Strings of droplets propelled by coherent waves

Bouncing walking droplets possess fascinating properties due to their peculiar wave/particule interaction. In order to study such walkers in a 1d system, we considered the case of one or more droplets in an annular cavity. We show that, in this geometry, walking droplets form a string of synchronized bouncing droplets that share a common coherent wave propelling the group at a speed faster than single walkers. The formation of this coherent wave and the collective behavior of droplets is captured by a model.Comment: 5 Pages, 5 Figures, 2 supplementary movies (identical), supplementary .pdf fil

Waveguides for walking droplets

When gently placing a droplet onto a vertically vibrated bath, a drop can bounce without coalescing. Upon increasing the forcing acceleration, the droplet is propelled by the wave it generates and becomes a walker with a well defined speed. We investigate the confinement of a walker in different rectangular cavities, used as waveguides for the Faraday waves emitted by successive droplet bounces. By studying the walker velocities, we discover that 1d confinement is optimal for narrow channels of width of $D \simeq 1.5 \lambda_F$. We also propose an analogy with waveguide models based on the observation of the Faraday instability within the channels.Comment: 8 pages, 6 figure

Pulse-Encoded Ultrasound Imagine of the Vitreous With an Annular Array

The vitreous body is nearly transparent both optically and ultrasonically. Conventional 10- to 12-MHz diagnostic ultrasound can detect vitreous inhomogeneities at high gain settings, but has limited resolution and sensitivity, especially outside the fixed focal zone near the retina. To improve visualization of faint intravitreal fluid/gel interfaces, the authors fabricated a spherically curved 20-MHz five-element annular array ultrasound transducer, implemented a synthetic-focusing algorithm to extend the depth-of-field, and used a pulse-encoding strategy to increase sensitivity. The authors evaluated a human subject with a recent posterior vitreous detachment and compared the annular array with conventional 10-MHz ultrasound and spectral-domain optical coherence tomography. With synthetic focusing and chirp pulse-encoding, the array allowed visualization of the formed and fluid components of the vitreous with improved sensitivity and resolution compared with the conventional B-scan. Although optical coherence tomography allowed assessment of the posterior vitreoretinal interface, the ultrasound array allowed evaluation of the entire vitreous body

Non-Planar Pad-Printed Think-Film Focused High-Frequency Ultrasonic Transducers for Imaging and Therapeutic Applications

Pad-printed thick-film transducers have been shown to be an interesting alternative to lapped bulk piezoceramics, because the film is deposited with the required thickness, size, and geometry, thus avoiding any subsequent machining to achieve geometrical focusing. Their electromechanical properties are close to those of bulk ceramics with similar composition despite having a higher porosity. In this paper, padprinted high-frequency transducers based on a low-loss piezoceramic composition are designed and fabricated. High-porosity ceramic cylinders with a spherical top surface are used as the backing substrate. The transducers are characterized in view of imaging applications and their imaging capabilities are evaluated with phantoms containing spherical inclusions and in different biological tissues. In addition, the transducers are evaluated for their capability to produce high-acoustic intensities at frequencies around 20 MHz. High-intensity measurements, obtained with a calibrated hydrophone, show that transducer performance is promising for applications that would require the same device to be used for imaging and for therapy. Nevertheless, the transducer design can be improved, and simulation studies are performed to find a better compromise between low-power and high-power performance. The size, geometry, and constitutive materials of optimized configurations are proposed and their feasibility is discussed

Correspondence - Characterization of the effective performance of a high-frequency annular-array-based imaging system using anechoic-pipe phantoms

A resolution integral (RI) method based on anechoic-pipe, tissue-mimicking phantoms was used to compare the detection capabilities of high-frequency imaging systems based on a single-element transducer, a state-of-the-art, 256-element linear array or a 5-element annular array. All transducers had a central frequency of 40 MHz with similar conventionally measured axial and lateral resolutions (about 50 and 85 μm, respectively). Using the RI metric, the annular array achieved the highest performance (RI = 60), followed by the linear array (47) and the single-element transducer (24). Results showed that the RI metric could be used to efficiently quantify the effective transducer performance and compare the image quality of different systems

Follow the guide

When gently placing a droplet onto a vertically vibrated bath, under specific conditions, coalescence may be avoided. The drop bounces permanently. Upon increasing the forcing acceleration, the drop interacts with the wave it generates, and becomes a walker [1,2]. Recently, some 2D confining systems for walking droplets have been developed: cylindrical cavity, harmonic potential or the use of Coriolis force [3,4]. In addition, the interactions between two identical walkers have been studied in a 2D case [5]. Nevertheless, no study focuses on 1D dynamics and their properties. In this work, we show it is possible to confine a walker in a quasi mono-dimensional geometry by using a submerged annular cavity. We focus on the interactions between droplets, and show the interdistance quantization. Then, we study the speed of pairs of walkers and show that the distance between the drops affects the group speed: the closer the drops are, the faster they move. We also propose a numerical model to characterize the distance quantization, and the evolution of the speed of a string of droplets. Finally, we investigate the case of a string of droplets. We discuss the influence of the number of droplets and the distance between droplets on the string speed. 1. Y. Couder, S. Protière, E. Fort, and A. Boudaoud, Nature 437, 208 (2005). 2. S. Protière, A. Boudaoud, and Y. Couder, J. Fluid Mech. 554, 85 (2006). 3. S. Perrard, M. Labousse, M. Miskin, E. Fort, and Y. Couder, Nat. Commun. 5, 3219 (2014). 4. M. Labousse and S. Perrard, Phys. Rev. E 90, 022913 (2014). 5. C. Borghesi, J. Moukhtar, M. Labousse, A. Eddi, E. Fort, and Y. Couder, Phys. Rev. E 90, 063017 (2014).Quandrop

Modélisation par une méthode pseudospectrale : différences-finies et fabrication de transducteurs ultrasonores pour l'imagerie médicale haute résolution

L'imagerie ultrasonore haute résolution est aujourd'hui indispensable à l'établissement de nombreux diagnostics médicaux. Elle est basée sur l'utilisation de transducteurs piézoélectriques haute fréquence (HF) qui sont fabriqués à l'aide de matériaux et de techniques adaptés. L'optimisation de leurs performances nécessite en général l'utilisation de plusieurs modèles numériques pour simuler à la fois la vibration du résonateur et son rayonnement. Ce travail de thèse présente un nouvel algorithme basé sur un couplage pseudospectral / différences-finies, permettant de modéliser la génération et la propagation des ondes acoustiques dans un transducteur et son environnement avec un seul modèle. Cet algorithme hybride a été utilisé pour simuler le fonctionnement de différents transducteurs fabriqués au cours de cette thèse. Les résultats ont été validés de façons théorique et expérimentale, et ils ont permis d'étudier les influences de différents paramètres sur les performances des transducteurs fabriquésHigh resolution ultrasonic imaging has become an essential tool to assist physicians for various medical diagnoses. This technique relies on the ability of piezoelectric transducers to generate a high frequency (HF) acoustic field into the scanned media. Such transducers are obtained using particular materials and fabrication processes.They are optimized through modeling of their electromechanical behaviour and acoustic radiation pattern, which usually requires the use of several models. A new algorithm has been developped and is presented in this report, which is based on the coupling of pseudospectral and finite-diffrence methods to simulate both the generation and the propagation of acoustic waves in the transducer and the surrounding media, using a single model. This hybrid algorithm has been used to simulate various transducers and the results were accordingly compared to theory and experiments. It has also been used to study the influence of various parameters on the performance of several single-element and array transducer devices