  The slow advance of a crack in sodo-silicate glasses was studied at nanometer
scale by in situ and real time atomic force microscopy (AFM) in a well
controlled atmosphere. An enhanced diffusion of sodium ions in the stress
gradient field at the submicrometric vicinity of the crack tip was revealed
through several effects: growth of nodules in AFM height images, changes in the
AFM tip/sample energy dissipation. The nodules patterns revealed a dewetting
phenomenon evidenced by 'breath figures'. Complementary chemical micro-analyses
were done. These experimental results were explained by a two-step process: i)
a fast migration (typical time: few milliseconds) of sodium ions towards the
fracture surfaces as proposed by Langford et al. [J. Mat. Res. 6 (1991) 1358],
ii) a slow backwards diffusion of the cations as evidenced in these AFM
experiments (typical time: few minutes). Measurements of the diffusion
coefficient of that relaxing process were done at room temperature. Our results
strengthen the theoretical concept of a near-surface structural relaxation due
to the stress-gradient at the vicinity of the crack tip. The inhomogeneous
migration of sodium ions might be a direct experimental evidence of the
presence of sodium rich channels in the silicate structure [A. Meyer et al.,
Phys. Rev. Let. 93 (2004) 027801].
