Ion Transport and Precipitation Kinetics as Key Aspects of Stress Generation on Pore Walls Induced by Salt Crystallization

The stress generation on pore walls due to the growth of a sodium chloride crystal in a confined aqueous solution is studied from evaporation experiments in microfluidic channels in conjunction with numerical computations of crystal growth. The study indicates that the stress buildup on the pore walls is a highly transient process taking place over a very short period of time (in less than 1 s in our experiments). The analysis makes clear that what matters for the stress generation is not the maximum supersaturation at the onset of the crystal growth but the supersaturation at the interface between the solution and the crystal when the latter is about to be confined between the pore walls. The stress generation is summarized in a simple stress diagram involving the pore aspect ratio and the Damkhöler number characterizing the competition between the precipitation reaction kinetics and the ion transport towards the growing crystal. This opens up the route for a better understanding of the damage of porous materials induced by salt crystallization, an important issue in Earth sciences, reservoir engineering, and civil engineering.

Figure 1

Schematic of the PDMS and glass microfluidic chip. Crystallization and wall deformations are observed in the pore channels.

Figure 2

Lateral crystal growth. (a) Kinetics of crystal growth; $r$ is the crystal lateral half size, $W$ is the initial channel half width; red scale bar represents $10\mu \mathrm{m}$. (b) Absolute deformation against the initial dissolved salt mass.

Figure 3

Numerical simulations of crystal growth. (a) Sketch of simulated problem. (b) Supersaturation at the point of crystal surface located the closest to the wall during crystal growth for different $k_R$ ($W=1\mu \mathrm{m}$ and $H=60\mu \mathrm{m}$).

Figure 4

“Stress” generation diagram: computed values of contact crystallization pressure $P_c$ as a function of Damkhöler number Da, and channel aspect ratio $H/W$. Colored lines are isolines of $P_c$ with values indicated in MPa. The inset shows a 3D representation.