Development of a novel ultrasonic measurement systemto investigate the viscoelastic properties of fluids

Abstract

The goal of this project is to develop an ultrasonic measurement system that canmeasure the viscoelastic properties of a fluid and use that system to investigate theproperties of magnetorheological (MR) fluid. MR fluid is made of microscopicferromagnetic particles dispersed in a carrier fluid. The substance fluid behaves as a fluidin the absence of a magnetic field, but when an external magnetic field is applied, itbecomes more solid-like. The ultrasonic system measures the resonant response of anoscillating 5 MHz AT cut (shear mode) quartz crystal. When the crystal is immersed in afluid, the oscillation is damped, causing the resonant frequency to decrease and thefrequency width of the resonant response to increase. Theoretical equations weredeveloped to relate these changes to the viscoelastic properties of the surrounding fluid,such as viscosity and shear stiffness. The ultrasonic system was validated by measuringfluids with viscosities that ranged from approximately 10 to 5000 centipoise (cP) andcomparing those measurements to measurements performed on the same fluids with aconventional mechanical viscometer. The viscoelastic properties of MR fluid wereinvestigated as a function of magnetic field orientation and as a function of time. Theorientation of the magnetic field relative to the face of the crystal was found to affect themeasured viscoelastic properties of the fluid. Measurements also revealed that theviscoelastic properties of the fluid in a magnetic field are not constant over time.