Elastic lens and mirror concepts that have been explored to date for enhanced structure-borne wave energy harvesting are suitable for relatively high-frequency waves (e.g. tens of kHz), which are very much outside the typical ambient structural frequency energy spectrum. One direct way of reducing the design frequency of such phononic crystal-based lens and reflector/mirror designs is to increase their size, which would yield very large dimensions to operate at ambient vibration frequencies (∼hundreds of Hz). In this work, we exploit locally resonant (LR) metamaterials to enable low-frequency elastic wave focusing via LR lens and mirror concepts with practical size limitations. LR lens is designed in a similar way to its phononic crystal counterpart by tailoring the refractive index profile of the LR unit cell distribution. However, LR approach enables altering the dispersion characteristics, and thereby the phase velocity distribution, at much lower frequencies right below the local resonance frequency. Other than the local resonance frequency of the unit cells, the key factor in design is the mass ratio of the resonators to achieve a desired refractive index profile and focusing. LR mirror uses the low-frequency bandgap which is right above the resonance frequency of the unit cells. LR unit cells arranged in the form of a parabola, for instance, makes a low-frequency LR mirror that operates in the bandgap for plane wave focusing. These LR focusing concepts can be used in vibration civil, aerospace, and mechanical systems to localize and harvest structure-borne wave energy.

In this paper, we explore structure-borne elastic wave energy harvesting, both numerically and experimentally, by exploiting a Gradient-Index Phononic Crystal Lens (GRIN-PCL) structure. The proposed GRIN-PCL is formed by an array of blind holes with different diameters on an aluminum plate where the orientation and size of the blind holes are tailored to obtain a hyperbolic secant gradient distribution of refractive index guided by finite-element simulations of the lowest asymmetric mode Lamb wave band diagrams. Under plane wave excitation from a line source, experimentally measured wave field successfully validates the numerical simulation of wave focusing within the GRIN-PCL domain. A piezoelectric energy harvester disk located at the first focus of the GRIN-PCL yields an order of magnitude larger power output as compared to the baseline case of energy harvesting without the GRIN-PCL on the uniform plate counterpart for the same incident plane wave excitation. The power output is further improved by a factor of five using complex electrical load impedance matching through resistive-inductive loading as compared to purely resistive loading case.

Vibration-to-electricity conversion has been heavily researched over the last decade with the ultimate goal of enabling self-powered small electronic components to use in wireless applications ranging from medical implants to structural health monitoring sensors. Regardless of the transduction mechanism used in transforming vibrational energy into electricity, the existing research efforts have mostly focused on deterministic or stochastic harvesting of direct vibrational energy available at a fixed location in space. Such an approach is convenient to design and employ linear and nonlinear vibration-based energy harvesters, such as base-excited cantilevers with piezoelectric laminates. Although the harvesting of local vibrations using linear and nonlinear devices has been well studied, there has been little effort to investigate power extraction from elastic waves propagating in host structures to gain a fundamental understanding of power flow and to best exploit not only standing but also traveling wave energy. This paper explores the problem of piezoelectric energy harvesting from one-dimensional bending waves involving propagating and evanescent components with a focus on infinitely long thin beams. A pair of electroded piezoelectric patches is implemented as the energy harvesting interface connected to a complex electrical load. An analytical modeling framework is given in order to relate the harvested power to incoming wave in the presence of a generalized resistive-reactive circuit. Effects of energy harvesting on the global wave dynamics as well as individual propagating and evanescent wave components are investigated with an emphasis on the wavelength matching concept. The electrical loading conditions for maximum power and efficiency are identified for several special cases in the low frequency range.

The impact of droplets onto a substrate in ink-jet printing is critical for control and optimization of the droplet deposition process to improve part quality and accuracy and to reduce the manufacturing time. However, most previous research on droplet impingement dynamics mainly utilized one metric — the droplet spreading radius, which does not provide enough information for manufacturing purposes. This paper presents a new metric that is relevant to manufacturing by characterizing the droplet shape by measuring the similarity between the droplet shape and a desired shape over time. This enables a model of droplet shape evolution and optimization of the droplet deposition process to build desired geometries. Meanwhile, analyses with this shape metric aids understanding the physics of droplet shape evolution during impingement. A 2-D shape metric is first proposed and test cases are given to validate the effectiveness of the shape metric. Then the definition is extended to characterize 3-D droplet shape. Results also show the 3-D shape metric is effective and robust.

We designed and fabricated a 64 element 1-D linear dual electrode Capacitive Micromachined Ultrasonic Transducer (CMUT) array operating at 9.5 MHz for Intracardiac Echocardiography (ICE). The dual electrode CMUT structure increases the overall sensitivity by 12.6dB (6.2dB in receive sensitivity; 6.4dB in output pressure) when compared to optimized single electrode CMUT. We report peak output pressure of 2.3MPa on the CMUT surface when 170V AC and 180V DC is applied. This significant performance increase makes the CMUT more competitive with their piezoelectric counterparts.

We investigate multiple-annular-ring CMUT array configuration for forward-looking intravascular ultrasound (FL-IVUS) imaging. This configuration has the potential for independent optimization of each ring and uses the silicon area more effectively without any particular drawback. We designed and fabricated a sample 1mm diameter dual annular ring CMUT test array which consists of 24 transmit and 32 receive elements. For imaging experiments, we designed IC chips that contain 8 transimpedance amplifiers, a multiplexer and a buffer. The real time-pulse echo experiments obtained with designed IC electronics show 26dB Signal to Noise Ratio (SNR) from a 3.5 mm away aluminum reflector in oil. This paper presents our first efforts in obtaining real time imaging with designed IC chips which is one step before CMUT on CMOS implementation.

Separation of hydrogen from the reaction products stream leaving fuel processor is an essential step prior to its introduction to the fuel cell. To this end, we are developing micromachined Pd/Ag alloy membranes for in-situ hydrogen separation suitable for integration with catalytic fuel reforming microreactors. In this work, we report an analysis of mass transport and kinetics of hydrogen permeation through a non-porous palladium membrane for the case of the sub-micron membrane thickness. A simplified model has been developed which divides the permeation into seven distinct regimes; gas phase mass transport to the surface, adsorption onto the surface, transition into the bulk material, solid-phase diffusion through the bulk material, transition to the effluent surface, desorption into the gas phase and diffusion away from the surface. Historically, this permeation process is limited by the bulk diffusion step and therefore membrane thickness controls permissible flux. Based on the present model applied to the sub-micron membrane, and utilizing accepted values from the literature, desorption of hydrogen from the Pd surface back into the gas phase has been identified as the rate-limiting step for this process below a ‘critical’ temperature. This result indicates that careful consideration of membrane packaging is critical for maximizing hydrogen flux for the sub-micron membrane and that further efforts to increase flux via thinner membranes would be futile.

According to the recent Laboratory News’ Proteomics Special article Mass Spectroscopy (MS) has become the technology of choice to meet today’s unprecedented demand for accurate bioanalytical measurements, including protein identification. Although MS can be used to analyze any biological sample, it must be first converted to gas-phase ions before it can be introduced into a mass spectrometer for analysis. It is transfer of a very small liquid sample (proteins are very expensive and often very difficult to produce in sizable quantities) into a gas-phase ions that is currently considered to be a bottleneck to high throughput proteomics. Electrospray ionization (ESI) is a technique developed in early 1990 th to generate a spray gas-phase ions by applying high voltage (from several hundreds volts and up to a few thousands kilovolts relative to the ground electrode of the MS interface) to a small capillary through which the liquid solution is pumped. The high electric field ionizes the fluid forming the converging Taylor cone of the exiting jet which eventually breaks into many small droplets when the repulsive Coulombic forces overcome the surface tension. Because of the focusing effect associated with the spraying the electrically charged fluid, the size of the electrospray cone and thus of the formed droplets is in a few tens of nanometers range although the inner diameter of the capillary is in the micrometer range.

Existing battery technologies have become a major obstacle to advances in the performance of portable energy-intensive devices primarily due to a limited lifetime between charge cycles. 1,2 Fuel-cell-based energy sources are a viable alternative due to the high energy density of liquid fuels and the potential for high efficiency power generation. The focus of recent work has been the development of two types of fuel cells for portable applications, hydrogen-based fuel cells with external fuel reformation, i.e., conversion to hydrogen, and direct-methanol fuel cells that oxidize methanol directly at the cell anode. 1,3 Regardless of whether internal or external fuel reformation is used, power-efficient atomization of liquid fuels ranging from methanol to higher hydrocarbons and diesel to kerosene and logistic fuels, e.g., JP-8, is an essential processing step for conversion of a fuel from liquid to gas phase. We present the experimental characterization and theoretical modeling of the fluid mechanics underlying the operation of a micromachined ultrasonic atomizer. This droplet generator utilizes fluid cavity resonances in the 0.5 to 3 MHz range along with acoustic wave focusing for low power atomization of liquids for fuel processing. The device comprises a fuel reservoir located between a bulk ceramic piezoelectric transducer for ultrasound generation and a silicon micromachined array of liquid horn structures as the ejection nozzles. The array size can be scaled to meet flow rate requirements for any application because a single piezoelectric actuator drives ejection from multiple nozzles. The atomizer is particularly well-matched to fuel processing applications because it is capable of highly controlled atomization of a variety of liquid fuels at low flow rates. This low-flow-rate requirement intrinsic to small-scale, portable power applications is especially challenging since one cannot rely on the conventional jet-instability-based atomization approach. Further, the planar configuration of the nozzle array is suited to integration with the planar design of fuel cells. Experimentally-validated finite element analysis (FEA) simulations of the acoustic response of the device are used to estimate the fraction of the electrical input power to the piezoelectric transducer that is imparted to the ejected fluid. Results of this efficiency analysis indicate that it is not optimal to design the ejector such that a cavity resonance (corresponding to acoustic wave focusing at the tips of the pyramidally-shaped nozzles and thus fluid ejection) coincides with the longitudinal resonance of the piezoelectric transducer. It also appears that the efficiency of the device increases with decreasing frequency. Atomization of methanol and kerosene from 5 to 25 μm diameter orifices is demonstrated at multiple frequencies between 0.5 and 3 MHz. In addition, high-resolution visualization of the ejection process is performed to investigate whether or not the proposed atomizer is capable of operating in either the discrete-droplet or continuous-jetting mode (see Figure 1). The results of the visualization experiments provide a basic understanding of the physics governing the ejection process and allow for the establishment of simple scaling laws that prescribe the mode of ejection; however, it is likely that the phenomena that dictate the mode of ejection (i.e., discretedroplet vs. continuous-jet) do not occur within the field of view of the camera. Further, the most important features that determine the initial interface evolution occur within the nozzle orifice itself. A detailed computational fluid dynamics (CFD) analysis of the interface evolution during droplet/jet ejection yields additional insight into the physics of the ejection process and provides further validation of the scaling laws. Figure 2 provides examples of simulation of both discrete-droplet and continuous-jet mode ejection.