Christopher S. Kwiatkowski

Resonator frequency shift due to ultrasonically induced microparticle migration in an aqueous suspension: Observations and model for the maximum frequency shift

Aqueous suspensions of plastic and hollow-glass microspheres are subjected to the radiation pressure of an ultrasonic standing wave. It is well known that the particles are attracted to the pressure nodes of the standing wave for the particle and host properties considered. We demonstrated that the radiation pressure induced migration of particles leads to a significant shift in the resonant frequency of a suitably designed chamber. This shift is easily resolved with a phase-locked loop even if the particle volume fraction is as small as 0.001. For sufficient ultrasonic amplitudes, the shift is found to saturate at a limiting value. To approximate the limiting frequency shift, the principle of adiabatic invariance is first applied to the case of a single compressible sphere in a standing wave and then superposition is used to give the collective shift for the suspension. The measured limiting shifts are typically within 25% of the calculated value and this method may have application to the characterizati...

Measurement and interpretation of the impulse response for backscattering by a thin spherical shell using a broad‐bandwidth source that is nearly acoustically transparent

A novel source was developed to produce a plane‐wave unipolar pressure impulse with a wide range of frequency components. The source consisted of a PVDF sheet with water in contact with both sides. The PVDF was driven by a step voltage. This source is nearly acoustically transparent and was used for backscattering from an empty stainless‐steel spherical shell. The shell was placed in the near field of the source where it experienced a plane‐wave pressure impulse followed much later by edge contributions resulting from the finite source size. A hydrophone was placed in the far field of the scatterer on the opposite side of the source. Prominent features in the shell’s calculated impulse response are observed over a wide frequency interval. Time records reveal an approximately Gaussian wave packet from the excitation of the subsonic a0− wave associated with the backscattering enhancement near the coincidence frequency (≊309 kHz). Superposed on the same records are large contributions from the low‐frequency ...

Direct deflection method for determining refractive-index profiles of polymer optical fiber preforms

We present a method for determining the refractive-index profile of polymer optical fiber preforms through a direct-deflection measurement. The method is simple to use, compact, and has good resolution. The profile is obtained from the deflection data by numerically integrating the differential-ray equation for a radial refractive-index gradient. Corrections for topographical deviations are also discussed. Results for both graded-index and step-index fibers are presented.

Refractive index profiles of polymer optical fiber preforms

We report a method for determining the refractive index profile of polymer optical fiber preforms by direct beam deflection measurements. The method is simple to use, compact, and has good resolution. The profile is obtained from the deflection data by numerically integrating the differential ray equation for a radial refractive index gradient. Refractive index profiles of both graded-index (GRIN) and step-index fiber preforms are reported.

Low‐power, cylindrical, air‐coupled acoustic levitation/concentration devices

The detection, identification, and separation of aerosols have applications in a wide variety of fields. Particle size, size distribution, and particle composition are quantities that are important to scientists studying both the environmental impact of industrial smog and the detection of chemical or biological agents in air. One common rule in aerosol detection is that most aerosol detection devices (particle sizers, optical classifiers, etc.) are more sensitive at higher aerosol concentrations. The present research describes a new class of acoustic aerosol concentration devices that alleviate many of the alignment and power consumption requirements of traditional acoustic levitators/concentrators [Kaduchak et al., Rev. Sci. Instrum. 73, 1332–1336 (2002)]. These devices are inexpensive, low‐power, and never require alignment. They are constructed from a single PZT tube. The devices operate on the premise that the lowest‐order structural modes of the tube are tuned to match a corresponding resonant mode ...

Ultrasonic aerosol concentration and positioning

The detection, identification, and separation of aerosols have potential applications in a wide vari…

Acoustic particle manipulation devices driven by resonantly excited, cylindrical structures

An overview of acoustic particle manipulation devices that use resonantly excited structures as the displacement generators will be discussed. These devices rely on coupling of the structural modes of a hollow, cylindrical tube to the acoustic modes set up in its interior cavity. For practical applications, discussed for both particles in air and in liquids, the structural vibrations are tuned such that desired modal responses of the cavity are obtained. The effects on particle manipulation due to different internal configurations and symmetry‐breaking of the cross‐sectional geometry will be shown in experimental data and videos. Specific designs of these devices have demonstrated that they can be engineered into power‐efficient assemblies. It will be shown that different structural and cavity modifications can be made to meet the resonance matching condition for several different azimuthal mode numbers. Material and geometry considerations for nonresonantly driven devices will also be discussed. Applicat...

Particle sedimentation measurements using active resonator shifts

A measurement of the properties of fluids with suspended particles is important in fields such as biology, chemistry, geology, and industrial processing. The limitation of being inside closed or otherwise inaccessible containers provides a challenge to typical measurement systems, especially where an invasive technique is either disadvantageous or impossible. These situations occur in opaque containers or within pre‐existing pipes. The technique presented describes using an active all‐ultrasonic technique, where the time‐averaged radiation force on particles in an acoustic standing wave causes them to migrate. The migration of particles has a measurable effect on the resonant frequency of the cavity that depends on the properties of the particles and the host fluid. This technique is used to initially determine the presence of suspended particles in a fluid, to characterize the particles, and provide a measurement of the total volume fraction of suspended particles. The measurement of the total volume fra...

Single‐transducer continuous‐wave noninvasive ultrasonic characterization of fluids

Ultrasonic fluid characterization studies that utilize a continuous wave (cw) frequency‐sweep technique typically employ a source transducer and a receiver transducer. This arrangement can lead to problems such as transducer misalignment, diffraction effects, and mechanical and electrical coupling. In contrast, the traditional tone‐burst technique that uses a single transducer for both transmit and receive functions can alleviate some of the problems associated with the cw technique. However, the frequency‐sweep technique allows for much higher signal‐to‐noise ratios and observation of relevant phenomena that occur early in time that can be missed by the tone‐burst technique. Reflection‐mode single‐transducer techniques have been developed in the past, but these usually require high‐precision bridge circuits and exact balancing circuitry to provide satisfactory results over a narrow frequency range. Due to impedance‐matching issues, the dynamic range of such measurements can be problematic. We describe a ...

A frequency‐domain technique for noninvasive, ultrasonic fluid detection in submerged sealed canisters

There are situations where canisters containing specialized parts and/or materials are submerged underwater for storage or safety purposes. In some cases, leaks in the canisters can be detrimental and must be detected as early as possible. The present research describes an ultrasonic method for determining the presence of fluid in sealed, submerged canisters. In this research, the canisters are metallic and are filled with a random array of loose objects. The technique is based on two phenomena: the radiation damping effect of the internal fluid on the container wall and spectral signatures that result from internal reflections within the canister. The experiment utilizes a pair of piezoelectric transducers with a lateral separation of 27 mm. The transducers are placed at a standoff distance of 5 mm from the container wall and traverse the vertical dimension of the canister on a stepper‐slide assembly. The transmitter is swept between 0.8–4.0 MHz and a frequency spectrum is collected. By analyzing the con...