Using Rheology to Determine the Phase Behavior: A Case Study of Colloidal Suspension of Charged Discotic Particles
Rheological principles are usually used to analyze response of the soft materials to an applied deformation field. However, the linear viscoelastic response that leads to spectrum of relaxation times is very sensitive to the microstructures of the soft materials and renders unique information about the same. In this work, we use rheology to discern phase behavior of aqueous suspension of Laponite that is composed of charged discotic nanoparticles, a highly contested system in the literature. The scattering studies on Laponite suspension suggest that for concentrations between 1 and 2 weight % and up to 4 mM NaCl there exists an attractive gel state, while beyond 2 weight % and up to 4 mM NaCl suspension is in a repulsive ‘Wigner’ glass state. In this work we perform time resolved rheometry on Laponite suspensions over a concentration range of 1 to 4 weight % and NaCl concentration range of 0 to 7 mM, wherein suspension is subjected to cyclic frequency sweep as its structure evolves. Very interestingly the linear viscoelastic response shows all the characteristic signatures of sol – gel transition observed for polymeric materials undergoing crosslinking reaction sampling the critical gel state with a percolated space spanning network. Significantly, the cryo-TEM images of the postgel state also clearly show the presence of a percolated network formed by interparticle bonds. Therefore, contrary to scattering studies, the present work clearly suggests Laponite suspensions to be in attractive gel state over the studied concentration ranges.
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