Magnetic Stirring Device for Limiting the Sedimentation of Cells inside Microfluidic Devices.

In experiments considering cell handling in microchannels, cell sedimentation in the storage container is a key problem because it affects the reproducibility of the experiments. Here, a simple and low-cost cell mixing device (CMD) is presented; the device is designed to prevent the sedimentation of cells in a syringe during their injection into a microfluidic channel. The CMD is based on a slider crank device made of 3D-printed parts that, combined with a permanent magnet, actuate a stir bar placed into the syringe containing the cells. By using A549 cell lines, the device is characterized in terms of cell viability (higher than 95%) in different mixing conditions, by varying the oscillation frequency and the overall mixing time. Then, a dedicated microfluidic experiment is designed to evaluate the injection frequency of the cells within a microfluidic chip. In the presence of the CMD, a higher number of cells are injected into the microfluidic chip with respect to the static conditions (2.5 times), proving that it contrasts cell sedimentation and allows accurate cell handling. For these reasons, the CMD can be useful in microfluidic experiments involving single-cell analysis.

Induction of Microstreaming by Nonspherical Bubble Oscillations in an Acoustic Levitation System.

When located near biological barriers, oscillating microbubbles may increase cell membrane permeability, allowing for drug and gene internalization. Experimental observations suggest that the temporary permeabilization of these barriers may be due to shear stress that is exerted on cell tissues by cavitation microstreaming. Cavitation microstreaming is the generation of vortex flows which arise around oscillating ultrasound microbubbles. To produce such liquid flows, bubble oscillations must deviate from purely spherical oscillations and include either a translational instability or shape modes. Experimental studies of bubble-induced flows and shear stress on nearby surfaces are often restricted in their scope due to the difficulty of capturing shape deformations of microbubbles in a stable and controllable manner. We describe the design of an acoustic levitation chamber for the study of symmetry-controlled nonspherical oscillations. Such control is performed by using a coalescence technique between two approaching bubbles in a sufficiently intense ultrasound field. The control of nonspherical oscillations opens the way to a controlled cavitation microstreaming of a free surface-oscillating microbubble. High-frame rate cameras allow investigating quasi-simultaneously the nonspherical bubble dynamics at the acoustic timescale and the liquid flow at a lower timescale. It is shown that a large variety of fluid patterns may be obtained and that they are correlated to the modal content of the bubble interface. We demonstrate that even the high-order shape modes can create large-distance fluid patterns if the interface dynamics contain several modes, highlighting the potential of nonspherical oscillations for targeted and localized drug delivery.

Acoustic microstreaming produced by nonspherical oscillations of a gas bubble. IV. Case of modes n and m.

This paper is the conclusion of work done in our previous papers [A. A. Doinikov et al., Phys. Rev. E 100, 033104 (2019)10.1103/PhysRevE.100.033104; Phys. Rev. E 100, 033105 (2019)10.1103/PhysRevE.100.033105]. The overall aim of the study is to develop a theory for modeling the velocity field of acoustic microstreaming produced by nonspherical oscillations of a gas bubble. In our previous papers, general equations were derived to describe the velocity field of acoustic microstreaming produced by modes m and n of bubble oscillations. Particular cases of mode interaction were derived, such as the 0-n, 1-1, 1-m, and n-n interactions. Here the general case of interaction between modes n and m, n>m, is solved analytically. Solutions are expressed in terms of complex mode amplitudes, meaning that the mode amplitudes are assumed to be known and serve as input data for the calculation of the velocity field of microstreaming. No restrictions are imposed on the ratio of the bubble radius to the viscous penetration depth. The n-m interaction results in specific streaming patterns: At large distance from the bubble interface the pattern exhibits 2|n-m| lobes, while 2min(m,n) lobes exist in the bubble vicinity. The spatial organization of the recirculation zones is unique for the interaction of two distinct nonspherical modes and therefore appears as a signature of the n-m interaction.

Subharmonic spherical bubble oscillations induced by parametric surface modes.

A potential source of subharmonic bubble emissions is revealed experimentally by high-speed imaging. When an acoustic bubble is driven at sufficiently large pressure amplitudes, energy transfer from surface to volume oscillations can lead to the triggering of subharmonic spherical oscillations. This experimental evidence is in agreement with recent theoretical modeling of nonspherical bubble dynamics accounting for nonlinear mode coupling. Implications for the monitoring of stable cavitation activity are discussed.

Acoustic microstreaming produced by nonspherical oscillations of a gas bubble. III. Case of self-interacting modes n-n.

This paper is the continuation of work done in our previous papers [A. A. Doinikov et al., Phys. Rev. E 100, 033104 (2019)2470-004510.1103/PhysRevE.100.033104; Phys. Rev. E 100, 033105 (2019)].2470-004510.1103/PhysRevE.100.033105 The overall aim of the study is to develop a theory for modeling the velocity field of acoustic microstreaming produced by nonspherical oscillations of an acoustically driven gas bubble. In our previous papers, general equations have been derived to describe the velocity field of acoustic microstreaming produced by modes m and n of bubble oscillations. After solving these general equations for some particular cases of modal interactions (cases 0-n, 1-1, and 1-m), in this paper the general equations are solved analytically for the case that acoustic microstreaming results from the self-interaction of an arbitrary surface mode n≥1. Solutions are expressed in terms of complex mode amplitudes, meaning that the mode amplitudes are assumed to be known and serve as input data for the calculation of the velocity field of acoustic microstreaming. No restrictions are imposed on the ratio of the bubble radius to the viscous penetration depth. The self-interaction results in specific streaming patterns: a large-scale cross pattern and small recirculation zones in the vicinity of the bubble interface. Particularly the spatial organization of the recirculation zones is unique for a given surface mode and therefore appears as a signature of the n-n interaction. Experimental streaming patterns related to this interaction are obtained and good agreement is observed with the theoretical model.

Acoustic microstreaming produced by nonspherical oscillations of a gas bubble. I. Case of modes 0 and m.

A theory is developed that allows one to model the velocity field of acoustic microstreaming produced by nonspherical oscillations of an acoustically driven gas bubble. It is assumed that some of the bubble oscillation modes are excited parametrically and hence can oscillate at frequencies different from the driving frequency. Analytical solutions are derived in terms of complex amplitudes of oscillation modes, which means that the mode amplitudes are assumed to be known and serve as input data when the velocity field of acoustic microstreaming is calculated. No restrictions are imposed on the ratio of the bubble radius to the viscous penetration depth. The present paper is the first part of our study in which a general theory is developed and then applied to the case that acoustic microstreaming is generated by the interaction of the breathing mode (mode 0) with a mode of arbitrary order m≥1. Examples of numerical simulations and a comparison with experimental results are provided.

Acoustic microstreaming produced by nonspherical oscillations of a gas bubble. II. Case of modes 1 and m.

This paper continues a study that was started in our previous paper [A. A. Doinikov et al., Phys. Rev. E 100, 033104 (2019)10.1103/PhysRevE.100.033104]. The overall aim of the study is to develop a theory for modeling the velocity field of acoustic microstreaming produced by nonspherical oscillations of an acoustically driven gas bubble. In the previous paper, general equations were derived that describe the velocity field of acoustic microstreaming produced by modes n and m of bubble oscillations. In the present paper, the above equations are solved analytically in the case that acoustic microstreaming is the result of the interaction of the translational mode (mode 1) with a mode of arbitrary order m≥1. Solutions are expressed in terms of complex mode amplitudes, which means that the mode amplitudes are assumed to be known and serve as input data for the calculation of the velocity field of acoustic microstreaming. No restrictions are imposed on the ratio of the bubble radius to the viscous penetration depth. Analytical results are illustrated by numerical examples.

Contrast Agent Microbubble Jetting during Initial Interaction with 200-kHz Focused Ultrasound.

The initial response of microbubbles flowing through a 500-µm polycarbonate capillary to a burst of 200-kHz focused ultrasound, at peak-negative pressure amplitudes from 0.7-1.5 MPa, was investigated with dual-perspective high-speed imaging. Directed jetting through the acoustic focus is demonstrated according to the pressure gradients acting across the cavitating microbubbles. At lower amplitudes, repeated microbubble-jetting is accompanied by sudden, intermittent translation. At higher amplitudes a rebound jet also forms, before disintegration into a cavitation cloud.

Cold wire constant voltage anemometry to measure temperature fluctuations and its application in a thermoacoustic system.

The knowledge of temperature fluctuations is essential for most thermoacoustic systems. In the present paper, cold wire constant-voltage anemometry (CVA) to measure temperature fluctuations is presented. Corrections for the thermal inertia and for the end losses of the wire are applied during the post-processing. The correction for the thermal inertia of the cold wire is achieved by applying a time dependent thermal lag as proposed originally for a constant-current anemometry (CCA) system. This thermal lag is measured in parallel by a hot wire. The thermal end losses of the wires to their supports are also considered and approximate corrections are proposed. The procedure for the cold wire CVA is validated in the acoustic field of an acoustic resonator with wires of different lengths. A comparison between a CVA and a CCA measurement also confirms the CVA measurement. Furthermore, the proposed measurement procedure is applied close to the stack of a thermoacoustic refrigerator. Supposing a two-dimensional flow, the simultaneous measurement of velocity and temperature fluctuations is possible. This allows a detailed examination of the acoustic field close to the stack, including the study of the correlation between temperature and velocity.

Dynamics of nonspherical microbubble oscillations above instability threshold.

Time-resolved dynamics of nonspherical oscillations of micrometer-sized bubbles are captured and analyzed using high-speed imaging. The axisymmetry of the bubble shape is ensured with certainty for the first time from the recordings of two synchronous high-speed cameras located at 90^{∘}. The temporal dynamics of finite-amplitude nonspherical oscillations are then analyzed for various acoustic pressures above the instability threshold. The experimental results are compared with recent theories accounting for nonlinearities and mode coupling, highlighting particular effects inherent to these mechanisms (saturation of the instability, triggering of nonparametric shape modes). Finally, the amplitude of the nonspherical oscillations is given as function of the driving pressure both for quadrupolar and octupolar bubbles.