Ultrasonic metamaterials for use in water

Abstract

This thesis describes several ways in which to manipulate ultrasound by means of Fabry-Perot resonant acoustic metamaterials (AMMs), Fresnel-type lenses and phononic crystals (PCs). These can be used to obtain properties such as sound focussing or sub-wavelength imaging in water, which are highly desirable characteristics in several applications such as biomedical imaging and flaw detection. The metamaterials and the other structures described in this thesis were mostly additively manufactured, with the further aim of validating the different layer-bylayer fabrication methods to build them. Experiments were conducted in a water tank and results were compared to numerical predictions obtained through Finite Element Modelling (FEM). Several tests on AMMs in the form of flat plates with periodic arrays of square holes were carried out. It was found that the use of tungsten to additively-manufacture patterned holey plates can lead to resonances in each channel, allowing sub-wavelength imaging below the conventional resolution limit. Periodic metamaterials are usually narrowband, but, in this work, irregularities due to additive manufacturing were seen to extend the bandwidth of usage, which can be useful in many applications. The concept of trapping air within a polymer shell for use in a metamaterial device will be described. Trapped air holey metamaterials were found to exhibit sub-wavelength imaging properties, leading to comparable performance with respect to metal metamaterials for a much lower fabrication cost. Moreover, the trapped-air concept was also applied to Fresnel-type lenses, leading to the possibility of focusing ultrasound in water using air volumes trapped within a polymeric shell. Additionally, additively-manufactured metallic two-dimensional PCs made of periodically arranged cylinders were investigated. These structures can block acoustic signals over a range of different frequencies, creating bandgaps. Other properties can arise from the periodicity of scatterers, such as negative refraction and sound focussing. Two properties of additively manufactured PCs, namely bandgaps and negative refraction, were compared to conventionally-manufactured ones to show similarities and differences. An additively manufactured crystal was then numerically simulated and tested to investigate its sound focusing properties. This led to the validation of selective laser melting as an additive manufacturing technique to fabricate PCs for use in water in the hundreds of kHz frequency range. It is expected that future advancements in additive-manufacturing resolution will stretch the present fabrication limits, making acoustic metamaterials even better candidates for applications in Nondestructive Testing, diagnostic medical ultrasound, and sensing