Abhinav Prasad

Characterization of mechanical properties of materials using ultrasound broadband spectroscopy.

This article explores the characterization of homogenous materials (metals, alloys, glass and polymers) by a simple broadband ultrasonic interrogation method. The novelty lies in the use of ultrasound in a continuous way with very low input power (0 dBm or less) and analysis of the transmitted acoustic wave spectrum for material property characterization like speed of sound, density and dimensions of a material. Measurements were conducted on various thicknesses of samples immersed in liquid where continuous-wave, frequency swept ultrasonic energy was incident normal to the sample surface. The electro-acoustic transmission response is analyzed in the frequency domain with respect to a specifically constructed multi-layered analytical model. From the acoustic signature of the sample materials, material properties such as speed of sound and acoustic impedance can be calculated with experimentally derived values found to be in general agreement with the literature and with pulse-echo technique establishing the basis for a non-contact and non-destructive technique for material characterization. Further, by looking at the frequency spacing of the peaks of water when the sample is immersed, the thickness of the sample can be calculated independently from the acoustic response. This technique can prove to be an effective non-contact, non-destructive and fast material characterization technique for a wide variety of materials.

Particulate mass sensing with piezoelectric bulk acoustic mode resonators

Current portable particle detection instruments typically rely on optical methods which are limited to 100 nm diameter particles. Microfabricated bulk acoustic resonators, when used as mass balances, could take particle detection below this limit. This study examines the collection of particles onto piezoelectric bulk acoustic mode resonators from gaseous flow using classical impaction. Collection of both polystyrene latex particles and α-pinene secondary organic aerosol particles was examined in terms of frequency shift and collection efficiency. A new experimental setup was introduced which allows for adjusting major impactor, resonator, and aerosol properties. Preliminary results show the setup works for both particles while the saturation limit was not reached within an hour despite highly elevated particle concentrations.

Micromechanical piezoelectric-on-silicon BAW resonators for sensing in liquid environments

This paper reports micromachined piezoelectric-on-silicon bulk acoustic wave resonators operating at a nominal frequency of approximately 3.15 MHz in fluidic media. Electrical measurements of the open-loop response of the resonators when one of the resonator surfaces is submerged in water indicate high quality factors in the range of 110-190. These values of quality factor are at least an order of magnitude higher than the flexural mode counterparts. The resonators are further exposed to Glycerol-Water mixtures of varying viscosity-density resulting in characteristic negative resonant frequency shifts. Experimental values are compared with a simplified liquid loading model and an agreement of up to 13% for highest and within 3-4% for lowest glycerol concentrations is established. These devices due to the relative ease of operation in liquid environments, scalability, high quality-factors and high mass-sensitivity have the potential for integration with microfluidics and electronics in order to realize an integrated platform for biochemical sensing and analysis.

Investigating biomechanical noise in neuroblastoma cells using the quartz crystal microbalance

Quantifying cellular behaviour by motility and morphology changes is increasingly important in formulating an understanding of fundamental physiological phenomena and cellular mechanisms of disease. However, cells are complex biological units, which often respond to external environmental factors by manifesting subtle responses that may be difficult to interpret using conventional biophysical measurements. This paper describes the adaptation of the quartz crystal microbalance (QCM) to monitor neuroblastoma cells undergoing environmental stress wherein the frequency stability of the device can be correlated to changes in cellular state. By employing time domain analysis of the resulting frequency fluctuations, it is possible to study the variations in cellular motility and distinguish between different cell states induced by applied external heat stress. The changes in the frequency fluctuation data are correlated to phenotypical physical response recorded using optical microscopy under identical conditions of environmental stress. This technique, by probing the associated biomechanical noise, paves the way for its use in monitoring cell activity, and intrinsic motility and morphology changes, as well as the modulation resulting from the action of drugs, toxins and environmental stress.

Studying particulate adsorption by drying droplets on a microfabricated electro-acoustic resonator

Microfabricated in-plane bulk acoustic resonators can be configured as gravimetric sensors wherein the mass of surface-bound species is transduced as a shift in the resonant frequency. Using this principle, we examine the adsorption of spherical polystyrene latex particles on the resonator surface by drying particulate laden droplets. Distribution of particles on the surface is governed by the coffee-ring effect which is correlated to the observed frequency response of the resonators. We also present an analytical approach to describe the spatial dependence of adsorbed mass on the induced frequency shift for square plate resonators which can be extended to model other resonator geometries.

Detection of phase transition in polyethylene glycol using a multimodal micromechanical acoustic resonator

A piezoelectrically transduced micromechanical beam acoustic resonator is utilized to detect solid to liquid phase transitions in sub-nanoliter volume of Polyethylene Glycol (PEG) 1000. A lower frequency flexural mode and a higher frequency length-extensional mode are simultaneously monitored for changes in the device response. The phase transition of the PEG drop perturbs the acoustic loading of the resonator, bringing about characteristic and repeatable changes in the frequency response. The phase transitions in the drop are verified optically, and the resonant parameters are compared with the control case when the resonator is pristine without the presence of any PEG drop.

Research Data Supporting "Quantifying measurement fluctuations from stochastic surface processes on sensors with heterogeneous sensitivity"

1) XQ10197_SI.pdf - the supplemental information, containing detail 1) the experimental details, 2) the derivation of the relative variance, 3) the transport and reaction of analytes and 4) the Monte Carlo simulation. 2) Lame1_10A_PS10um_LOD.xlsx - original data of the measured admittance (before feedthrough cancellation) for the particles positioned in different location on the resonator. The two port measurements were obatined using an Agilent 4936B network analyzer.

High-frequency piezoelectric-on-Si MEMS resonator and numerical method for parameter extraction

This paper presents the design and characterization of a piezoelectrically-transduced (AlN) on silicon micro-mechanical resonator operating in its lateral bulk acoustic width-extensional mode at 28.73 MHz. The equivalent m-BVD model of the resonator is extracted using a least-squares-error algorithm which is presented in this paper. We report a mechanical Q factor of 5970 and motional resistance Rx of 273 Ω in vacuum (p0 = 30 mTorr) for the fundamental bulk acoustic mode for a 240 μm × 149 μm resonator. A good fit between the m-BVD model and the experimental data is obtained using the numerical fitting algorithm.