Biomedical subjects

Zhengyou Liu

Publications and source records attributed to Zhengyou Liu.

6 recordsLinked to original sources

PubMed2006

Acoustic quasimodes in two-dimensional dispersed random media.

Using the generalized coherent-potential-approximation approach, we present the dispersion relation of the two-dimensional dispersed random media. In the intermediate-frequency regime, two acoustic modes are found in colloidal suspensions including cylindrical plastic rod in water background. The scattering cross section offers a good explanation for the two modes and the observed frequency gaps in the excitation spectra.

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PubMed2006

Effective mass density of fluid-solid composites.

We show through rigorous derivation and experimental support that the dynamic effective mass density of an inhomogeneous mixture, used in the prediction of wave velocities in the long wavelength limit, can differ from the static version--the volume average of the component mass densities. The physical reason for this difference is explained. The dynamic mass density expression, first derived by Berryman more than two decades ago, is shown to give a closer correspondence between the acoustic and electromagnetic metamaterials by allowing for negative mass densities at frequencies around resonances.

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PubMed2004

Focusing of sound in a 3D phononic crystal.

We present a combined experimental and theoretical study of phonon focusing phenomena in a pass band above the complete band gap in a 3D phononic crystal. Wave propagation was found to depend dramatically on both frequency and incident direction. This propagation anisotropy leads to very large negative refraction, which can be used to focus a diverging ultrasonic beam into a narrow focal spot with a large focal depth. The experimental field patterns are well explained using a Fourier imaging technique, based on the 3D equifrequency surfaces calculated from multiple scattering theory.

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PubMed2004

Splitting and tuning characteristics of the point defect modes in two-dimensional phononic crystals.

Point defect modes of acoustic wave in two-dimensional square arrays of square water rods in a mercury host were studied. The defects are created by three kinds of geometry, namely, square defect, circular defect, and rectangular defect, respectively. The results show that for both square defect and circular defect, the defect modes are only related to the defect filling fraction F(d), but not with the geometry of defects (square or circular), as well as the orientations of the square defect. For the rectangular defect, the defect modes could be tuned by changing the ratio of edge width of the defect, moreover, the double degenerate one will split into two nondegenerate modes when the ratio of edge widths Lx / L(y) >9.0. Meanwhile the corresponding pressure distributions also will be changed.

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PubMed2002

Acoustic band gaps created by rotating square rods in a two-dimensional lattice.

Acoustic band gaps can be opened and tuned by rotating square rods in two-dimensional liquid sonic crystals. For the systems of mercury rods with square cross section in a water host, the width of lowest gaps increases as the rotation angle of the square rods increases. But opposite results are found for the inverse systems of water rods in mercury, where the lowest gaps narrow with an increase in the rotation angle. This gap-tuning effect becomes more evident with the filling fraction increase. Such an effect should open up a new way for designing acoustic band gaps in two-dimensional phononic crystals.

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PubMed2002

Ultrasound tunneling through 3D phononic crystals.

We report the study of ultrasound tunneling in 3D phononic crystals, consisting of fcc arrays of close-packed tungsten carbide beads in water. The transmission coefficient, phase velocity, and group velocity were measured along the [111] direction, allowing us to systematically investigate the tunneling of ultrasound at frequencies in the lowest band gap. Our experimental data are interpreted using multiple scattering theory, which provides a good explanation of our results. The effect of absorption and the difference between the tunneling of classical waves and quantum waves are discussed.

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