  Na-montmorillonite is a natural clay mineral and is available in abundance in
nature. The aqueous dispersions of charged and anisotropic platelets of this
mineral exhibit non-ergodic kinetically arrested states ranging from soft
glassy phases dominated by interparticle repulsions to colloidal gels
stabilized by salt induced attractive interactions. When the salt concentration
in the dispersing medium is varied systematically, viscoelasticity and yield
stress of the dispersion show non-monotonic behavior at a critical salt
concentration, thus signifying a morphological change in the dispersion
microstructures. We directly visualize the microscopic structures of these
kinetically arrested phases using cryogenic scanning electron microscopy. We
observe the existence of honeycomb-like network morphologies for a wide range
of salt concentrations. The transition of the gel morphology, dominated by
overlapping coin (OC) and house of cards (HoC) associations of clay particles
at low salt concentrations to a new network structure dominated by face-face
coagulation of platelets, is observed across the critical salt concentration.
We further assess the stability of these gels under gravity using
electroacoustics. This study, performed for concentrated clay dispersions for a
wide concentration range of externally added salt, is useful in our
understanding of many geophysical phenomena that involve the salt induced
aggregation of natural clay minerals.
