A spiral wave front beacon for underwater navigation: transducer prototypes and testing

Transducers for acoustic beacons which can produce outgoing signals with wave fronts whose horizontal cross sections are circular or spiral are studied experimentally. A remote hydrophone is used to determine its aspect relative to the transducers by comparing the phase of the circular signal to the phase of the spiral signal. The transducers for a "physical-spiral" beacon are made by forming a strip of 1-3 piezocomposite transducer material around either a circular or spiral backing. A "phased-spiral" beacon is made from an array of transducer elements which can be driven either in phase or staggered out of phase so as to produce signals with either a circular or spiral wave front. Measurements are made to study outgoing signals and their usefulness in determining aspect angle. Vertical beam width is also examined and phase corrections applied when the hydrophone is out of the horizontal plane of the beacon. While numerical simulations indicate that the discontinuity in the physical-spiral beacon introduces errors into the measured phase, damping observed at the ends of the piezocomposite material is a more significant source of error. This damping is also reflected in laser Doppler vibrometer measurements of the transducer's surface velocity.     

Modeling of thermal effects in antivascular ultrasound therapy

Antivascular ultrasound consisting of low-intensity sonication in the presence of circulating microbubbles of an ultrasound contrast agent has been demonstrated to disrupt blood flow in solid cancers. In this study a mathematical framework is described for the microbubble-induced heating that occurs during antivascular ultrasound. Biological tissues are modeled as a continuum of microbubble-filled vasculature, cells, and interstitial fluids with compressibility equal to the sum of the compressibility of each component. The mathematical simulations show that the absorption of ultrasound waves by viscous damping of the microbubble oscillations induced significant local heating of the tissue vasculature. The extent and the rate of temperature increase not only depends on the properties of the microbubbles and the sonication parameters but is also influenced markedly by the blood flow. Slow flow conditions lead to higher tissue temperatures due to a stronger interaction between microbubbles and ultrasound and reduced heat dissipation. Because tumors have slower blood flow than healthy tissue, the microbubble-induced ultrasound antivascular therapy is likely to affect cancerous tissue more extensively than healthy tissue, providing a way to selectively target the vasculature of cancers.     

Using feature vectors to detect frog calls in wireless sensor networks

A method for detecting vocalization of giant barred frogs (Mixophyes iteratus) in noisy audio is proposed. Audio recordings from remote wireless sensor nodes were segmented into individual sounds and from each sound a small set of features was extracted. Feature vectors were compared to those of example calls using a Euclidean distance formula as a detection system. The system achieved a sensitivity of 0.85 with specificity of 0.92 when distinguishing M. iteratus calls from other species' calls and sensitivity of 0.88 with specificity 0.82 against background noise.     

A dimension gap for continued fractions with independent digits

Kinney and Pitcher (1966) determined the dimension of measures on [0, 1] which make the digits in the continued fraction expansion i.i.d. variables. From their formula it is not clear that these dimensions are less than 1, but this follows from the thermodynamic formalism for the Gauss map developed by Walters (1978). We prove that, in fact, these dimensions are bounded by 1−10⁻⁷. More generally, we considerf-expansions with a corresponding absolutely continuous measureμ under which the digits form a stationary process. Denote byE _δ the set of reals where the asymptotic frequency of some digit in thef-expansion differs by at leastδ from the frequency prescribed byμ. ThenE _δ has Hausdorff dimension less than 1 for anyδ>0.     

X-ray crystal structure* and kinetics of oxidation of 1,4,8,11-tetraazaundecanenickel(II), [NiII(2,3,2-tet)]2+ by peroxodisulphate: Formation of a stable non-macrocyclic nickel(III) polyamine

While macrocyclic ligands are well-known to stabilize tervalent nickel, it is very unusual to form stable nickel(III)-amine species in aqueous solution. We describe the kinetics of oxidation of 1,4,8,11-tetraazaundecanenickel(II), [Ni^II(2,3,2-tet)]²⁺ by peroxodisulphate in aqueous solution to the tervalent state. The reaction follows a second order rate law, first order in [Ni^II(2,3,2-tet)]²⁺ and first order in peroxodisulphate. The activation enthalpy and entropy are 55 ± 3 kJ mol^–1 and –38 ± 10 J K^–1 mol^–1 respectively. The X-ray crystal structure of the [Ni^II(2,3,2-tet)]²⁺ cation is also described. The title compound crystallizes in the monoclinic space group P2(1)/n. The cation has a slightly distorted square planar structure, with the bite angle of the 5-membered chelate rings being slightly less than 90°, and that of the 6-membered chelate ring being correspondingly greater than 90°.     

Mean topological dimension

In this paper we present some results and applications of a new invariant for dynamical systems that can be viewed as a dynamical analogue of topological dimension. This invariant has been introduced by M. Gromov, and enables one to assign a meaningful quantity to dynamical systems of infinite topological dimension and entropy. We also develop an alternative approach that is metric dependent and is intimately related to topological entropy.     

MOLECULAR AND SUPRAMOLECULAR ORDERING IN CONFINED ENVIRONMENTS

Crystal and phase morphologies and structures determined by self-organization of crystalline-amorphous diblockcopolymers, crystallization of the crystallizable blocks, and vitrification of the amorphous blocks are reviewed through asystematic study on a series of poly(ethylene oxide)-b-polystyrene (PEO-b-PS) diblock copolymers. On the base ofcompetitions among these three processes, molecular and supramolecular ordering in confined environments can beinvestigated. In a concentration-fluctuation-induced disordered (D_(CF)) diblock copolymer, the competition between crystalli-zation of the PEO blocks and vitrification of the PS blocks is momtored by time-resolved simultaneous small angle X-rayscattering (SAXS) and wide angle X-ray diffraction (WAXD) techniques. In the case of T_c相似文献   

Controlling the Spontaneous Precipitation of Silver Nanoparticles in Sol-Gel Materials

The mechanisms responsible for spontaneous silver precipitation in silver-doped sol-gel materials are identified. The chemistry of the solvent phase is found to be the critical factor in controlling this phenomenon. The addition of HCl as catalyst leads to the formation of AgCl and subsequent formation of silver upon light exposure. Another factor leading to silver precipitation is the reducing capability of methanol radicals. Silver precipitation is inhibited by simply washing out the pore solvents by a solvent exchange method.     

Electrospinning of Hyaluronic Acid (HA) and HA/Gelatin Blends

Summary: In the present study, electrospinning of hyaluronic acid (HA) and hyaluronic acid/gelatin (HA‐GE) blends in N,N‐dimethylformamide (DMF)/water‐mixed solvents have been investigated. When the volume ratio of DMF to water was in the range of 1.5–0.5, HA solutions could be electrospun into fibrous membranes successfully. The average diameter of HA fibers was about 200 nm. The HA‐GE composite nanofibrous membranes with varied HA/GE weight ratio in the range of 100/20–100/100 have also been successfully fabricated. The average diameter of HA‐GE fibers was in the range of 190–500 nm. The decrease in surface tension could promote fiber formation. Thus, an introduction of DMF that could decrease the surface tension distinctively, without significant change or increase in viscosity of the solution, could bypass the use of blowing‐assisted electrospinning. Our postulated picture is that the lower surface tension could help the ejection of stream with relatively high viscosity and reduce or prevent the droplet formation during the spinning process.

HA/GE (100/80) nanofibrous membrane produced by electrospinning.