M. Pluta

Age-Dependent Acoustic and Microelastic Properties of Red Blood Cells Determined by Vector Contrast Acoustic Microscopy

Variations of the mechanical properties of red blood cells that occur during their life span have long been an intriguing task for investigations. The research presented is based on noninvasive monitoring of red blood cells of different ages performed by scanning acoustic microscopy with magnitude and phase contrast. The characteristic signature of fixed cells from groups of three different ages fractionated according to mass density is obtained from the acoustic microscope images, with the data represented in polar graphs. The analysis of these data enables the determination of averaged values for the velocities of ultrasound propagating in the cells from the different groups ranging from (1,681 ± 16) m s(-1) in the youngest to (1,986 ± 20) m s(-1) in the oldest group. The determined bulk modulus varies with age from (3.04 ± 0.05) GPa to (4.34 ± 0.08) GPa. An approach to determine for an age-mixed population of red blood cells, collected from a healthy person, the age of the individual cells and the age dependence of the cell parameters including density, velocity, and attenuation of longitudinal polarized ultrasonic waves traveling in the cells is demonstrated.

Imaging of acoustic waves in piezoelectric ceramics by Coulomb coupling

The transport properties of bulk and guided acoustic waves travelling in a lead zirconate titanate (PZT) disc, originally manufactured to serve as ultrasonic transducer, have been monitored by scanned Coulomb coupling. The images are recorded by excitation and detection of ultrasound with local electric field probes via piezoelectric coupling. A narrow pulse has been used for excitation. Broadband coupling is achieved since neither mechanical nor electrical resonances are involved. The velocities of the traveling acoustic waves determined from the images are compared with characteristic velocities calculated from material properties listed by the manufacturer of the PZT plate.

Scattering and attenuation of surface acoustic waves and surface skimming longitudinal polarized bulk waves imaged by Coulomb coupling

Coulomb coupling has been applied for vector contrast imaging of surface acoustic waves and surface skimming longitudinal polarized waves travelling on or in a 5 mm thick, Y-Z cut Lithium Niobate single crystal. The excitation and detection of acoustic waves was performed by Coulomb coupling. For excitation 20 oscillations at a frequency of 89.9 MHz were used. The scattering and attenuation of both, surface skimming and surface acoustic waves are studied by imaging in vector contrast. Objects employed for interaction with the waves are layers of absorptive and scattering materials. The interaction allows a clear differentiation of volume waves skimming the surface and guided waves traveling at the surface.

Lattice dynamics approach to determine the dependence of the time-of-flight of transversal polarized acoustic waves on external stress

Based on the lattice dynamics approach the dependence of the time-of-flight (TOF) on stress has been modeled for transversal polarized acoustic waves. The relevant dispersion relation is derived from the appropriate mass-spring model together with the dependencies on the restoring forces including the effect of externally applied stress. The lattice dynamics approach can also be interpreted as a discrete and strictly periodic lumped circuit. In that case the modeling represents a finite element approach. In both cases the properties relevant for wavelengths large with respect to the periodic structure can be derived from the respective limit relating also to low frequencies. The model representing a linear chain with stiffness to shear and additional stiffness introduced by extensional stress is presented and compared to existing models, which so far represent each only one of the effects treated here in combination. For a string this effect is well known from musical instruments. The counteracting effects are discussed and compared to experimental results.

Metamorphosis of bulk waves to Lamb waves in anisotropic piezoelectric crystals

Acoustic bulk waves were excited by local electric field probe in an anisotropic piezo-electric crystal Lithium Niobate (X-cut). A narrow pulse with a width of 25 ns was used for excitation to obtain wide frequency content in the Fourier domain. A wide spectrum ensures metamorphosis of bulk waves into Lamb waves for scan lengths comparable to the involved wavelengths. The low frequency content experiences multiple reflections from the two surfaces of the plate and disperses along the propagation direction. Acoustic bulk waves evolution and transformation to Lamb waves are illustrated and explained with the aid of the Lamb wave dispersion phenomenon. The holographic images in the Fourier domain exemplify the metamorphosis of waves during propagation following the excitation at an approximate point source.

Generalized representations and universal aspects of Lamb wave dispersion relations

Due to the dependence on a limited amount of parameters, the dispersion relations of Lamb waves can be presented in a generalized way. This is exemplified for the different established typical representations. Special attention is given to the representation of the momentum on energy, which is well suited to discuss basic features since energy as well as momentum is the properties which are strictly conserved in loss free homogeneous materials. Representations involving the phase and group velocity are discussed. Features related to level crossing of interacting modes and relations to basic mechanical properties especially relevant to Lamb waves are included in the presentation and discussion.

Dispersion Relations and Stress Dependencies of Acoustic Waves Travelling on a Chain of Point Masses Connected by Linear and Torsional Springs

The mechanical dynamics of a linear chain model consisting of identical mass points subject to a direction and distance dependent potential are investigated analytically and numerically. The model employs the usual linear springs between mass points and in addition torsional springs at each point mass to represent directional forces. Dynamic equations based on interactions with next and second next neighbors are derived. Results for the dispersion relation of longitudinal and transversal polarized acoustic waves are exemplified. For selected parameters and respective classes of solutions the waves phase and group velocities as well as the dependence of the time-of-flight on externally applied stress are discussed. The obtained theoretical results are compared with experimental observations of the acousto-elastic coefficient for Lamb waves propagating in an aluminum plate under variable in plane stress. The developed models are capable to demonstrate general features of the mode and frequency dependence of the acousto-elastic coefficients.

Stress dependent dispersion relations of acoustic waves travelling on a chain of point masses connected by anharmonic linear and torsional springs

The propagation of a deformation along a flexural beam or plate depends on material properties, geometrical conditions like the beam cross-section, effects of stiffening or softening due to external stress, and last but not least the mode of the wave including its polarization. The time-of-flight (TOF) of acoustic waves is influenced by any of the above listed parameters. This effect is utilized in ultrasonic NDE and structural health monitoring applications. It was shown in earlier publications that the solutions of wave equations for a linear chain model consisting of identical mass points, subject to a direction and distance dependent potential, show the dispersion properties and dependencies on externally applied stress of the lowest longitudinal and transversal plate modes. In the model presented here anharmonic potentials are introduced. The potentials are represented by torsional springs at each mass point and linear springs between them. Dynamic equations are derived, based on interactions with ne...

Characterization of acoustic lenses with the Foucault test by confocal laser scanning microscopy

In this work, the Foucault knife-edge test, which has traditionally been known as the classic test for optical imaging devices, is used to characterize an acoustic lens for operation at 1.2 GHz. A confocal laser scanning microscope (CLSM) was used as the illumination and detection device utilizing its pinhole instead of the classical knife edge that is normally employed in the Foucault test. Information about the geometrical characteristics, such as the half opening angle of the acoustic lens, were determined as well as the quality of the calotte of the lens used for focusing. The smallest focal spot size that could be achieved with the examined lens employed as a spherical reflector was found to be about 1 μm. By comparison to the idealized resolution a degradation of about a factor of 2 can be deduced. This limits the actual quality of the acoustic focus.

Surface acoustic wave generation and detection by Coulomb excitation

Excitation and detection of acoustic waves in piezoelectric materials relies on a gradient in the piezoelectric properties respectively a gradient in the electric field. The relatively weak coupling is usually enhanced for established practical applications by mechanical, geometrical and electrical resonances. The geometrical resonances, as present for the commonly used inter digital transducer (IDT), lead to limitations concerning the spatial and temporal resolution that can be achieved with such devices. Concentration of the electric field by geometrical means and point like conversion at the surface of piezoelectric materials is the basis for the novel scheme presented here. The principles of the developed method together with instrumental details are discussed. Applications involving two dimensional imaging with time resolved recording for each pixel of the image for phase and magnitude of the transfer and echo signals are presented.