Parametric instability of a standing gravity–capillary wave is observed in conditions of irradiation of a surface of a liquid by a plane ultrasound beam modulated by amplitude. A threshold value of ultrasound intensity for excitation of the parametric oscillations of the surface is determined, and the frequency response of the process is measured. A mechanism of instability based on sound radiation pressure applied to a liquid surface curved with menisci is suggested.

The nonlinear pressure profile at the focus of an ultrasonic beam radiated into water at a frequency of 2.0 MHz and having a shock front with amplitude of 50 MPa is reconstructed for the first time using Raman lidar signals. This is possible due to the relationship between the degree of deformation of the O–H stretching vibration band in the Raman spectrum of water, the amplitude of elastic scattering, and the pressure at the sounding site. The lidar profile is in good agreement with a PVDF hydrophone profile.

It has been demonstrated for the first time that pulsed laser Raman spectroscopy can be used for diagnostics of a local acoustic pressure profile with a peak pressure drop of 50 MPa and a carrier frequency of 2.0 MHz in the focus of an ultrasound beam propagating in water. A 527-nm 10-ns laser pulse has been focused into the waist of the ultrasound beam at an angle of 90°. Backscattered photons have been recorded in a gated spectrum analyzer. It has been found that the Raman spectra at the times corresponding to the maximum and minimum acoustic pressures are significantly different. This feature has been used for point-to-point reconstruction of the acoustic pressure profile; for this purpose, the delay between the ultrasound and laser pulses is consequently increased with a step of 50 ns. It has been shown that, within the measurement error, the resulting changes in the position of the center of the stretching OH vibration band of water molecules in the Raman spectrum reproduce the acoustic pressure profile directly measured using a PVDF hydrophone at the laser sensing point. The results obtained can be used to develop a new method for remote diagnostics of the time profile of acoustic pressure and monitoring the local dynamic of the compression-tension processes in water up to critical pressures corresponding to the cavitation rupture, when the use of the hydrophone can lead to its damage.

Nonlinear parametric coupling of gravity-capillary waves (GCW) by ultrasound wave impinging on a liquid surface is studied experimentally. Modulation of the plane wave radiation pressure by dynamically curved surface of the liquid is considered as a mechanism of the coupling. The standing GCW modes are excited near antinodes by ultrasound beam of carrier frequency 1.030 MHz and coupled parametrically by a plane ultrasound wave with a frequency 1.035 MHz. The beam was modulated at resonance frequencies of GCW overtones f(2) = 5.265 Hz and f(4) = 7.873 Hz while intensity of the plane wave was modulated at frequency f(p) = 2.657 Hz that corresponds to the nonlinear parametric resonance f(p) = 2f(2) - f(4). The amplitudes of the parametric interaction of GCW triads are measured and compared with critical condition for GCWs explosive instability.

For the first time, nanoparticles are observed that result from ablation of a bulk solid-state sample in water as its surface is affected by high-intensity focused ultrasound pulses with a carrier frequency of 1.8 MHz and peak-to-peak acoustic pressure of 50 MPa in the focus. As photo and video recordings show, formation of ablation particles and their ejection into water under these extreme conditions is of local and spallation-explosive character. Size and mass distributions of the ablation particles measured using a laser particle analyzer reveal that ultrasound ablation results in formation of nanometer-sized particles among others. The size of these particles mostly ranges between 20 and 60 nm with the distribution maximum at 35 nm.

A method dedicated to the measurement of low surface tension coefficients in binary fluids is proposed. The method which is not intrusive and does not interfere with the volume/weight proportion of liquid composing the binary mixture utilizes excitation of progressive capillary waves by pulsed radiation pressure force of focused ultrasound beam. A known dispersion relationship for gravity-capillary waves is used to evaluate the surface tension coefficient. The experiments are carried out in microgravity conditions in order to make the capillary term of the relationship strongly dominant and thus to reduce artifacts in acquired data.

Ultrasonic separation of two immiscible liquids under microgravity conditions has been demonstrated on example of liquid mixtures of FC-70 or FC-72 with silicone oils of 1.5 or 1.0 cSt viscosities, respectively. A 3-MHz focused ultrasound beam was utilized to form an emulsion from these liquid combinations, and a plane wave of the same frequency served to separate (demix) fluids. Droplet deformation under pulsed ultrasound in microgravity conditions is reported, and the observed singular behavior is discussed.

The paper presents experimental results of observing the structurization effect for one of the formed elements of blood—erythrocytes—in the field of standing surface acoustic waves. Characteristic images of the striated structures formed by erythrocytes on the surface of lithium niobate as result of ultrasound action have been obtained. The results on the ultrasound structurization of erythrocytes in a blood sample and of calcium carbonate particles in an aqueous colloid solution have been comparatively analyzed. It has been noted that the achieved effect agrees qualitatively with the theoretical model of the behavior of colloid particle ensembles in an acoustic field developed by O.V. Rudenko et al.

The experimental characterization of gravity-capillary waves excited at an interface between two immiscible liquids by a periodic sequence of focused ultrasound pulses propagating perpendicular to the interface is presented. The experiments have been performed in a glass cylinder filled with two liquids: Fluorinert FC70 and silicone oil. The spatial and temporal evolution of the interface deformation is recorded by a high-speed video camera. The effect of the duration and amplitude of ultrasound pulses on the amplitude and shape of interfacial oscillations is analyzed. Prospects of the proposed approach and possible applications of the observed phenomena are discussed.

The emphasis of this study is on the ejection of single droplets of a certain size under pulsed ultrasound. Droplet ejection from an interface of two immiscible liquids in this mode, which differs from the well-known ultrasonic fountain (where liquid droplets arise spontaneously), has been experimentally implemented and investigated. The spatial and time evolution of the interface deformation and violation of interface integrity, caused by pulsed acoustic radiation pressure, has been recorded with a high-speed video camera. It is shown that, depending on the ultrasound intensity, three characteristic modes of interface response can be distinguished. In the first (low-intensity)mode, the interface undergoes forced oscillations, without violation of its integrity. In the second (intermediate-intensity) mode, which is in the focus of our study, the interface integrity is violated due to the ejection of a single droplet of a certain size; the latter continuously changes its shape when moving in the second liquid. In the third (high-intensity) mode, the predictable ejection of droplets of a predictable size turns into stochastic ejection of multiple droplets with unpredictable sizes. The dependence of the sizes of single droplets on the parameters of focused ultrasound beam have been measured in the second (stable) mode of ultrasound ejection. Based on these measurements, the range of ultrasound parameters providing controlled generation of single droplets of a specified size is estimated. Differences in the dynamics of interface motion and specific features of droplet generation for the liquid/liquid interface in comparison with the liquid/gas interface are indicated. Possible applications of the observed effects are discussed.

The nonlinear interaction between internal and sound waves in a two-component fluid due to the time modulation of the convective component of fluid interface velocity is considered.

Synchronous heating of two local regions of an absorbing medium by phase conjugate ultrasound beams focused on them has been experimentally demonstrated. A polymeric biological tissue phantom with two small air cavities scattering sound has been used as the medium irradiated by a 5-MHz “probe” ultrasound beam. The scattered field is incident on a parametric device for ultrasonic wave phase conjugation. The conjugate and amplified field is self-adaptive focused on scatterers and heats the medium owing to the absorption of the ultrasonic energy. In this case, these regions are heated by about 5°C in 70 s. Only an insignificant increase in the temperature owing to the heat conduction effect is observed in the remaining volume of the phantom. The implemented effect can be used in medical applications of phase conjugate ultrasound beams.

Experimental results on heating a biological tissue phantom using phase conjugate ultrasonic beams are presented. A polymer with acoustic properties close to those of biological tissues is used as the phantom. Heating is measured by thermocouples. The advantages of the proposed method of heating via wave phase conjugation include self-targeting of an ultrasonic beam on the region of heating and the weak effect the sample’s position or movement has on the heating parameters within a certain region.

The possibility of acoustic imaging of objects was experimentally demonstrated for the practically important case when a phase-inhomogeneous medium is placed between a transducer and an object. In such configuration, focusing of the probing beam as a rule violated because of phase distortions introduced by a layer. This makes inefficient both conventional methods and the method using the phenomenon of ultrasonic phase conjugation (PC) to compensate for phase distortions. To overcome these difficulties, it is proposed to use parametric PC of one of the higher harmonics of the probing beam. In this case, focusing is provided due to small phase distortions at the fundamental frequency. At the same time, necessary resolution and compensation for introduced distortions can be achieved by PC of one of the higher harmonics generated by the probing beam in a nonlinear medium. The proposed method was experimentally tested at parametric PC of the fifth harmonic of the focused ultrasonic beam in the transmission mode. A comparative analysis of object images obtained by various acoustic methods showed that the proposed method in certain cases is unique opportunity to obtain reliable acoustic data on an object hidden inside an inhomogeneous medium.

For several pairs of immiscible liquids, a new opportunity to excite oscillations of their interface by ultrasound pulses propagating parallel to the interface has been discovered experimentally. A plane ultrasound transducer is placed so that the interface between liquids halves its aperture. The evolution of the shape of the interface oscillations under the variation of the amplitude and duration of excitation pulses, as well as of the distance from the transducer, has been analyzed. The possibility of the excitation of various modes of the interface oscillations in a bounded volume has been revealed.

Проведены измерения акустического поглощения в сериях образцов биологических тканей – мышечной, почечной и жировой ткани свиньи с помощью стандартного метода сравнения с эталоном и с использованием обращения волнового фронта ультразвука. Сравнение полученных экспериментальных результатов и выявленные различия подтверждают перспективность использования обращенных волн для измерений акустических потерь в биологических объектах. Показано, что в неоднородных тканях метод с обращением волнового фронта позволяет получить более достоверную оценку диссипативных потерь.