Development of a Low Stress Creep Rheometer

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9802793ColbyThere are many examples of fluids that are actually weak gels due to the presence of structure in the materials on length scales much larger than molecular dimensions. Below some critical yield stress, these structured fluids are solids, characterized by a modulus. In most cases, this modulus cannot be measured in commercially available rheological instrumentation because those rheometers cannot apply low enough stresses. However, by simply combining technologies that have been proven to work in various rheometers, it is possible to construct a creep rheometer capable of applying three orders of magnitude lower stress levels than "state of the art" commercial rheometers. This new rheometer will allow application of stresses below the yield stress, to measure the modulus of the unbroken structure. Application of stresses in the vicinity of the yield stress will allow detailed studies of yielding in structured fluids. This information is needed for needed for modeling the rheology of structured fluids, as the entire rheological response (even at high stresses) is due to structure breaking. Examples of such structured fluids that we are currently studying are: swollen gels, liquid crystal polymers, block copolymers, polyelectrolyte-surfactant complexes, suspensions, surfactant solutions and foods. The new rheometer will lead to breakthroughs in our understanding of each of these classes of materials because it will allow, for the first time, characterization of the linear response and the first departures from linear response. This rheometer is, therefore, vital to the long-term research and education plans of the Penn State Rheology Group.%%%***