Dynamics and geometry of solid-liquid reaction interface

The spreading of small droplets (50-200 μm) of Hg on thin Ag films has been studied. The Hg front formation mechanism has been determined using optical microscope, Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM) analyses. The dynamics and geometry of the growing rough surface observed in our experiment, have been analyzed using the concepts of dynamic scaling and self-affine fractal geometry, in order to determine the corresponding roughness (α) and growth (β) exponents. We have studied two systems, one in which the silver thickness was 500-2000 angstrom and the other where the silver thickness was 0.1 mm. We have found that α = 0.66 ± 0.03 and β = 0.46 ± 0.02 in the first system, and α = 0.77 ± 0.04 and β = 0.6 ± 0.02 in the second one. Each of these pairs satisfies the scaling law α + α/β = 2 within the experimental error. In addition, it was found, in both systems, that for final stages of the experiment, the roughness exponent α crosses over to α ≈ 0.5 for relatively long length scales (order of a few microns). These findings indicate that different mechanisms govern the process in various time and length regimes: diffusion and wetting at early stages, and chemical reaction at final stages and short-length scales.