High-speed ultrasound imaging of bubbly flows and shear waves in soft matter.

In this methods paper, we explore the capabilities of high-speed ultrasound imaging (USI) to study fast varying and complex multi-phase structures in liquids and soft materials. Specifically, we assess the advantages and the limitations of this imaging technique through three distinct experiments involving rapid dynamics: the underwater flow induced by an external jet, the dissolution of sub-micron bubbles in water, and the propagation of shear waves in a soft elastic material. The phenomena were simultaneously characterized using optical microscopy and USI. In water, we use compounded USI for tracking a multi-phase flow produced by a jetting bubble diving into a liquid pool at speeds around 20 m s-1. These types of jets are produced by focusing a single laser pulse below the liquid surface. Upon breakup, they create a bubbly flow that exhibits high reflectivity to the ultrasound signal, enabling the visualization of the subsequent turbulent flow. In a second experiment, we demonstrate the potential of USI for recording the diffusive shrinkage of micro- and nanobubbles in water that could not be optically resolved. Puncturing an elastic material with a liquid jet creates shear waves that can be utilized for elastography measurements. We analysed the shape and speed of shear waves produced by different types of jetting bubbles in industrial gelatin. The wave characteristics were simultaneously determined by implementing particle velocimetry in optical and ultrasound measurements. For the latter, we employed a novel method to create homogeneously distributed micro- and nanobubbles in gelatin by illuminating it with a collimated laser beam.

Clean production and characterization of nanobubbles using laser energy deposition.

We have demonstrated the production of laser bulk nanobubbles (BNB) with ambient radii typically below 500 nm. The gaseous nature of the nanometric objects was confirmed by a focused acoustic pulse that expands the gas cavities to a size that can be visualized with optical microscopy. The BNBs were produced on demand by a collimated high-energy laser pulse in a "clean" way, meaning that no solid particles or drops were introduced in the sample by the generation method. This is a clear advantage relative to the other standard BNB production techniques. Accordingly, the role of nanometric particles in laser bubble production is discussed. The characteristics of the nanobubbles were evaluated with two alternative methods. The first one measures the response of the BNBs to acoustic pulses of increasing amplitude to estimate their rest radius through the calculation of the dynamics Blake threshold. The second one is based on the bubble dissolution dynamics and the correlation of the bubble's lifetime with its initial size. The high reproducibility of the present system in combination with automated data acquisition and analysis constitutes a sound tool for studying the effects of the liquid and gas properties on the stability of the BNBs solution.

Bullet jet as a tool for soft matter piercing and needle-free liquid injection.

The collapse of a laser-induced vapor bubble near a solid boundary usually ends in a liquid jet. When the boundary is from a soft material the jetting may pierce the liquid-solid interface and result in the injection of liquid into it. A particular impulsive jet flow can be generated when a laser pulse is focused just below the free surface of a thin liquid layer covering a gelatin sample used as a surrogate of biological tissue. Here, a downwards jet forms from a liquid splash at the free surface and then penetrates through the liquid layer into the soft boundary. In the present manuscript we report on the use of this novel jet, termed "bullet" jet, to pierce soft materials and we explore its potential to become an optical needle-free injection platform. The dynamics and depth of the injection is studied as a function of the elasticity of the solid and the liquid properties. Injections of up to 4 mm deep into 4 %w/w gelatin within 0.5 ms are observed. The advantages of the bullet jet over other kinds of impulsively generated jets with lasers are discussed.

On-Demand Bulk Nanobubble Generation through Pulsed Laser Illumination.

We demonstrate the temporally and spatially controlled nucleation of bulk nanobubbles in water through pulsed laser irradiation with a collimated beam. Transient bubbles appear within the light exposed region once a tension wave passes through. The correlation between illumination and cavitation nucleation provides evidence that gaseous nanobubbles are nucleated in the liquid by a laser pulse with an intensity above 58 MW/cm^{2}. We estimate the radius of the nanobubbles through microscopic high-speed imaging and by solving the diffusion equation to be below 420 nm for ∼80% of the bubble population. This technique may provide a novel approach to test theories on existence of stable bulk nanobubbles.

Nanosecond timing and synchronization scheme for holographic pump-probe studies at the MID instrument at European XFEL.

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 light is reported. A fully electronic delay-and-trigger system has been implemented at the MID station at the European XFEL, and applied to the study of emerging laser-driven cavitation bubbles in water. Synchronization and timing precision have been characterized to be better than 1 ns.

Cross-frequency couplings in non-sinusoidal dynamics of interacting oscillators: Acoustic estimation of the radial position and spatial stability of nonlinear oscillating bubbles.

In this work, the analysis of cross-frequency couplings (CFC) is introduced in the context of nonlinear acoustics related to the dynamics of bubble(s)-resonator systems. The results obtained from experiments specifically designed to untangle the causal connection between the CFC patterns observed at the signal level and the underlying physical processes, are discussed. It was found that "causal" amplitude-to-amplitude (AAC) and amplitude-to-phase (APC) couplings emerge in the system dynamics as a consequence of the bubble(s)-resonator mechanistic interaction in the oscillatory steady-state. In these CFC patterns, the amplitude of the fundamental frequency component (f0) effectively modulates the amplitude and relative phase of the harmonic components (Nf0). Moreover, these AAC and APC couplings give rise to "epiphenomenal" phase-to-amplitude (PAC) and phase-to-phase (PPC) couplings, in which the link between modulating and modulated parameters represents a correlation rather than a causal connection. It is shown that these CFC patterns can be exploited to determine the presence, spatial stability and radial position of nonlinear oscillating bubble(s) trapped within the acoustic chamber. Potential applications of the proposed techniques are also discussed. Substantial evidence is presented showing that CFC patterns emerging from quasi-periodic non-sinusoidal waveforms are informative on the interaction between underlying oscillators.

Energy concentration and positional stability of sonoluminescent bubbles in sulfuric acid for different static pressures.

In this study we report several experimental and numerical results on the influence of static pressure (P_{0}) over the main parameters in single bubble sonoluminescence (SBSL), using a sulfuric acid aqueous solution (SA) with low concentrations of argon gas dissolved. Bifrequency driving was used in the experiments to enhance spatial stability of the bubbles. The experimental results were compared with simulations provided by a numerical code that models the radial dynamics of the bubbles. The results showed that an increase on the static pressure of the system shifts the Bjerknes instability threshold, allowing the bubble to access higher acoustic pressures (P_{Ac}^{}). Furthermore, a decrease in the measured ambient radius R_{0} and the calculated relative gas concentration c_{∞}/c_{0} were observed. A notorious increment in the bubble collapse violence and energy focusing for P_{0} above 1 bar was achieved. These were mainly indicated by the growth of the bubble expansion ratio (R_{max}/R_{0}), the bubble mechanical energy density, and the maximum bubble wall velocity dR/dt. In agreement with the previous statement, the maximum temperature during the bubble collapse predicted by the model is augmented as well. The use of different harmonics in the ultrasound pressure field regarding energy focusing is also discussed. Finally, we analyzed the stability regions of the R_{0}-P_{Ac}^{} parameter space via numerical predictions for P_{0} above the measured, identifying the shape instabilities as the main limiting agent to obtain further energy concentration in SA systems at high static pressures.

Upscaling energy concentration in multifrequency single-bubble sonoluminescence with strongly degassed sulfuric acid.

Single-bubble sonoluminescence (SBSL) was explored under a variety of multifrequency excitations. In particular, biharmonic excitation was used to produce SBSL for unprecedented low dissolved noble gas concentrations in a sulfuric acid solution. Reducing the amount of dissolved noble gas makes it possible to reach higher acoustic pressures on the SL bubble, which otherwise are not attainable because of the Bjerknes instability. By using biharmonic excitation, we were able to experimentally trap and to spatially stabilize SL bubbles for xenon pressure overhead as low as 1 mbar. As a result, we have access to regions in phase space where the plasma temperatures are higher than the ones reached before for bubbles driven at ≈30 kHz.