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29 Oct 2012

Volume 101, Issue 18, Articles (18xxxx)

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Appl. Phys. Lett. 101, 183101 (2012); http://dx.doi.org/10.1063/1.4761935 (4 pages)

S. J. Kim, J. J. Lee, H. J. Kang, J. B. Choi, Y.-S. Yu, Y. Takahashi, and D. G. Hasko
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Broadband sound absorption by lattices of microperforated cylindrical shells

Victor M. García-Chocano, Suitberto Cabrera, and José Sánchez-Dehesa

Appl. Phys. Lett. 101, 184101 (2012); http://dx.doi.org/10.1063/1.4764560 (4 pages)

Online Publication Date: 29 October 2012

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Absorption of broadband noise by sonic crystals consisting of microperforated cylindrical shells is proposed and experimentally demonstrated. The theoretical study has been performed in the framework of multiple scattering method, where a model for the T matrix of the microperforated shells has been developed. It has been predicted an extraordinary broadband sound absorption that is explained in terms of the multiple scattering phenomena occurring at the surfaces of the absorptive units—the microperforated panels. Our proposal has been supported by experiments performed on a structure consisting of 3 rows of cylindrical shells 3 meters height.
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43.20.Fn Scattering of acoustic waves
43.40.-r Structural acoustics and vibration
43.50.Gf Noise control at source: redesign, application of absorptive materials and reactive elements, mufflers, noise silencers, noise barriers, and attenuators, etc.

Nanoscale surface roughness affects low Reynolds number flow: Experiments and modeling

R. Jaeger, J. Ren, Y. Xie, S. Sundararajan, M. G. Olsen, and B. Ganapathysubramanian

Appl. Phys. Lett. 101, 184102 (2012); http://dx.doi.org/10.1063/1.4764293 (5 pages)

Online Publication Date: 29 October 2012

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Most micro-channel fabrication strategies generate nano-to-micro-scale, stochastic surface roughness. This inherent stochasticity can potentially be harnessed to direct microfluidic operations such as self-cleaning behavior and localized mixing. This work investigates the effect of stochastic nanoscale roughness on low to moderate Reynolds number Newtonian flow using concurrent modeling and experiments. We fabricate a microscopic channel with tailored hydrofluoric-acid-etched rough surfaces. Optical profilometry and micro-particle-image-velocimetry (micro-PIV) are used to characterize the surface roughness and flow field and is integrated with direct numerical simulation that resolves effects of nanoscale roughness. Results indicate that nanoscale roughness causes flow perturbations that extend up to the mid-plane and is insensitive to flow-rates.
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47.60.Dx Flows in ducts and channels
47.85.Np Fluidics
47.11.Fg Finite element methods
47.80.Jk Flow visualization and imaging
47.15.G- Low-Reynolds-number (creeping) flows
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Dynamic photoacoustic spectroscopy for trace gas detection

C. M. Wynn, S. Palmacci, M. L. Clark, and R. R. Kunz

Appl. Phys. Lett. 101, 184103 (2012); http://dx.doi.org/10.1063/1.4764515 (4 pages)

Online Publication Date: 2 November 2012

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We present a method of photoacoustic spectroscopy in which a laser beam tuned to an absorption feature of a gas is swept through its plume at the speed of sound. The resulting coherent addition of acoustic waves leads to an amplification of the signal without the need for a resonant chamber, thus enhancing the ability to remotely sense the gas. We demonstrate the concept using a tunable CO2 laser and SF6 gas in conjunction with a microphone. Sound pressure levels of 83 dB (relative to 20 μPa) are generated from a 15-ppm plume.
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82.80.Kq Energy-conversion spectro-analytical methods (e.g., photoacoustic, photothermal, and optogalvanic spectroscopic methods)
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing

Upper limit power for self-guided propagation of intense lasers in plasma

Wei-Min Wang, Zheng-Ming Sheng, Ming Zeng, Yue Liu, Zhi-Dan Hu, Shigeo Kawata, Chun-Yang Zheng, Warren B. Mori, Li-Ming Chen, Yu-Tong Li, and Jie Zhang

Appl. Phys. Lett. 101, 184104 (2012); http://dx.doi.org/10.1063/1.4765056 (4 pages)

Online Publication Date: 2 November 2012

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It is shown that there is an upper-limit laser power for self-focusing of a laser pulse in plasma in addition to the well-known lower-limit critical power set by the relativistic effect. This upper limit is caused by the transverse ponderomotive force of the laser, which tends to expel plasma electrons from the laser propagating area. Furthermore, there is a lower-limit plasma density for a given laser spot size, below which self-focusing does not occur for any laser power. Both the lower-limit density and the upper-limit power are derived theoretically and verified by two-dimensional and three-dimensional particle-in-cell simulations. It is also found that plasma channels may be unfavorable for stable guiding of lasers above the upper-limit power.
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52.38.Hb Self-focussing, channeling, and filamentation in plasmas
52.65.Rr Particle-in-cell method
52.25.-b Plasma properties
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