Experimental Studies of Instabilities Near the Sol-Gel Transition

Abstract

This master thesis presents a new experimental way to dynamically determinerheological properties of a complex fluid. The fluid investigated is asuspension of the synthetic clay mineral laponite of 3 wt%. This is mixedtogether with distilled water or a NaCl solution with concentrations bellow10^2 M. Laponite gels are thixotropic and shear thinning with a yield stress.In the experiments done during this thesis the transition between liquid andsolid behavior is investigated using a Hele-Shaw cell and a rheometer. Threedifferent phases is observed with respect to waiting time and NaCl concentration.These are recognized as a liquid phase for small concentration ofNaCl and short waiting times and a solid phase for high concentration ofNaCl and long waiting time. In the intermediate range a viscoelastic phaseis observed.Some experiments were done with the sample placed between crossed polarizers,and birefringence was observed in all three phases. In the solid phaseit is observed only a small area around the fracture tip. In the liquid phasethis is observed in front of the finger, but this is not a stable phase. Inthe viscoelastic regime a stable nematic phase occur along the stress fieldaround the interface of the bubble. This nematic phase will align normal tothe glass surface. Fracture like patterns in the nematic phase are observed.This is an elastic behavior with the clay platelets aligning normal to thestress field to prepare for fractures to propagate there.Different NaCl concentrations will give different arrested states, respectivelya repulsive glass and a attractive gel. It is expected that glass and gels behavedifferently in this experiment, but this was not confirmed. A gel phasehave been observed in concentrations of NaCl above 5*10^3 for waitingtimes up to 605 hours. For smaller concentrations a even longer waitingtime is expected and experiments on this needs to be performed in futurestudies.In addition this experiment could investigate and give insight of flow incomplex fluid. To investigate this further, tracer particles have been usedin a few experiments. The results are promising for further studies.In this thesis work, a large range in waiting times and concentrations of NaClhave been investigated. In future studies it should be carried out experimentsin a smaller range of these parameters along with other parameters.Also a more quantitative analysis should be carried out. Especially thetransitions between fluid/viscoelastic and viscoelastic/solid is interesting toinvestigate further.