Abel L. Thangawng

A simple sheath-flow microfluidic device for micro/nanomanufacturing: fabrication of hydrodynamically shaped polymer fibers

A simple sheath flow microfluidic device is used to fabricate polymer micro/nanofibers that have precisely controlled shapes and sizes. Poly(methylmethacrylate) (PMMA) was used as the model polymer for these experiments. The sheath-flow device uses straight diagonal and chevron-shaped grooves integrated in the top and bottom walls of the flow channel to move sheath fluid completely around the polymer stream. Portions of the sheath stream are deflected in such a way as to define the cross-sectional shape of the polymer core. The flow-rate ratio between the sheath and core solution determines the fiber diameter. Round PMMA fibers with a diameter as small as 300 nm and flattened fibers with a submicron thickness are demonstrated.

Thin Fresnel zone plate lenses for focusing underwater sound

A Fresnel zone plate (FZP) lens of the Soret type creates a focus by constructive interference of waves diffracted through open annular zones in an opaque screen. For underwater sound below MHz frequencies, a large FZP that blocks sound using high-impedance, dense materials would have practical disadvantages. We experimentally and numerically investigate an alternative approach of creating a FZP with thin (0.4λ) acoustically opaque zones made of soft silicone rubber foam attached to a thin (0.1λ) transparent rubber substrate. An ultra-thin (0.0068λ) FZP that achieves higher gain is also proposed and simulated which uses low-volume fraction, bubble-like resonant air ring cavities to construct opaque zones. Laboratory measurements at 200 kHz indicate that the rubber foam can be accurately modeled as a lossy fluid with an acoustic impedance approximately 1/10 that of water. Measured focal gains up to 20 dB agree with theoretical predictions for normal and oblique incidence. The measured focal radius of 0.68λ (peak-to-null) agrees with the Rayleigh diffraction limit prediction of 0.61 λ/NA (NA = 0.88) for a low-aberration lens.

Hydrodynamically directed multiscale assembly of shaped polymer fibers

A long-sought goal of material science is the development of fabrication processes by which synthetic materials can be made to mimic the multiscale organization many natural materials utilize to achieve unique functional and material properties. Here we demonstrate how the microfluidic fabrication of polymer fibers can take advantage of hydrodynamic forces to simultaneously direct assembly at the molecular and micron scales. The microfluidic device generates long fibers by initiating polymerization of a continuously flowing fluid via UV irradiation within the microfluidic channel. Prior to polymerization, hydrodynamic shear forces direct molecular scale assembly and a combination of hydrodynamic focusing and advection driven by grooves in the channel walls manipulate the cross-sectional shape of the pre-polymer stream. Polymerization subsequently locks in both molecular scale alignment and micron-scale fiber shape. This simple method for generating structures with multiscale organization could be useful for fabricating materials with multifunctionality or enhanced mechanical properties.

Underwater acoustic omnidirectional absorber

Gradient index media, which are designed by varying local element properties in given geometry, have been utilized to manipulate acoustic waves for a variety of devices. This study presents a cylindrical, two-dimensional acoustic “black hole” design that functions as an omnidirectional absorber for underwater applications. The design features a metamaterial shell that focuses acoustic energy into the shells core. Multiple scattering theory was used to design layers of rubber cylinders with varying filling fractions to produce a linearly graded sound speed profile through the structure. Measured pressure intensity agreed with predicted results over a range of frequencies within the homogenization limit.

Experimental Demonstration of Underwater Acoustic Scattering Cancellation.

We explore an acoustic scattering cancellation shell for buoyant hollow cylinders submersed in a water background. A thin, low-shear, elastic coating is used to cancel the monopole scattering from an air-filled, neutrally buoyant steel shell for all frequencies where the wavelength is larger than the object diameter. By design, the uncoated shell also has an effective density close to the aqueous background, independently canceling its dipole scattering. Due to the significantly reduced monopole and dipole scattering, the compliant coating results in a hollow cylindrical inclusion that is simultaneously impedance and sound speed matched to the water background. We demonstrate the proposed cancellation method with a specific case, using an array of hollow steel cylinders coated with thin silicone rubber shells. These experimental results are matched to finite element modeling predictions, confirming the scattering reduction. Additional calculations explore the optimization of the silicone coating properties. Using this approach, it is found that scattering cross-sections can be reduced by 20 dB for all wavelengths up to k0a = 0.85.

Underwater sound transmission through arrays of disk cavities in a soft elastic medium.

Scattering from a cavity in a soft elastic medium, such as silicone rubber, resembles scattering from an underwater bubble in that low-frequency monopole resonance is obtainable in both cases. Arrays of cavities can therefore be used to reduce underwater sound transmission using thin layers and low void fractions. This article examines the role of cavity shape by microfabricating arrays of disk-shaped air cavities into single and multiple layers of polydimethylsiloxane. Comparison is made with the case of equivalent volume cylinders which approximate spheres. Measurements of ultrasonic underwater sound transmission are compared with finite element modeling predictions. The disks provide a deeper transmission minimum at a lower frequency owing to the drum-type breathing resonance. The resonance of a single disk cavity in an unbounded medium is also calculated and compared with a derived estimate of the natural frequency of the drum mode. Variation of transmission is determined as a function of disk tilt angle, lattice constant, and layer thickness. A modeled transmission loss of 18 dB can be obtained at a wavelength about 20 times the three-layer thickness, and thinner results (wavelength/thickness ∼ 240) are possible for the same loss with a single layer depending on allowable hydrostatic pressure.

Low-frequency resonance of an oblate spheroidal cavity in a soft elastic medium

Axisymmetric monopole resonances of an oblate spheroidal cavity in a soft elastic medium are computed using both separation of variables and finite-element approaches. The resonances are obtained for compression wavelengths much longer than the cavity size and thus have a low-frequency character. Resonant frequencies for high-aspect-ratio oblate spheroids (either air-filled or evacuated) are found to be significantly lower than their spherical counterparts with equivalent volume. This finding contrasts with the case of an air bubble in water which features weak shape dependence. The results are relevant to the design of locally-resonant acoustic media using soft-lithography techniques with elastomers.

Underwater sound transmission through thin soft elastomers containing arrays of pancake voids: Measurements and modeling

Measurement of underwater sound transmission through thin (~750 micron) layers of the soft elastomer polydimethylsiloxane (PDMS) containing microfabricated arrays of pancake-shaped cavities is presented. Cavities are 120 microns in diameter and 5 microns in height with a nominal lattice spacing of 300 microns. A sound transmission minimum is found at 282 kHz which agrees with predictions of a finite-element model of the array and the value for monopole resonance frequency of an air-filled single pancake cavity in unbounded PDMS. This resonance is a factor of 0.62 lower than the null that would occur for spherical cavities of equivalent volume. The width of the null is also significantly broader than that which would be obtained with spherical voids. Modeling results incorporate careful measurements of attenuation for both shear and compression waves in PDMS done in a separate effort. Acoustic transmission variation as a function of lattice spacing and the number of layers is discussed. [Work sponsored by ...

Disk cavities in soft materials: Their bubble-like resonance and use in thin underwater sound blocking materials

The acoustics of gas-filled cavities in soft viscoelastic solids, such as rubber and gels, has been a renewed subject of research in recent years owing to usefulness in studies of multiple scattering and importance to sound insulating and anechoic materials. A brief review is followed by a presentation of recent research on disk cavity resonance done at the Naval Research Laboratory [J. Acoust. Soc. Am. 138, 2537–2547]. A lumped parameter analysis of the breathing mode of a disk cavity is presented which yields a natural frequency expression valid for a high-aspect ratio cavity embedded in an elastic medium. A verification approach using finite-element methods is also described which directly computes resonance in the framework of COMSOL Multiphysics. Calculation of scattering cross sections and visualization of the elastic displacement field indicates the importance of shear wave radiation. As an application example, a specially designed single layer array of disk cavities in a thin silicone rubber (PDMS...

Thin Fresnel zone plate lenses for underwater acoustics: Modeling and experiments

In situations where size, weight, and system complexity are important, Fresnel zone plate (FZP) lenses have potential advantages in acoustic systems. Numerical modeling and laboratory characterization of focusing by a Soret type FZP lens are presented in this study. Both rubber foam and resonant air cavity designs are considered for the opaque zones. This study extends previous work by examining a wider range of incident field angles. Measurements of a higher-order focus, more narrow than the primary focus but with reduced gain, are also presented that compare favorably with modeling predictions. Comparison is also made between standard diffraction theory predictions (Fourier acoustics) and finite-element, oblique wave scattering simulations for the axisymmetric FZP geometry. The latter accounts for edge effects and acoustic penetration through the lens material, and predicts a higher focal gain for oblique plane wave insonification. Finally, the directivity of an FZP lens and hydrophone combination is compared with that of an ideal pressure sensitive disk of equivalent diameter to assess gain vs. side-lobe tradeoffs. Calculations demonstrate 10 dB reduction of the first sidelobe level using the FZP with a 15% main lobe broadening.