Robert Mettin

University of Göttingen | GAUG · III. Physical Institute

Light emissions from cavitating liquids serve as a diagnostic tool for chemical activity, bubble collapse conditions, or excited species. Here we demonstrate the influence of mechanical stirring on sonoluminescence (SL) and sonochemiluminescence (SCL) emissions emerging in the presence of dissolved sodium salts and luminol in different sonicated li...

Cavitation in supercooled water has been induced by the short ultrasound pulses of an ultrasonic horn driven at 20 kHz. The cavitation during the ultrasonic pulses and occasionally the crystallization events thereafter have been imaged by a high-speed camera. The probability of ice crystallization in dependence on the pulse duration and temperature...

Single bubble sonoluminescence (SBSL) is the phenomenon of synchronous light emission due to the violent collapse of a single spherical bubble in a liquid, driven by an ultrasonic field. During the bubble collapse, matter inside the bubble reaches extreme conditions of several gigapascals and temperatures on the order of 10 000 K, leading to picose...

In this work, we study the jetting dynamics of individual cavitation bubbles using x-ray holographic imaging and high-speed optical shadowgraphy. The bubbles are induced by a focused infrared laser pulse in water near the surface of a flat, circular glass plate, and later probed with ultrashort x-ray pulses produced by an x-ray free-electron laser...

(Abstract according to original publication under OpenAccess CC BY license in Fluids (8) https://www.mdpi.com/2369726): A bubble expanding and collapsing near a solid boundary develops a liquid jet toward the boundary. The jet leaves a torus bubble and induces vortices in the liquid that persist long after the bubble oscillations have ceased. The v...

The dynamics of a single, laser-induced cavitation bubble on top of a solid cylinder and right at a plane solid boundary is studied both experimentally and numerically. The most intriguing phenomenon that occurs for a millimeter sized bubble right at a flat solid boundary in water is the formation of a fast jet that is directed towards the solid wi...

We explore the pressure evolution and structural dynamics of transient phase transitions in a microfluidic water jet after a laser-induced dielectric breakdown. To this end, we use a combined approach of nearfield holography with single femtosecond X-ray free-electron laser pulses and X-ray diffraction. We observe chaotic perturbations with thin fi...

A model for acoustic cavitation flows able to depict large geometries and time scales is proposed. It is based on the Euler-Lagrange approach incorporating a novel Helmholtz solver with a non-linear acoustic attenuation model. The method is able to depict a polydisperse bubble population, which may vary locally. The model is verified and analyzed i...

Cavitation bubbles close to solid surfaces have been studied intensively in the last decades. Nevertheless, the issue is still good for surprises. A striking example is the formation of very fast jets from bubbles oscillating in close proximity to a flat solid surface. These jets result from self-impact of annular inflow at the axis of symmetry and...

The dynamics of a laser-induced bubble on top of a solid cylinder is studied both experimentally and numerically. When the bubble is generated close to the flat top along the axis of the cylinder and its maximum radius exceeds the one of the flat top surface, it collapses in the form of a mushroom with a footing on the cylinder, a long stem and a h...

Cavitation bubbles can be seeded from a plasma following optical breakdown, by focusing an intense laser in water. The fast dynamics are associated with extreme states of gas and liquid, especially in the nascent state. This offers a unique setting to probe water and water vapor far-from equilibrium. However, current optical techniques cannot quant...

It is shown that a liquid slug in gas-liquid segmented flow in microchannels can act as an acoustic resonator to disperse large amounts of small liquid droplets, commonly referred to as atomization, into the gas phase. We investigate the principles of acoustic resonance within a liquid slug through experimental analysis and numerical simulation. A...

Single-pulse holographic imaging at XFEL sources with 10¹² photons delivered in pulses shorter than 100 fs reveal new quantitative insights into fast phenomena. Here, a timing and synchronization scheme for stroboscopic imaging and quantitative analysis of fast phenomena on time scales (sub-ns) and length-scales (≲100 nm) inaccessible by visible li...

Experimental analysis of cavitation bubble dynamics typically uses optical imaging and photographic recording. However, the images are often severely affected by distortions and shadows due to refraction and total reflection of the illuminating light at the liquid–gas interface of the bubble. Optical ray tracing may become a powerful tool for the a...

X-ray free-electron lasers (XFELs) have opened up unprecedented opportunities for time-resolved nano-scale imaging with X-rays. Near-field propagation-based imaging, and in particular near-field holography (NFH) in its high-resolution implementation in cone-beam geometry, can offer full-field views of a specimen’s dynamics captured by single XFEL p...

Bubble dynamics and luminol emissions of cavitation in sub-millimeter-sized PFA flow tubes, submerged in an ultrasonic bath reactor, are studied at 27 kHz driving frequency. Nucleation of cavitation inside the tubes only takes place via a free interface, realized here in form of an alternating water-air slug flow. High-speed recordings show that ca...

A small spherical bubble of high internal pressure is inserted into water at constant ambient pressure as a model of a laser-induced bubble. Its subsequent dynamics near a flat solid boundary is studied in dependence on the distance of the bubble to the boundary by numerically solving the Navier-Stokes equations with the help of the open source sof...

A feedforward control technique is presented to steer a harmonically driven, non-linear system between attractors in the frequency–amplitude parameter plane of the excitation. The basis of the technique is the temporary addition of a second harmonic component to the driving. To illustrate this approach, it is applied to the Keller–Miksis equation d...

A feedforward control technique is presented to steer a harmonically driven, non-linear system between attractors in the frequency–amplitude parameter plane of the excitation. The basis of the technique is the temporary addition of a second harmonic component to the driving. To illustrate this approach, it is applied to the Keller–Miksis equation d...

A feedforward control technique is presented to steer a harmonically driven, non-linear system between attractors in the frequency–amplitude parameter plane of the excitation. The basis of the technique is the temporary addition of a second harmonic component to the driving. To illustrate this approach, it is applied to the Keller–Miksis equation d...

Ultrasonic microreactors are increasingly applied to particle synthesis, crystallization processes, or organic synthesis involving solids due to the clogging prevention offered by ultrasound irradiation. However, the further application of this technology is limited by the scalability of ultrasonic flow reactors. In this work, we combined experimen...

Experimental analysis of cavitation bubble dynamics typically uses optical imaging. However, images are often severely affected by distortions and shadows due to refraction and total reflection at the liquid--gas interface. Here it is shown that optical ray-tracing can be a powerful tool for the analysis process by assisting in the comparison of ex...

A small, spherical bubble of high internal pressure is inserted into water at constant ambient pressure as a model of a laser-induced bubble. Its subsequent dynamics near a flat solid boundary is studied in dependence on the distance of the bubble to the boundary by numerically solving the Navier-Stokes equations with the help of the open source so...

The main aim of this paper is to demonstrate the benefit of the application of high-performance computing techniques in the field of non-linear science through two kinds of dynamical systems as test models. It is shown that high-resolution, multi-dimensional parameter scans (in the order of millions of parameter combinations) via an initial value p...

The mutual interaction and structure formation of cavitation bubbles in strong sound fields is still not fully understood. Here, we analyse the temporal evolution of an ensemble of bubbles seeded by a laser pulse in concentrated phosphoric acid and driven by an ultrasonic standing wave field. The bubbles nucleate at random positions in a cylindrica...

The active cavitation threshold of a dual-frequency driven single spherical gas bubble is studied numerically. This threshold is defined as the minimum energy required to generate a given relative expansion (Rmax-RE)/RE, where RE is the equilibrium size of the bubble and Rmax is the maximum bubble radius during its oscillation. The model employed i...

Bubble dynamics and luminol emissions of cavitation in sub-millimeter-sized perfluoroalkoxy alkane (PFA) flow tubes, submerged in an ultrasonic bath reactor, are studied at 27.2 kHz driving frequency. Nucleation of cavitation inside the tubes only takes place via a free interface, realized here in form of an alternating water-air slug flow. High-sp...

A parameter identification technique of an underlying bubble model of an experimentally observed single bubble in a cluster under dual-frequency external forcing is presented. The measurements are carried out via high-speed camera recordings at a rate of 162750 frames per second. The used frequencies during the experiment are 25 kHz and 50 kHz. Wit...

This book presents the latest research on fundamental aspects of acoustic bubbles, and in particular on various complementary ways to characterize them. It starts with the dynamics of a single bubble under ultrasound, and then addresses few-bubble systems and the formation and development of bubble structures, before briefly reviewing work on isola...

A bubble expanding and collapsing in water near a flat solid boundary is studied numerically. According to current knowledge, it develops a high-speed liquid jet towards the boundary of typically ∼100 m/s. However, the character of jet formation and the jet properties alter strongly when the bubble expands and collapses very close to a solid bounda...

Bubble dynamics and cavitation have been recognized as a relevant topic of physics and engineering for more than 100 years. Starting with erosion problems at ship propellers end of the nineteenth century [1, 2], experimental and theoretical research went on to intense ultrasound fields in liquids after World War I [3]. However, the phenomena are in...

Zusammenfassung Ultraschall kann in Flüssigkeiten Lichtblitze erzeugen. Diese Sonolumineszenz stammt aus implodierenden Kavitationsblasen. Das Gas in kollabierenden Blasen kann sich dabei so stark erwärmen, dass kurzzeitig ein Plasma mit blitzartiger Lichtemission auftritt. Aus dem optischen Spektrum des Leuchtens schließt man auf die physikalische...

Sonochemical splitting of thermodynamically very stable water molecule provides the evidence for drastic conditions inside the cavitation bubble. Kinetics of OH• radicals or H2O2 molecules formation during sonolysis of water can be used for quantification of acoustic power delivered to the system. This chapter focuses on the influence of several fu...

A detailed experimental study on acoustically-induced bubble jets produced far from any solid wall is presented. This particular type of bubble jets is observed when a laser-induced gas cavity is seeded in a liquid by an optical dielectric breakdown while applying a strong ultrasonic field. This generation method allows a high degree of control ove...

Video corresponding to Figure 3 of the article Acoustically induced bubble jets, https://doi.org/10.1063/1.5063011

A novel method to control multistability of nonlinear oscillators by applying dual-frequency driving is presented. The test model is the Keller–Miksis equation describing the oscillation of a bubble in a liquid. It is solved by an in-house initial-value problem solver capable to exploit the high computational resources of professional graphics card...

In analytical and numerical studies on bubbles in liquids, often the Rayleigh initial condition of a spherical bubble at maximum radius is used: the Rayleigh case. This condition cannot be realized in practice, instead the bubbles need first to be generated and expanded. The energy-deposit case with its initial condition of a small, spherical bubbl...

An electrochemical high-speed wall shear raster microscope is presented. It involves chronoamperometric measurements on a microelectrode that is flush-mounted in a submerged test specimen. Wall shear rates are derived from the measured microelectrode signal by numerically solving a convection-diffusion equation with an optimization approach. This w...

Acoustic cavitation typically forms a variety of bubble structures of generally unknown and broad size distributions. As the bubbles strongly oscillate, their (equilibrium) sizes are not directly observable. Here, a method is presented to experimentally determine the size distribution in bubble populations from high-speed imaging of the bubbles in...

The interest in application of ultrasonic cavitation for cleaning and surface treatment processes has increased greatly in the last decades. However, not much is known about the behavior of cavitation bubbles inside the microstructural features of the solid substrates. Here we report on an experimental study on dynamics of acoustically driven (38.5...

The acoustic waves being generated during the motion of a bubble in water near a solid boundary are calculated numerically. The open source package OpenFOAM is used for solving the Navier-Stokes equation and extended to include nonlinear acoustic wave effects via the Tait equation for water. A bubble model with a small amount of gas is chosen, the...

The application of ultrasonic cavitation in chemical processing is a widespread method to accelerate dissolution processes. In recycling technology, often bulk material is introduced into a reactor forming a packed bed of granular particles. Thereby, the cavitation activity at the material surfaces is of interest. In this numerical study, the packe...

When a cavitation bubble oscillates and collapses in the vicinity of a solid boundary (a substrate), it induces intense microconvection in the surrounding liquid and—of high practical importance—directly at the substrate. As the involved flows are fast, highly unsteady, and possess an impressive shear, experiments are difficult and data are scarce....

Multibubble sonoluminescence (MBSL) is the emission of light from imploding cavitation bubbles in dense ensembles or clouds. We demonstrate a technique of high-speed recording that allows imaging of bubble oscillations and motion together with emitted light flashes in a nonstationary multibubble environment. Hereby a definite experimental identific...

A laboratory filtration plant for drinking water treatment is constructed to study the conditions for purely mechanical in-situ cleaning of fouled polymeric membranes by the application of ultrasound. The filtration is done by suction of water with defined constant contamination through a membrane module, a stack of five pairs of flat-sheet ultrafi...

The dynamics of acoustic cavitation bubbles can be complicated due to their nonlinear nature. They comprise several aspects on different spatial and temporal scales: The interplay of bubble and sound field leads to volume oscillations and partly strong implosion of the gas phase, which induces further effects like chemical reactions and luminescenc...

Despite the increasing use of high frequency ultrasound in heterogeneous reactions, knowledge about the spatial distribution of cavitation bubbles at the irradiated solid surface is still lacking. This gap hinders controllable surface sonoreactions. Here we present an optimization study of the cavitation bubble distribution at a solid sample using...

The detailed link of liquid phase sonochemical reactions and bubble dynamics is still not sufficiently known. To further clarify this issue, we image sonoluminescence and bubble oscillations, translations, and shapes in an acoustic cavitation setup at 23 kHz in sufuric acid with dissolved sodium sulfate and xenon gas saturation. The colour of sonol...

Cavitation bubbles collapsing in the vicinity to a solid substrate induce intense micro-convection at the solid. Here we study the transient near-wall flows generated by single collapsing bubbles by chronoamperometric measurements synchronously coupled with high-speed imaging. The individual bubbles are created at confined positions by a focused la...

In single-bubble experiments in acids, frequently moving bubbles are observed, termed m-SBSL. In a setup in phosphoric acid, driven at 25 kHz, we closer inspect the bubble dynamics. It is found that the bubble can split into two symmetric fragments, recovering into one bubble again in the rebound. This process is repeated in every period, and then...

The importance of bubble dynamics in acoustic cavitation of sound-irradiated liquids is emphasized. Starting from a discussion of cavitation thresholds, the oscillation of single, spherical bubbles in sound fields is described for various parameters (sound pressure amplitude, bubble radius, and acoustic frequency), including response curves and hab...

The aim of this study is to investigate acoustic streaming in water at very high ultrasonic frequencies, namely beyond 100MHz. At such high frequencies, the dissipation length of acoustic waves shrinks considerably, and the acoustic streaming transforms from the well-known Eckart type into a Stuart-Lighthill type: While Eckart streaming is driven b...

Laser generated cavitation bubbles are numerically modeled with the finite volume method using the open source software package OpenFOAM. The alterations applied to the native code are described and validated by solving the spherical bubble collapse problem and comparing the solution with the Gilmore model and experiments. Problems of spherical sta...

Acoustic cavitation at a 20 kHz ultrasonic horn is investigated by means of high-speed imaging and particle image velocimetry. In one experimental set-up, bubble dynamics is visualized synchronously with the acoustic streaming liquid flow to reveal their connection. By switching an elevated static pressure, cavitation can be turned off and on for o...

We report on the numerical modeling of laser-induced cavitation bubbles with the finite volume method using the open source software package OpenFOAM. The numerical model is validated by comparison to experimental data for the two cases of a bubble collapsing under normal ambient conditions in an unbounded liquid as well as close to a solid wall. (...

The dynamics of collapsing bubbles close to a flat solid is investigated with respect to its potential for removal of surface attached particles. Individual bubbles are created by nanosecond Nd:YAG laser pulses focused into water close to glass plates contaminated with melamine resin micro-particles. The bubble dynamics is analysed by means of sync...

The interaction of two laser-induced bubbles in bulk water is investigated. The strength and direction of the emerging liquid jets can be controlled by adjusting the relative bubble positions, the time difference between bubble generation, and the laser pulse energies determining the bubble sizes. Experimental and numerical studies are performed fo...

The impact of a droplet onto a dry or wet surface leads to a rapid formation of a shear flow at the boundary. We present a novel method to experimentally resolve this flow in time at different heights above the solid. The radial flow field close to the substrate is reconstructed by evaluation of streak images of fluorescent tracer particles in the...

For the first time, we apply a high speed imaging technique to record the activity of acoustically-driven cavitation bubbles (86 kHz) on micro patterned surfaces with hydrophobic and hydrophilic stripes. The width of the hydrophobic stripes lies between 3.5 and 115 ?m. This work provides the first direct visualization of preferential location of bu...

Technologies including (3D-) (bio-)printing, diesel engines, laser-induced forward transfer, and spray cleaning require optimization and therefore understanding of micrometer-sized droplets impacting at velocities beyond 10 m s(-1). However, as yet, this regime has hardly been addressed. Here we present the first time-resolved experimental investig...

The dynamics of acoustic cavitation bubbles can be complicated due to their nonlinear nature. They comprise several aspects on different spatial and temporal scales: The interplay of bubble and sound field leads to volume oscillations and partly strong implosion of the gas phase, which induces further effects like chemical reactions and luminescenc...

High-speed recordings reveal peculiar details of the oscillation and translation behavior of cavitation bubbles in the vicinity of an ultrasonic horn tip driven at 20 kHz. In particular, a forward jump during collapse that is due to the rapid reduction of virtual mass is observed. Furthermore, frequently a jetting in the translation direction durin...

Ultrasonic horn transducers are frequently used in applications of acoustic cavitation in liquids. It has been observed that if the horn tip is sufficiently small and driven at high amplitude, cavitation is very strong, and the tip can be covered entirely by the gas/vapor phase for longer time intervals. A peculiar dynamics of the attached cavity c...

The interaction of acoustically driven bubbles with a wall is important in many applications of ultrasound and cavitation, as the close boundary can severely alter the bubble dynamics. In this paper, the non-spherical surface oscillations of bubbles near a surface in a weak acoustic standing wave field are investigated experimentally and numericall...

A liquid jet that is ejected from a nozzle into air will disintegrate into drops via the well-known Plateau-Rayleigh instability within a certain range of Ohnesorge and Reynolds numbers. With the focus on the micrometer scale, we investigate the control of this process by superimposing a suitable ultrasonic signal, which causes the jet to break up...

Ultrasonic horn transducers are frequently used in applications of acoustic cavitation in liquids, for instance, for cell disruption or sonochemical reactions. They are oper- ated typically in the frequency range up to about 50 kHz and have tip diameters from some mm to several cm. It has been observed that if the horn tip is sufficiently small and...