Zhemin Zhu

Sensitivity of Lamb wave sensors in liquid sensing

The differences of the sensitivities of liquid- and solid-sensing of Lamb wave sensors are discussed. It is shown that the sensitivity of S/sub 0/ mode in liquid sensing is much smaller than that in solid sensing, and also much smaller than that of the A/sub 0/ mode.

Acoustic nonlinearity of liquid containing encapsulated microbubbles

Recently, a high value nonlinear parameter of liquid containing contrast agents (encapsulated microbubbles) has been reported by some measurements, however an effective way to study it theoretically has not been found so far. In this paper, an approach, based on the concept of the equivalent nonlinear parameter is presented to estimate the nonlinear property of such media. The liquid containing encapsulated microbubbles is treated as a homogenous liquid with a equivalent nonlinear parameter. An experimental approximate linear relationship for attenuation and nonlinear parameter with bubble volume is confirmed from our calculation. It is also found that when a microbubble is encapsulated by a shell, the nonlinear oscillation of the bubble is restrained; the second and higher harmonics are generated not so effectively compared to a free bubble. Naturally this causes the nonlinear parameter of such a liquid to decrease greatly. Favorable agreement between the theory and Wu et al.’s experiments shows this app...

A general dispersion relation for Lamb-wave sensors with liquid-layer loading

Abstract Lamb-wave devices have been exploited to measure biochemical processes in liquids. Proteins or other biomolecules that absorb to the plate surface can be treated as mass loading to the devices; consequently, the plate-wave velocity decreases. A general dispersion equation for Lamb waves in a plate with asymmetric inviscid liquid-layer loading on two sides of the plate is derived. A thin plate bordered with one inviscid liquid layer, which is the case more relevant to Lamb-wave sensors, is treated as a special case of the general equation and discussed in detail. The numerical results calculated from the dispersion relation are presented.

Theory of acoustic streaming generated by ultrasonic Lamb waves

A theory on acoustic streaming generated by Lamb waves propagating in a thin composite membrane was developed. Using this theory, the mass-transport velocity in water, which is loaded on a 4-μm-thick membrane, due to Lamb waves at 4.7 MHz was calculated. The results are in good agreement with the experimental values measured by Moroney et al. [Appl. Phys. Lett. 59, 774 (1991)].

A perturbation analysis of Lamb-wave sensors

Abstract General dispersion relations for the antisymmetrical (A) and symmetrical (S) Lamb-wave-like modes of a thin plate symmetrically bordered by two thin layers have been derived using a perturbation approach. The derived dispersion relations were applied to treat the effects of thin layers (liquid layers, polymer layers or solid layers), which were in direct contact with the plate on the propagation of Lamb waves. The analytical expressions of the first-order approximation of the change of wave-numbers and the attenuation coefficients for non-solid layers were obtained. It has been shown that for non-solid-layer loading, the A0 mode is more sensitive than the S0 mode. For the A0 mode, the sensitivity of mass loading for any layers is approximately equal to 1 m s for two layers, where ms is the mass per unit area of the plate.

A note about acoustic streaming: Comparison of C. E. Bradley’s and W. L. Nyborg’s theories

Some comparisons of C. E. Bradley’s and W. L. Nyborg’s theories on acoustic streaming are given.

Acoustic Nonlinearity of Porous Viscoelastic Medium

An approach based on the equivalent medium method is used to describe the propagation of acoustic longitudinal wave in porous viscoelastic medium. Combining the wave equation for the porous medium and the dynamic equation for micropores and using the perturbation method, equations for fundamental and second harmonic are obtained. Treating medium containing micropores as a homogeneous medium, the effective nonlinearity parameter is defined, and a formula for it is achieved. The frequency dependence of effective nonlinear parameter is studied in detail, and its low frequency limit consists with Ostrovsky’s static results.

The shock formation distance in a bounded sound beam of finite amplitude

This paper investigates the shock formation distance in a bounded sound beam of finite amplitude by solving the Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation using frequency-domain numerical method. Simulation results reveal that, besides the nonlinearity and absorption, the diffraction is another important factor that affects the shock formation of a bounded sound beam. More detailed discussions of the shock formation in a bounded sound beam, such as the waveform of sound pressure and the spatial distribution of shock formation, are also presented and compared for different parameters.

Theory on mass transport generated by ultrasonic Lamb waves

It was reported that acoustic streaming generated by a Lamb‐wave sensor (LWS) can be used to pump fluids and transport solids in a small‐scale system [Appl. Phys. Lett. 59 (1991)]. The speed, observed by attracting small polystyrene spheres suspended in water, is proportional to the square of the wave amplitude and is about 100 μm/s for 6.5‐nm surface displacement of the plate. In this paper, a fairly practical model, i.e., a thin isotropic plate in contact with a viscous liquid layer on its side, is presented. Based on the rigorous propagation theory of Lamb waves and Nyborg’s [Phys. Acoust. B 2, 265–295] and Bradley’s [J. Acoust. Soc. Am. 100, 1399–1408 (1996)] theories of acoustic streaming, the first‐order ultrasonic field in the viscous liquid layer and the second‐order mass‐transport velocity distribution of the liquid are calculated. Comparison between our theoretical values of the Ao‐mode Lamb wave and Moroney et al.’s experimental result is given. It shows good agreement. [Work supported by NSF o...

A perturbation method applied in a study of Lamb wave sensors

There are basically two approaches in studies of sensing using Lamb wave sensors [J. Acoust. Soc. Am. 91, 861–867 (1992)]: the dispersion equation method and the acoustic impedance method. The first approach is exact but complicated; the latter is simple but approximate. In many Lamb wave sensing applications, changes of the boundary conditions to be detected can be considered to be small perturbations to the propagation of free Lamb wave in a thin plate. General dispersion equations for Lamb waves in a plate with proper perturbed boundary conditions are derived. The equations are then applied to various sensing applications including viscous mass loading. [Work of JWU supported by NSF and Vermont EPSCoR.]