Rheometer for the study of structured soft materials

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Macroscopic properties of fluids and solids are determined by microscopic interactions. In my research programme, I study structure and dynamics in self-assembling colloidal systems. Colloidal suspensions are suspensions of particles in a fluid where the particles are small enough that they are constantly kept jostling by the molecules in the surrounding fluid. This is termed Brownian motion - in 1905 Einstein postulated that Brownian motion in colloids was indeed a proof of the molecular nature of the fluid.Microscopic colloidal structure modifies the macroscopic flowing characteristics of the fluid. Tomato ketchup is a colloidal fluid that exhibits viscoelastic properties: more solid-like in the absence of shear but thinning in the presence of shear. While even colloidal fluids can exhibit a complex response to shear ("rheological behaviour"), it is expected that the approach to an amorphous solid ("glass") phase is characterized by a breakdown of a classic relationship, known as the Stokes-Einstein relation, between microscopically observed diffusion coefficients and the macroscopic viscous properties.Colloidal suspensions can exhibit glassy behaviour when crystallization is suppressed. However, colloidal suspensions form many anisotropic structures including crystalline colloidal phases in the presence of external electric fields. This microscopic phenomenon is coupled with an increase in solid-like rheological response: the solid does not yield below a limiting stress, and such a material is termed an electrorheological fluid. In the study of both the colloidal glass transition and the electrorheological effect, the primary goals are the understanding of microscopic mechanisms and the relationship between micro-structure and macroscopic properties. A rheometer will enable the study of viscoelastic properties of bulk colloidal suspensions in the presence and absence of an external electric field, and will provide the crucial link between microscopic and macroscopic properties.The rheometer will benefit other members of the research community as a valuable research and training tool.