We have investigated the dissolution of a single cylinder of soluble mineral by microfluidic experiments and numerical simulations. The experiments used gypsum chips (10 mm radius, 0.5 mm thick) cast from rehydrated CaSO4 hemihydrate. The numerical simulations used a finite-volume discretization of the reactive-transport equations with a mesh that conforms to the evolving shape of the solid gypsum. The conforming mesh gives a precise computation of the reactive fluxes with a minimal computational effort – fully converged simulations, up to the point where the mineral vanishes entirely, can be completed in less than 6 hours on a quad-core processor. Despite the simplicity of the system (gypsum and water), our studies indicate that a number of issues stand in the way of a quantitative comparison between the evolution of the size and shape of the sample predicted by simulation and what is observed experimentally. Our initial comparisons indicated a factor of two difference in the dissolution time scale, which could not be explained by uncertainties in the surface reaction rate. Further investigation revealed that, after about 10 hours, additives in the plaster reduced the surface dissolution rate five fold in comparison with samples prepared from the pure (95.6%) hemihydrate. Because of diffusional hindrance, a factor of 5 change in the surface reaction rate translates to about a factor of two change in the overall dissolution rate. Agreement between experiment and simulation was further improved by including activity corrections to the diffusion coefficient, which reduces the dissolution rate by about 20%. There are very few exact, but non-trivial, solutions of transport equations that can be used to rigorously test the predictions of a numerical simulation. Conformal mapping offers the possibility of near-exact solutions of closely related problems, which can be used to understand the limitations of numerical codes, particular with regard to errors introduced by the meshing. In the talk I will indicate how we are using conformal mapping to both provide physical insight into the dissolution process and precise test data for code validation.

Evolution of grain shapes by dissolution: Comparison of pore-scale simulations, microfluidic experiments and conformal mapping

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.