Chemical and mechanical effects in SFA force measurements between rough and reactive calcite surfaces
Seminar by PhD Student Joanna Dziadkowiec
Nm-range forces acting between mineral surfaces in aqueous solutions are significant for cohesion between individual mineral grains and contribute to the overall mechanical strength of rocks and mineral-based materials. We explore chemical and mechanical effects that influence the forces between rough and reactive calcite surfaces. Smooth calcite surfaces in water repel each other due to surface hydration effects. However, such repulsion can be screened in high-concentration electrolyte solutions, and in this case, adhesive forces may dominate at nm-range separations. Surface roughness can also affect interactions between calcite grains because of mechanical repulsive effects. Such repulsion is related to elastic and plastic deformation of surface asperities, and for very rough surfaces, it may dominate the overall interaction. Additionally, calcite surfaces are generally more reactive when exposed to fluids, in comparison with silicate and aluminosilicate minerals. This is evidenced in quite rapid nanoscale surface recrystallization of calcite in water, which occurs to a greater extent when ionic strength of the solution is higher. Thus, although in high-ionic strength salt solutions, the adhesive forces should dominate at short separations (due to ELD and hydration repulsion screening), this effect can be counteracted by the more pronounced calcite roughening that gives rise to the repulsive mechanical forces. We investigated these interactions using the Surface Forces Apparatus (SFA) by measuring normal forces and following the surface reactivity in situ through the coupled Multiple Beam Interferometry.