Band 13, Heft 5

Self-sensing is the technique of using a transducer to both actuate and sense concurrently, therefore eliminating the need for separate sensors. A sensorless rheometer could be much smaller, simpler and more robust than traditional designs. One application where such a rheometer would be desired is the in-situ measurement of curing bone cement in orthopaedic surgery. A set of equations was developed that models the relationship between force, motion and back-e.m.f. generation for a class of electromagnetic actuators. This enables velocity, displacement and force to be self-sensed from voltage measurements only. This self-sensing was validated on a conventional linear electromagnetic actuator, and a small rotary moving magnet device, which was designed to be a small self-sensing rheometer. The accuracy of the estimation was assessed and shown to compare favourably with measured data. The actuators were then used to construct simple rheometers to measure bone cement. Rheological models were used to calculate storage and loss moduli and dynamic viscosity from the self-sensed values of displacement, velocity and torque. The accuracy of these self-sensing rheometers was verified against a traditional rheometer using a silicone fluid and a polyethylene oxide solution. The self-sensing rheometers were used to characterise acrylic bone cements during curing, reinforcing and extending upon previous results. The elimination of sensors meant that it was possible to produce a small, inexpensive rheometer with a very simple structure. This indicates there is potential to develop small rheo-transducers for certain applications.

A general constitutive relation describing the change of viscoelastic behavior during the liquid - solid (sol - gel) transition which takes place in preceramic polymers is derived on the basis of Jeffrey’s 3-constants model with time dependent viscosities and elasticity. It is postulated that the sol - gel - transition can be analyzed analogous to the solutions of the Avrami equation used for modeling crystallization processes. Two different polymer systems used as precursor for the production of ceramic materials are investigated here: i) a mixture based on polysiloxane, alumatrane and isopropanol; ii) a non-oxidic carbodiimide gel based on the reaction of chlorosi-lanes with bis(trimethylsilyl)carbodiimide. Continuous measurements of the dynamic moduli versus reaction time, as well as creep tests at constant shear stress, evidenced both qualitative similarities and quantitative differences associated with the sol - gel transition of the two polymer systems. The shear rate and viscosity dependence of reaction time in creep tests, respectively the evolution of Lissajous figures associated with oscillatory experiments, are found to be consistent with the numerical simulations of the proposed constitutive relation.

Recently a new theory of viscosity of concentrated emulsions dependency on volume fraction of droplets (Starov V, Zhdanov G: J. Colloid Interface Sci, 258, 404 (2003)) has been suggested that relates the viscosity of concentrated emulsions to formation of clusters. Through experiments with milk at different concentrations of fat, cluster formation has been validated using optical microscopy and their properties determined using the mentioned theory. Viscometric studies have shown that within the shear rate range studied, both the packing density of fat droplets inside clusters and the relative viscosity of milk (viscosity over skim milk viscosity) are independent of shear-rate, but vary with volume fraction. Comparison of the experimental data with previous theories that assumed that the particles remained discrete shows wide variation. We attribute the discrepancy to cluster formation.