Architected materials present an opportunity to overcome the limited ability of brittle piezoelectric ceramics to strain under electromechanical load. In the absence of a commercially available resin containing piezoelectric nanoparticles, this work seeks to investigate the printability and thermal processability of a prepared piezoelectric particle loaded slurry using laser stereolithography. This was accomplished by comparing the cure depth, in-plane resolution, and the dimensional accuracy achieved with a piezoelectric slurry prepared with barium titanate, to a commercially available silica and alumina-based suspension. The study of thermal processability revealed the dimensional sensitivity of fabricated open architectures to sintering temperature and duration. The prepared piezoceramic slurry, containing barium titanate, was successfully polymerized using laser-stereolithography and its cure depth exhibited a similar response to exposure duration and fluence level as the commercial slurries. The in-plane resolution of the barium titanate-based slurry was unexpectedly high, and may be due to the opacity of the piezoelectric particles. Open architectures with millimeter sized features were successfully fabricated using laser stereolithography. Dimensional accuracy was highest for the alumina-based material system, and the inverse relationship between cured depth and in-plane resolution was reflected in the dimensions of the silica-based parts. Open architected structures further withstood thermal processing. During thermal processing, there was a greater reduction in height for all parts due to higher in-plane concentrations of ceramic particles, relative to the z-direction. Both an increase in sintering temperature and duration resulted in a uniform change of 1%, respectively, in part dimensions. Data collected in this work is to be used as a benchmark to inform formulation requirements, laser stereolithography parameter settings and thermal processing procedures for a custom ceramic slurry containing piezoelectric nanoparticles.
We present the design, fabrication, and testing of stretchable pressure sensing membranes. Two sensing techniques are demonstrated: resistive and capacitive. Both designs are incorporated in 400μm-thick films and are fabricated with thin film application of silicone and stencil/mask deposition of conductive materials. The resistive sensor utilizes room temperature liquid metal while the capacitive sensor utilizes multi-walled carbon nanotubes. Tests are performed with 18mm-diameter samples of each. Point load tests and acoustic response in an impedance tube provide feedback on sensor performance. The resistive sensor demonstrates a sensitivity of 0.045Ω/mm, and the sensor’s response has been characterized for in the 30Hz to 10kHz range with varying degrees of sensitivity. The capacitive sensor has a small point-load-deflection sensitivity ranging from 0.018pF/mm to 0.044pF/mm depending on capacitor diameter. Acoustic response are shown for 5Hz to 40 Hz, limited by external electronics. These devices are progress towards developing sensor networks capable of tracking aqueous turbulence.
We present a liquid flow sensor inspired by cupula structures found on a variety of fish. Our 5mm x 5mm x 1.75mm artificial cupula uniquely comprises a pair of differential liquid metal capacitors encased in silicone. Deflection of the structure – manually or by fluid flow – increases capacitance on one side and decreases on the other. To fabricate the complex internal structure, a commercial 3D printer is used to create a mold out of a sacrificial wax-like material. After casting uncured rubber, internal mold structures are melted and dissolved away, leaving channels and voids for liquid metal vacuum injection. The measured sensitivity of ~0.05pF/mm is compared to theoretical capacitance versus deflection values based on kinematics. To test behavior under water flow, a custom flow channel consisting of a 7.5mm x 7.5mm cross-section is employed with rates up to 1L/min. The parabolic capacitive response as a function of flowrate is compared to analytic theory based on kinematics and drag as well as to fluid-structure interaction (FSI) simulations using COMSOL. This device has future applications in the control of bio-inspired soft robotics. [Work sponsored by the Office of Naval Research.]
We design and acoustically simulate additively manufactured, flat acoustic membranes (also called metasurfaces) which can be reconfigured into 3-dimentional solids. Using finite element simulations, we design frequency selective acoustic ‘window’ membranes. These transmit narrow frequency bands near flexure resonances. The frequency range of coverage was chosen to be in the audible range and spans from 2,500Hz to 10,000Hz with first order resonances only. We demonstrate selective, non-overlapping acoustic transmission through each membrane window in its flat configuration, and directional selectively when the flat metasurface is folded into the truncated-octahedron with an omnidirectional microphone placed on the interior of the solid form. This work was supported by the Office of Naval Research.
The limitations on resolution due to the effects of diffraction have presented a significant barrier to generating and observing small features with acoustic or electromagnetic waves. Previously proposed methods to overcome this limit, and therefore achieve superresolution, have largely been restricted to operating within the near-field region of the aperture. In this work, we will describe how acoustic helicoidal waves generated using a phased acoustic aperture (such as a traditional phased array or acoustic metasurface) can create acoustic vortices that are well below the resolution limit, and how this can enable far-field superresolution acoustic imaging. The acoustic vortices generated in this manner propagate from the near-field into the far-field through an arrangement of stable integer mode vortices, thereby enabling the generation of far-field superresolved features in the acoustic pressure field. Through the use of non-axisymmetric vortex beam distributions, splitting of the on-axis vortex occurs. This leads to arbitrary off-axis arrangements of vortices, enabling more complicated superresolved structures to be created such as squares, triangles and multi-point stars. In this paper, theoretical and numerical results will be presented for an acoustic aperture which is capable of generating superresolved far-field features in the radiated acoustic pressure, and results will be shown illustrating the superresolution capability of this novel technique.
We have previously demonstrated an acoustic vortex wave antenna (VWA) based on a metamaterial aperture. This system produced arbitrary angular mode number waveforms,1 but did not produce pure integer mode vortex waves. In this work we extend our previous result to a design which radiates pure integer vortex modes. By combining an acoustic leaky wave antenna with a ring resonator waveguide we produce integer mode acoustic vortex waves. Further, we computationally show that this spatial mode can be transferred between two opposing acoustic vortex wave antenna in a "pitch - catch" configuration.
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