1.7. We suggest that on these scales a self-similar interface is generated by the process of invasion percolation. The invasion of Hg through the grain boundaries, which is observed at a high magnification, might be the origin of the roughening of the front interface. On larger scales the roughness saturates and the corresponding exponents are α = 0 and ν = 1.