| Summary: | Details of the design and development of an improved high frequency virtual gap rheometer, VGR, are presented. This rheometer utilises multiple path shear wave interferometry to determine the shear wave propagation characteristics (shear phase velocity (v) and damping length (<I>x<SUB>o</SUB></I>) of colloidal dispersions from which viscoelastic parameters are derived. This VGR incorporates a data acquisition system (1MHz sampling rate; ≃ 8x10<SUP>6</SUP> samples) which allows measurement at frequencies (<I>f</I>) 200 ≤ <I>f</I> ≤ 2000Hz. The effect of digital quantisation, its relation to the operation of an electrostrictive actuator (source of shear waves), and to the design of test waveforms is considered. Protocols for the correct generation of waveforms are established. Measurements on a 'Newtonian' fluid (silicone oil) show that the VGR is able to determine the frequency dependent viscoelastic characteristics of dispersive systems. Test signals are analysed with high precision digital Fourier transform methods. This forms the basis for Fourier Transform Mechanical Spectroscopy (FTMS) which is developed for the VGR herein, allowing the use of up to eleven different frequency components simultaneously. A novel method for establishing linearity of the viscoelastic response is also reported. A low frequency (10<SUP>-3</SUP>→10Hz) rheological characterisation (conducted with a controlled stress rheometer) of aqueous, electrostatically stabilised dispersions of a clay colloid (Laponite XLG) is reported and establishes the suitability of these dispersions as a test material for the VGR. Measurements of <I>v</I> and δ(→ <I>x</I><SUB>o</SUB>) with the VGR using sequential single frequencies and those conducted with simultaneous multiple frequencies (VGR FTMS), on equilibrium laponite dispersions, are in good agreement. The evolution of viscoelastic parameters with time, determined by VGR FTMS for equilibrating dispersions at various weight fractions, is reported. These results show that these dispersions exhibit marked viscoelastic wave dispersion over the frequency range of the VGR. Possible origins of wave dispersion are identified.
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