Fractal analysis of surface roughness of montmorillonite clay self-supported films: Effects of exchanged cations and of mechanical tensile stress

• Charge and size of interfoliaire cation control films texture.
• The intensity of the surface forces controls the macroscopic mechanical properties.
• Mechanical loading shows pronounced plastic deformation for monovalent samples.
• Films with highly charged cations show stable roughness under mechanical load.
• Strong correlation of films surface H parameter with the parent cakes permeability.

In this paper, surface roughness of self-supported montmorillonite clay films was studied by the use of fractal analysis to study its links with various inter-foliar cations, mechanical properties, and permeability. The Hurst exponent of fractional Brownian motion was estimated by maximum likelihood from optical images in reflection mode. Images corresponding to films with different inter-foliar exchanged cations visually show significant texture variations. It is confirmed by a continuous decrease of H parameter toward less correlated textures as one goes from the samples exchanged with monovalent ions to those exchanged with divalent and finally trivalent ions. The same films were analyzed while they were submitted to mechanical tensile stress. The films exchanged with the most charged cations or less hydrated ones do not show any evidence of texture change and the H parameter is almost constant. Concerning the permeability study, less permeable parent cakes – with hydrated monovalent cations – lead to relatively smooth and less disturbed surface films with high H parameter. In the opposite, very permeable cakes give birth to anti-persistent relief.

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