Biomedical subjects
Publications and source records attributed to X L Wu.
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We use measurements of swimming bacteria in an optical trap to determine fundamental properties of bacterial propulsion. In particular, we directly measure the force required to hold the bacterium in the optical trap and determine the propulsion matrix, which relates the translational and angular velocity of the flagellum to the torques and forces propelling the bacterium. From the propulsion matrix, dynamical properties such as torques, swimming speed, and power can be obtained by measuring the angular velocity of the motor. We find significant heterogeneities among different individuals even though all bacteria started from a single colony. The propulsive efficiency, defined as the ratio of the propulsive power output to the rotary power input provided by the motors, is found to be approximately 2%, which is consistent with the efficiency predicted theoretically for a rigid helical coil.
Efficient resonant electron transfer from the surface bonding structure to the conduction band of quantum confined Si nanocrystals is observed by Si nanocrystals in a toluene suspension. Based on the electron transfer mechanism, the enhancement of photoluminescence originates from the band-to-band recombination in the p-type Si nanocrystals suspended in a toluene solution. The energy levels of the electrons in the Si nanocrystals chemisorbed with toluene molecules are calculated using the method of linear combination of atomic orbitals, and the characteristics of the obtained density of states is in good agreement with the observed photoluminescence properties.
111-oriented p-type Si wafer with a resistivity of 1-5 Omega cm was implanted with Fe+ and then annealed at 1100 degrees C in N2 for 60 min, followed by anodization in a solution of HF to form porous structure with beta-FeSi2 nanocrystallites. Photoluminescence (PL) spectral measurements show that a strong PL peak appears in the range of 610-670 nm. The position of the PL peak remains unchanged, but its intensity increases with the storage time in air until about three months and then saturates. C60 molecules were chemically coupled on the porous structure through a kind of silane coupling agent to form a nanocomposite. It is revealed that the stable PL peak monotonically shifts to a pinning wavelength at 570 nm. Experimental results from PL, PL excitation, Raman scattering, and x-ray diffraction measurements clearly show that the pinned PL originates from optical transition in C60-related defect states, whereas the photoexcited carriers occur in the beta-FeSi2 nanocrystallites formed during anodization. This work opens a new way to tailor nanometer environment for seeking optimal luminescent properties.
Photoluminescence (PL) measurements have been performed on the nanocomposites of higher fullerene-coupled porous silicon (PS) nanocrystals. For the C70PS and C76(78)PS nanocomposites, the PL spectra show a pinning wavelength at approximately 565 nm and for the C84PS and C94PS nanosystems the pinning wavelength is at approximately 590 nm. The PL pinning property is closely related to the sorts of the coupled fullerenes. A band mixing model of direct and indirect gaps in a nanometer environment consisting of nc-Si core, SiO2 surface layer, and coupled fullerene has been proposed for calculation of electronic states. Good agreement is achieved between the experiments and theory.
The single-point (SP) velocity statistics are investigated in forced and decaying two-dimensional turbulence in a flowing soap film. It is shown that the probability distribution functions (PDF) in both cases deviate from a Gaussian distribution, which is normally anticipated in turbulent fluid flows. In the forced turbulence case, the tail of the SP velocity PDF decays faster than Gaussian (termed the sub-Gaussian) and can be correlated with the forcing statistics on small scales. In the decaying-turbulence case, the SP velocity PDF evolves from a sub-Gaussian to a super-Gaussian behavior as a function of time. However, in all times, the locally averaged vorticity remains normally distributed. While our forced turbulence data may be explained by a recent theory proposed by Falkovich et al., the decaying-turbulence data remain unexplained.
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It is generally believed that two-dimensional turbulence is immune to intermittency possibly due to the absence of vortex stretching. However, in turbulence created in a freely suspended soap film by electromagnetic forcing, it is found that intermittency is not insignificant. We draw this conclusion based on the measured velocity structure function Sp(l) (identical to ) proportional to l(zeta(p)) on scales l greater than the energy injection scale l(inj) . The scaling exponent zeta(p) vs p deviates from the expected linear relation and shows intermittent behavior comparable to that observed in fully developed three-dimensional turbulence in wind tunnels. Our measurements demonstrate that intermittency can be accounted for by the nonuniform distribution of saddle points in the flow.
In a two-dimensional soap film convection experiment, the velocity fields are found to be strongly intermittent in the buoyancy subrange, which was reported to be nonintermittent in a recent numerical simulation. The structure functions Sq(l)(= ) exhibit self-similar structures and can be described by power laws l(zetaq) for integers 8 > or = q < or = 1. By extending Kolmogorov's refined similarity hypothesis to our system, an analytical form is derived for the scaling exponent zeta(q) = q/2 + (mu/18)(3q - q2). Our measurements yield mu = 0.42, which is significantly greater than 0.2 found in high Reynolds number turbulence in wind tunnels.
We present a study of density fluctuations in two-dimensional soap-film convection. When the temperature difference (DeltaT) imposed vertically along the film is smaller than a critical value (DeltaT(C) approximately 48 K), the convective motion is in a strongly stratified state and the frequency power spectrum of density fluctuations shows a Bolgiano-like scaling f(-7/5) in the buoyancy subrange. When DeltaT>DeltaT(C), the fluid motion crosses over to a strongly mixed state characterized by the emergence of a large-scale circulation. The density power spectrum in this state has a passive-scalar-like scaling f(-1.0).
The wake in the flow past a circular cylinder has posed a long-standing challenge to scientists since the late 19th century. Many aspects of this seemingly simple phenomenon remain unexplained. Of particular interest is the relationship between the dimensionless vortex shedding frequency (the Strouhal number St) and the ratio of inertial to viscous forces in the fluid (the Reynolds number Re). We propose a new St-Re relation based on the observations of the structure of a vortex street in flowing soap films. The measurements suggest a simple two-parameter form St=1/(A+B/Re) that describes laminar vortex shedding remarkably well for bulk fluids as well as for two-dimensional flowing soap films.
Using electrochemical etching of a polycrystalline 3C-SiC target and subsequent ultrasonic treatment in water solution, we have fabricated suspensions of 3C-SiC nanocrystallites that luminesce. Transmission electron microscope observations show that the 3C-SiC nanocrystallites, which uniformly disperse in water, have sizes in the range of 1-6 nm. Photoluminescence and photoluminescence excitation spectral examinations show clear evidence for the quantum confinement of 3C-SiC nanocrystallites with the emission band maximum ranging from 440 to 560 nm. Tunable, composite polystyrene/SiC film can be made by adding polystyrene to a toluene suspension of the 3C-SiC nanocrystallites and then coating the resulting solution onto a Si wafer.
Identification of major genes, that genetically impact fat tissue formation is important for successful selection of lean animals with good meat quality. Because of its central role in fat cell differentiation and muscle fibre type determination, PPARGC1 is a potential candidate gene affecting fattening traits and pig meat quality. In this study, a T/A substitution at position 1378 (GenBank accession no. AY346131) in the porcine PPARGC1 gene causing a Cys430Ser amino acid substitution at position 430 was genotyped on a total of 239 animals, including 101 from seven Chinese and 138 from six Western pig breeds. Bayesian analysis revealed that the mean frequency of allele T (Cys) was 92.64 +/- 4.82% in Chinese pigs, and 45.99 +/- 4.13% in Western pigs. The 95% interval of the posterior mean frequency of allele T was 0.82-1.00 in Chinese pigs and 0.38-0.54 in Western pigs, indicating these two groups of pigs diverged at this locus during genetic evolution of the breed. Because marked differences in fat and lean tissue deposition exist between Western and Chinese pig breeds, this Cys430Ser exchange in the PPARGC1 gene deserves further evaluation to determine its phenotypic effect on fattening and carcass traits in commercial pig populations.
Several reports have demonstrated that bovine chromosome 26 (BTA26) harbours significant or suggestive quantitative trait loci (QTL) for milk production and composition traits in dairy cattle. Our previous study showed that a C/T substitution in the bovine TCF7L2 gene on BTA26 was significantly linked to QTL for protein yield (PY) in a Canadian dairy cattle population. Actually, this polymorphism was one of the markers derived from a genome-wide screening of QTL for milk PY using an amplified fragment length polymorphism technique combined with a DNA pooling strategy. In the present study, 990 Holstein bulls with complete genotype and phenotype data from 14 sire families were analysed to confirm, if the QTL effects exist in other populations. Statistical analysis revealed that this marker was significantly associated with PY, protein percentage, milk yield and fat yield (FY) (p < 0.001) in the US Holstein population. These results indicate that this QTL region has a pleiotrophic effecton different milk traits and is portable in different populations.
Two-dimensional (2D) colloids embedded in a soap film are employed to study the structures and dynamics of grain boundaries (GBs). While a soap film is drawn vertically from a concentrated bulk suspension, the colloidal particles self-assemble to form polycrystalline structures with a typical grain size of about 100 lattice spaces. Studies of these grain boundaries reveal that the GB lines are faceted at "atomic" scale, leading to a constant energy per unit length along each line. The measured GB energy as a function of misfit angle is in agreement with Reed-Shockley's theoretical predictions. When an external mechanical vibration is applied to the bulk suspension, the grain boundaries are observed to migrate and grains to rotate under the stress field induced by the vibration. The total length of the grain boundaries is found to diminish logarithmically with time, with the rate constant being a function of the excitation strength. This phenomenon, called "mechanical annealing," provides a robust means to grow large 2D single-domain crystals.
Using C60 molecule as a kind of surface-passivated agent to modify the electronic structure of Si nanocrystallites in porous silicon, we disclose that this kind of C60/nanocrystalline Si coupling system can show a strong blue emission at approximately 460 nm when stored in air for more than one year. After a full characterization of the photoluminescence properties, we propose a luminescent center in the SiOx layer at the surface of a Si nanocrystallite. It is a pair consisting of an oxygen vacancy and an interstitial oxygen. The interstitial oxygen also forms a peroxy linkage with a neighboring lattice oxygen. Radiative recombination of carriers photogenerated from Si nanocrystallite cores in the luminescent centers results in the observed blue photoluminescence. Neutron irradiation experiments support our assignment of the blue emission mechanism. This work improves the understanding of the origin of blue emission from silicon/oxygen-related nanostructured materials.
According to the HRTEM study, the UHP jadeite-quartzite mineral (Rutile, TiO(2)) in Anhui Province, Dabie Mountains, China, has ultrastructures such as 011 two-dimensional commensurable modulated structures or superstructures, [011] twin domain structures, dislocations and crystal deformations. The SAED patterns and HRTEM images indicate the existence of the deformations and stacking faults on the interface of [011] twin crystal of rutile and its two-dimensional commensurate modulated structures with repetition period 0.753 nm (3d(011)) has tetragonal symmetry, cell parameters a = 3a0 = 1.377 nm (a0 = 0.459 nm), c = c0 = 0.3 nm. The modulated structures of rutile were probably caused by the isomorphic replacement of Ti(4+) and position modulation or occupation modulation of oxygen atoms in different degree; the deformation structures reveal that during the process of crystallization and mineralization, this mineral may be affected by the geological environment (such as temperature, pressure and stress), metamorphism and deformation.