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Formation and growth of labyrinthine drying patterns in 2-D porous model media

Pascal Panizza Matière Molle, Institut de Physique de Rennes

Drying of colloidal solutions in porous media is relevant to many fields such as soil physics, and civil engineering. For all these applications it is essential to understand and to predict the kinetics and topology of the crystal deposits that form during the drying process.
This problem is important for civil engineering owing to salt weathering which damages buildings and other engineering structures and in material science to engineer smart
nanoscale and microscale materials using evaporation as a novel route of controlled selfassembly. Despite the importance of all these applications, this issue still remains elusive notably because of complex couplings between the formation of crystal deposits and the various transport phenomena involved in the drying process which are difficult to unravel.
To address this issue, we investigate drying of colloidal suspensions or surfactant solutions confined in a 2D porous model medium which permits to overcome optical limitation inherent to bulk systems. We observe the formation of thin solid films that interconnect to form a maze structure. Our experiments show that the dynamics of formation of these unusual out of equilibrium patterns is triggered by a regular succession of Haines jumps. We identify the elementary mechanisms leading to the formation of the crystal deposits at the pore scale and then establish a pore-network model that well captures the topology of the labyrinthine structures observed in our experiments as well as their dynamics. Using simple arguments, we predict the topological changes in the drying pattern at the scale of an individual pore that occur when the geometrical parameters of the problem such as the sample thickness and the contact angle that the liquid makes with the substrate are varied. Our experiments confirm our theoretical predictions.

Fluorescent microscopy image of the maze structure observed after the drying of a colloidal suspensions in a 2 D model porous medium made of cylindrical posts whose height and diameter are respectively 30 and 100 micrometers. The interdistance between posts is 175 micrometers