ETCHING OF MASS, SURFACE, AND POROUS FRACTAL SOLIDS

To study fractal aspects of reactive etching, we consider three types of solids with surface, mass, and pore fractality. An analytical solut ion for etching of the solid boundary in the form of a Koch curve demo nstrates that in the absence of diffusion resistance bifractal structu res are formed due to screening effects. Over a certain domain the sur face area scales like t(1-Df). Stochastic simulation of etching of sol id clusters, obtained by diffusion-limited aggregation (DLA), shows th at the dynamics strongly depend on the value of fractal dimension. Ana lytical approximation for the dynamics of etching based on the DLAs de nsity distribution was obtained, showing that the mass scales like (1- Kt)(Df), where K is a certain constant, and was in excellent agreement with the simulation. The simulation of etching of a Sierpinski-gasket fractal and the corresponding uniform-pore object of the same size, p orosity, and reactive area demonstrates the combined influence of diff usion resistance optimization and fractal geometry on the character of etching. The etching of fractal structures is significantly faster in the regions of strong and moderate diffusion resistance. In the regio n of strong diffusion resistance the rates of etching declined monoton ically with time. The reaction occurred in a narrow penetration layer, and fractality did not break. No simple scaling is evident in this ca se.