We currently have a multitude of experimental methods to investigate isolated mineral surfaces immersed in water, even down to a molecular scale! Yet, it is still not so straightforward to measure what happens when two such surfaces are placed in very close proximity – nanometers apart from each other. This is especially the case when the two solid surfaces become reactive in contact with an undersaturated solution. As the gap between the surfaces is very narrow, the spatial confinement can modify how equilibration with the thin solution film progresses: dissolution and recrystallization may become affected by the transport of ionic species in the gap. This may in turn influence adhesive and repulsive forces between the surfaces. And it is definitely useful to find out how.
Nucleation and crystal growth in confinement happen everywhere: in sedimentary or metamorphic rocks, in buildings and monuments, but also in living organisms. If uncontrolled, it can cause substantial damage and can even act as a precursor to earthquakes. But studying how reactive surfaces interact with each other at small scales, requires special experimental methods. Recently, during a stay at the University of Oslo, I’ve learned how to measure these interactions with the Atomic Force Microscopy (AFM).
In a typical AFM experiment, we usually use an extremely sharp silicon tip, with a typical spike radius of several nanometers. This lets us resolve details of the interfacial structure of minerals immersed in water. But in order to understand the effects of confinement, we need a mesoscale experimental setup: our surfaces should be at least microns-large, but we should be able to bring them into contact with nanometer-distance resolution. With such good distance resolution, we should be able to resolve even weak forces acting between the surfaces. Since there are no commercial calcite-modified AFM cantilevers, we have to prepare one.
The recipe is not too difficult: We use tipless AFM cantilevers that can be customized with various particles and surfaces. We place such cantilever above the freshly cleaved calcite surface inside the AFM and look for micron-sized particles that lie on the surface. Once a suitable particle is located, we pick a small amount of epoxy glue. We then quickly go back and press the glue-wet cantilever against the chosen particle. After several hours, the calcite AFM probe is ready. You can read about the application of this method in several recent works: (1, 2, 3).
Although the shape of this custom calcite probe varies between experiments and the probe’s resolution is limited in comparison with sharp AFM tips, the method is highly relevant for geological environments and mineral-based materials. We can now probe reactions in confinement, which can have a decisive influence on the mechanical properties of solid contacts.
References: Dziadkowiec, J. et al. (2019). Scientific Reports, 9(1), 8948. https://doi.org/10.1038/s41598-019-45163-6 Javadi, S., & Røyne, A. (2018). Journal of colloid and interface science, 532, 605-613. https://doi.org/10.1016/j.jcis.2018.08.027 Pourchet, S. et al. (2013). Cement and Concrete Research, 52, 22-30. https://doi.org/10.1016/j.cemconres.2013.04.002 Røyne, A. et al. (2011). Journal of Geophysical Research: Solid Earth, 116(B4). https://doi.org/10.1002/2015GL064365 Söngen, H. et al. (2018). Physical review letters, 120(11), 116101. https://doi.org/10.1103/PhysRevLett.120.116101