University of Connecticut |
Polymer Program SeminarRelaxation in Soft Glassy Materials Friday, February 15, 2008 11:00 am , IMS Room 20 I will present experimental data which shows that colloidal soft materials can be used as a model system to understand glass formation in molecular materials. A molecular glass is formed when a liquid is cooled so rapidly that crystallization cannot occur. During cooling, the viscosity of the liquid increases dramatically until, at the glass transition temperature, the time scales for any flow of the material exceed experimental time scales. One puzzle in the understanding of glasses is related to the so called fragility of glass forming liquids, a measure for how fast the relaxation time or the viscosity of a liquid is increasing upon approach to the glass transition. A fundamental understanding of this variation in fragility is missing. I will show that we can gain new insight into the origin of fragility and into the glass transition in general by studying glass formation in soft materials; here particles on the colloidal scale play the role of atoms in molecular glass formers. We use soft, deformable microgel particles of controlled elasticity and thereby access the full range of arrest scenarios seen in molecular glasses. Our light scattering data and rheological measurements strongly indicate that the dynamic arrest behavior is dictated by the elastic properties of the individual colloidal particles and suggest an equivalent interpretation of dynamic arrest in molecular systems. We demonstrate, for the first time, that an understanding of glass-formation in colloids provides fundamentally new insight into glass-formation in molecular systems. To further study the behavior of glass-like materials I have developed a new rheological technique, Strain-Rate Frequency Superposition (SRFS). This technique extends the frequency range accessible to oscillatory rheology and provides a direct explanation for the nonlinear viscoelastic response of these materials. Our data indicates that strain rate acts as an effective temperature, driving relaxation processes to faster time scales, without affecting the characteristic shape of the relaxation spectrum. I will offer some practical examples as this technique is now being applied to study the behavior of a wide range of glass-like materials.
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