Modelling single- and tandem-bubble dynamics between two parallel plates for biomedical applications.

Carefully timed tandem microbubbles have been shown to produce directional and targeted membrane poration of individual cells in microfluidic systems, which could be of use in ultrasound-mediated drug and gene delivery. This study aims at contributing to the understanding of the mechanisms at play in such an interaction. The dynamics of single and tandem microbubbles between two parallel plates is studied numerically and analytically. Comparisons are then made between the numerical results and the available experimental results. Numerically, assuming a potential flow, a three-dimensional boundary element method (BEM) is used to describe complex bubble deformations, jet formation, and bubble splitting. Analytically, compressibility and viscous boundary layer effects along the channel walls, neglected in the BEM model, are considered while shape of the bubble is not considered. Comparisons show that energy losses modify the bubble dynamics when the two approaches use identical initial conditions. The initial conditions in the boundary element method can be adjusted to recover the bubble period and maximum bubble volume when in an infinite medium. Using the same conditions enables the method to recover the full dynamics of single and tandem bubbles, including large deformations and fast re-entering jet formation. This method can be used as a design tool for future tandem-bubble sonoporation experiments.

Nonlinear coupling of surface and quasitransverse bulk modes in cubic crystals.

In crystals the speed of the surface acoustic wave mode may approach that of the lowest-speed quasitransverse bulk mode in some directions of propagation. Under these circumstances, it is possible for energy to be transferred from the surface mode to the bulk mode by nonlinear coupling. In the present paper we investigate the possibilities for mode coupling in the (001), (110), and (111) planes of cubic crystals. A condition is given for determining the range of propagation directions with significant coupling, and numerical results are provided for eight different crystals with a range of anisotropy ratios. It is shown that even for significant excitation amplitudes the coupling is negligible for most propagation directions in the aforementioned surface cuts.

Effects of harmonic phase on nonlinear surface acoustic waves in the (111) plane of cubic crystals.

Spectral evolution equations are used to perform numerical studies of nonlinear surface acoustic waves in the (111) plane of several nonpiezoelectric cubic crystals. Nonlinearity matrix elements which describe the coupling of harmonic interactions are used to characterize velocity waveform distortion. In contrast to isotropic solids and the (001) plane of cubic crystals, the nonlinearity matrix elements usually cannot be written in a real-valued form. As a result, the harmonic components are not necessarily in phase, and dramatic variations in waveforms and propagation curves can be observed. Simulations are performed for initially monofrequency surface waves. In some directions the waveforms distort in a manner similar to nonlinear Rayleigh waves, while in other directions the velocity waveforms distort asymmetrically and the formation of shocks and cusped peaks is less distinct. In some cases, oscillations occur near the shocks and peaks because of phase differences between harmonics. A mathematical transformation based on the phase of the matrix elements is shown to provide a reasonable approximation of asymmetric waveform distortion in cases where the matrix elements have similar phase.

Directional dependence of nonlinear surface acoustic waves in the (001) plane of cubic crystals.

Spectral evolution equations are used to perform analytical and numerical studies of nonlinear surface acoustic waves in the (001) plane of a variety of nonpiezoelectric cubic crystals. The basic theory underlying the model equations is outlined, and quasilinear solutions of the equations are presented. Expressions are also developed for a characteristic length scale for nonlinear distortion and a nonlinearity coefficient. A time-domain equation corresponding to the spectral equations is derived. Numerical calculations based on measured second- and third-order elastic constants taken from the literature are performed to predict the evolution of initially monofrequency surface waves. Nonlinearity matrix elements that indicate the coupling strength of harmonic interactions are shown to provide a useful tool for characterizing waveform distortion. The formation of compression or rarefaction shocks can be strongly dependent on the direction of propagation, and harmonic generation is suppressed or increased in certain directions.

Time-reversed sound beams of finite amplitude.

Numerical simulations based on the nonlinear parabolic wave equation are used to investigate time reversal of sound beams radiated by unfocused and focused sources. Emphasis is placed on nonlinear propagation distortion in the time-reversed beam, and specifically its effect on field reconstruction. Distortion of this kind, due to amplification during time reversal, has been observed in recent experiments [A. P. Brysev et al., Acoust. Phys. 44, 641-650 (1998)]. Effects of diffraction introduced by time-reversal mirrors with finite apertures are also considered. It is shown that even in the presence of shock formation, the ability of time reversal to retarget most of the energy on the source or focal region of the incident beam is quite robust.

Second-harmonic generation in sound beams reflected from, and transmitted through, immersed elastic solids.

Second-harmonic generation in sound beams reflected from and transmitted through thick isotropic elastic solids is investigated experimentally. Measurements of diffraction patterns are compared with a theoretical model based on integral solutions for harmonic generation in sound beams. The solutions are connected by classical linear theory for reflection and transmission at fluid-solid interfaces. Nonspecular phenomena associated with rapid phase variations near critical angles are accurately described. The principal restriction is that the solid is sufficiently thick that internal reflections may be ignored.

Second-harmonic generation in a sound beam reflected and transmitted at a curved interface.

This article presents a model for second-harmonic generation in a sound beam that is reflected from or transmitted through a curved interface. Propagation in homogeneous fluids is assumed. Simple analytic solutions are derived for the case of focused Gaussian beams. The solutions are used to illustrate the effects of focusing or defocusing due to curvature of the interface, in combination with impedance change at the interface, on energy transfer from the fundamental to the second harmonic.

PTSD and comorbid psychotic disorder: comparison with veterans diagnosed with PTSD or psychotic disorder.

Symptoms of posttraumatic stress disorder (PTSD), psychosis, general psychopathology, role functioning, violence potential, and cognitive and emotional aspects of psychotic states were compared in three groups of veterans. Groups were defined on the basis of their DSM-IV diagnoses: Psychotic disorder and war-related PTSD, war-related PTSD without psychotic symptoms, and psychotic disorder without PTSD. Veterans with PTSD and a comorbid psychotic disorder showed significantly higher levels of positive symptoms of psychosis, general psychopathology, paranoia, and violent thoughts, feelings, and behaviors than the other two groups. These data show that patients with comorbid PTSD and psychotic disorder show levels of cognitive, emotional, and behavioral disturbance that far exceed the levels of disturbance seen in patients with PTSD without psychosis or in patients with psychotic disorder.

Nonlinear distortion of short pulses radiated by plane and focused circular pistons.

Detailed measurements of finite-amplitude pulses radiated by plane and focused circular pistons in water are presented. Comparisons of time waveforms and frequency spectra, both on and off axis, are made with numerical calculations based on the nonlinear parabolic wave equation. Emphasis is on nonlinear distortion of amplitude- and frequency-modulated tone bursts. Use of short pulses enabled resolution of the direct and diffracted waves prior to their coalescence and subsequent shock formation along the axis of the source. Because of its relevance to investigations of cavitation inception, attention is devoted to variation of the peak positive (p+) and negative (p-) pressures along the axis of a focused source. It is shown that with increasing source amplitude, the maximum of each shifts away from the focal plane, toward the source. This effect is more pronounced for p- than for p+.

Analytical method for describing the paraxial region of finite amplitude sound beams.

A special analytical method, which combines the parabolic approximation (KZ equation) with nonlinear geometrical acoustics, is developed to model nonlinear and diffraction effects near the axis of a finite amplitude sound beam. The corresponding system of nonlinear equations describing waveform evolution is derived. For the case of an initially sinusoidal wave radiated by a Gaussian source, an analytic solution of the coupled equations is obtained for the paraxial region of the beam. The axial solution is expressed in both the time and frequency domains, and is analyzed in detail for both unfocused and focused beams in their preshock regions. Harmonic propagation curves are compared with finite difference solutions of the KZ equation, and good agreement is obtained for a variety of parameter values.

Measurements of harmonic generation in a focused finite-amplitude sound beam.

Measurements of harmonic generation in the sound field radiated by a focused circular piston source are compared with numerical calculations based on the nonlinear parabolic wave equation. The large dynamic range and high spatial resolution of the measurements enable precise comparisons to be made with the theoretical predictions.

A comparison of the absorption and metabolism of isopropyl alcohol by oral, dermal and inhalation routes.

The admission of a child with an almost lethal level of isopropyl (alcohol 380 mg/dl) after sponge bathing for fever, and questionable ingestion, prompted the present experiment in animals. Twelve rabbits (2-2.6 kg) were divided into 4 groups of 3 each. Groups 1 and 2 were given isopropyl alcohol, 2 and 4 ml/kg respectively, by gavage. Groups 3 and 4 were placed in an inhalation chamber with group 3 having a towel soaked with isopropyl alcohol applied to the chest. Group 4 had a similar towel placed on the chest but with a plastic layer to preclude skin contact. Average blood levels (mg/dl) of isopropanol/acetone were then measured over 4 hr. Oral absorption produced the highest levels of isopropyl alcohol and acetone, followed by inhalation and dermal. Inhalation alone was of little significance. Acetone levels continued to rise even as isopropyl levels fell following oral exposure. With the inhalation plus skin absorption group, both isopropyl and acetone levels continued to rise throughout the 4 hr time period. These results indicate that skin absorption is a significant factor in isopropyl alcohol toxicity, and the delayed rise in acetone levels may be responsible for prolonged activity.

The bird Mark 8 time cycled servo ventilator.

A bird Mark 8 ventilator has been modified by a time cycled servo mechanism to function as an inexpensive ventilator with high peak pressure capabilities. This system was designed to deliver IMV-PEEP therapy at levels not usually available on commercial ventilators. IMV rate can be varied from 1 to 60/min, Vt from 0.1 to 2 liters. Inspiratory pressure in excess of 200 torr and PEEP from 1 to 25 torr can be generated. Modifications to the Mark 8 cost $750.00. The ventilator has been successfully used in over 100 patients. Transport and operating room use are facilitated because it is solely pneumatically powered. Thus, an inexpensive, reliable and versatile ventilatory support system with high peak pressure capabilities has been developed and successfully used.