An improved measurement of frequency-dependent shear properties of highly compliant materials using a dynamic Kibble’s method

Abstract

While methods exist to characterize the Young’s modulus and loss factor of viscoelastic materials, there are fewer methods to characterize the shear modulus and loss factor in shear. We previously presented an experimental apparatus that applied dynamic torque on rod-like viscoelastic materials with diameters of 0.5 cm to 2 cm and lengths of 10 cm to 30 cm. The torque was applied using Lorentz forces and measured using a dynamic version of Kibble’s method (similarly, the Watt method). Here, we present a modified version of this apparatus that has several notable improvements over the previously reported version. Improvements include a stronger, more uniform magnetic field and a direct measurement of the angular response of the specimen using a laser and photo-diode configuration. Moreover, the direct measurement of the angular response eliminates the need to know other geometric quantities (such as the radius of the electromagnetic coil), which further reduces the uncertainty of the measured torque. The measurement of the angular response and applied torque are fit to models in order to determine the shear moduli and loss factor. We demonstrate this improved technique on rod-like specimens of different highly-compliant viscoelastic materials.