Development of Acoustic Wave Devices to Characterize Viscosity and its Nonlinearity

Abstract

Fluid has many physical properties, such as conductivity, density, heat capacity, surface tension, thermal conductivity and viscosity. Viscosity is one of the most important physical properties of fluid, and it needs to be studied while choosing proper fluids for specific applications. Viscosity measurements are essentially connected with product quality and consistency. When concerned with fluids characterization in design, development, quality control or just liquid transportation, viscosity measurements are always involved. For example, the quality of engine oil is critical to its lubricating property and also plays an important role in engine performance. Therefore, it is important to monitor the oil quality in an engine by measuring its viscosity over time. Some microacoustic devices have been developed to measure viscosity. It should be mentioned that all these devices have been developed to measure the viscosity only. As it is well known that some small change in viscosity may not affect the engine very much, therefore, it is reasonable to assume that the non-linearity of viscosity may be the real key of determining engine oil conditions. Among these acoustic technologies, magnetostrictive strip and cantilever have some unique advantages over others, such as wireless and good performances in liquids by choosing magnetostrictive strip, and large vibration displacement by selecting cantilever. In this research, the resonance behaviors of magnetostrictive strip are systemically studied to improve the sensor applications. Different factors which could have influences on resonance behaviors of magnetostrictive strip are examined intensively, such as DC bias field influence and locations effect. And the view that the AC-driving field could influence resonance frequencies of sensor is proposed. Proofs from three independent methods (lock-in amplifier, impedance analyzer and network analyzer) are given to support the view that resonance frequencies decrease with an increase in the AC-driving field. This discovery shows that AC driving field should be strong enough to avoid the influence on the resonance frequencies. The measurement results of the lock-in amplifier and impedance analyzer are compared to study the principle difference between these two methods. And also by using different coils in impedance analyzer method, the view that the impedance analyzer measures the signal from sensor and equivalent circuits is proved. Combined with the former results from the AC-driving field, it is found that coils with a small diameter and the same length as the sensor are good for resonance behaviors measurement. The location effects study shows the center of the pick-up coil is the best position of measuring the vibration signals of the magnetostrictive strip sensor. A new method by using properly chosen function of characteristic frequencies versus one of three frequencies is developed to identify the non-linearity of viscosity and also differentiate Newtonian oils and non-Newtonian oils. The resonance behaviors of magnetostrictive strips with different lengths in different oils are studied, and the performances of same length magnetostrictive strips with different length-width ratios are also investigated. All studies show that geometry of magnetostrictive strip is improtant in identifying non-linearity of viscosity. In order to apply magnetostrictive strips in a real circumstance, the performances of strip sensor are studied under different temperatures, and it is found that a magnetostrictive strip of this size can determine the non-linearity of viscosity well within broad temperature ranges. The resonance behaviors of cantilever sensors with different lengths in different oils are studied, and the performances of same length cantilever sensors with different length-width ratios are also investigated. The results show that a longer cantilever with the same width and thickness has better performance in identifying non-linearity of viscosity, and a cantilever sensor of the same length and thickness with a smaller width has better performance in differentiating Newtonian oils and non-Newtonian oils. And numerical simulation of the vibration of the strip sensor shows non-Newtonian liquids behave like Newtonian liquids at a rather low shear rates and high shear rates in which the range’s viscosity does not change with the shear rate and acts totally different with Newtonian liquids at an intermediate shear rate. So based on this conclusion, similar results can be achieved by the method I proposed which can distinguish these two types of liquids. And this also approves the validity of the method proposed in this study.