Generation of Fractal Silicas by Negative Pressure Rupturing of SiO₂ Glass

Abstract

Stimulated by the finding that SiO₂ aggregates (e.g. SiO₂ “smokes” or “snow”) assembled under highly non-equilibrium conditions have fractal geometries and dynamics—and retain them under annealing densification almost until the density of silica glass is reached –we have examined the consequences of an inverse procedure. Using the power of molecular dynamics to perform otherwise difficult experiments we have subjected normal density vitreous SiO₂ at 300 K to isotropic expansions (which generate negative pressures) to and well beyond the tensile limit at –70 kbar. As the tensile limit is exceeded, void-containing structures are produced which indeed prove to be fractal in character, and the pressure trends towards zero. The fractal dimension is determined concordantly from two different density correlation relationships and is found to change smoothly with density from 3.0 at the tensile limit down to 1.7 at the limiting density of 0.1 g/cm³ set by our system size. Our study suggests that the path of least resistance to the rupturing of an initially stable structure may be the path to fractal geometries.

Key Words:

• Silica
• fractal
• isotropic expansion
• rupture