Pressure Solution at the Molecular Scale

The topological evolution of the cleavage surface of a gypsum single crystal during its dissolution in a flowing undersaturated aqueous solution has been observed with an atomic force microscope. The matter transfer from solid to liquid proceeds through the migration of atomic steps. The step velocity has been measured and appears to depend on the force applied by the tip on the surface. Whereas the high force velocity enhancement is likely to stem from corrosive wear, the speed behavior at low force ($<10\mathrm{nN}$) differs drastically and can be interpreted as a consequence of the pressure solution of the crystal induced by the tip force. The step velocity evolution with the force obeys the known kinetic law of pressure solution. Hence these experiments enable us to evidence a first atomic mechanism at the origin of pressure solution.

Figure 1

(a) AFM image of the (010) face of a gypsum single crystal experiencing dissolution in a flowing calcium sulfate aqueous solution with $\Omega =42\%$ (relief height: 5 nm). (b) Space-time representation of the migration of a [001] step for $\Omega =88\%$ (relief height: 2 nm). The consequence of a change of the force applied by the tip is also shown.

Figure 2

Evolution with the saturation index of the step velocity in the [001] (black dots) and [100] (grey dots) directions. The results of Bosbach & Rammensee [20] (triangles), Hall & Cullen [21] (squares) and Fan & Teng [13] (diamonds) have been added to ours (circles).

Figure 3

Step velocity in the [001] direction as a function of the force applied by the AFM tip on the surface for various saturation indices. Each dot stands for an average of a few values. The curves are a fit of the experimental values by a law based on the inclusion of strain energy in the transition state theory.

Figure 4

Step velocity in the [001] direction for large forces applied by the AFM tip on the surface and for $\Omega =71\%$. Curves are guides for the eyes. Insert: AFM topographic image at the end of the 32 nN velocity measurement ($15\times 15\mu {\mathrm{m}}^2$, relief height: 5 nm). The black segment indicates the tip route during the velocity measurement.