Outer Acoustic Streaming Flow Driven by Asymmetric Acoustic Resonances

While boundary-driven acoustic streaming resulting from the interaction of sound, fluids and walls in symmetric acoustic resonances have been intensively studied in the literature, the acoustic streaming fields driven by asymmetric acoustic resonances remain largely unexplored. Here, we present a theoretical and numerical analysis of outer acoustic streaming flows generated over a fluid–solid interface above which a symmetric or asymmetric acoustic standing wave is established. The asymmetric standing wave is defined by a shift of acoustic pressure in its magnitude, i.e., S0, and the resulting outer acoustic streaming is analyzed using the limiting velocity method. We show that, in symmetric acoustic resonances (S0 = 0), on a slip-velocity boundary, the limiting velocities always drive fluids from the acoustic pressure node towards adjacent antinodes. In confined geometry where a slip-velocity condition is applied to two parallel walls, the characteristics of the obtained outer acoustic streaming replicates that of Rayleigh streaming. In an asymmetric standing wave where S0 ≠ 0, however, it is found that the resulting limiting velocity node (i.e., the dividing point of limiting velocities) on the slip-velocity boundary locates at a different position to acoustic pressure node and, more importantly, is shown to be independent of S0, enabling spatial separation of acoustic radiation force and acoustic streaming flows. The results show the richness of boundary-driven acoustic streaming pattern variations that arise in standing wave fields and have potentials in many microfluidics applications such as acoustic streaming flow control and particle manipulation.

Visualization of Three-Dimensional Acoustic Streaming Flow Patterns around an Inclined Triangular Obstruction using Digital In-Line Holographic Micro-Particle Tracking Velocimetry

Acoustic Streaming is a flow phenomenon with many applications in the field of microfluidics, such as micro mixing[1, 2] and particle manipulation[3]. With the manufacturing techniques evolves, more complicated geometries can be designed for microfluidic device and 3-D acoustic streaming patterns may occurs. In this study, 3-D trajectories of particle induced by acoustic streaming around an inclined triangular obstruction in a microchannel were visualized by a volumetric tracking method using Digital Inline Holographic Microscopy (DIHM)[4-6]. The triangular obstruction has a tip angle of 20° and an inclined angle of 30°. The acoustic streaming is created under 12 kHz oscillation of a piezo plate driven by 20V voltage. Illuminated by a 450nm continuous laser, the magnified hologram of the motion of 1.79μm tracer particles was recorded by a low-cost 10X industrial microscope with a machine vision camera of 10 fps (frames per second). Using RayleighSommerfeld back-propagation method[7], particle locations was reconstructed frame by frame and 3-D tracking of individual particles was performed afterwards. The trajectories of each particle were reconstructed to reveal the vortical structure of the acoustic streaming flow. For the current system setup, the measurable range was estimated to be 550×685×840 μm3. The 3-D location reconstruction accuracy was verified with a calibration target and the location sensitivity was found to be linear throughout the measurable range. Reconstruction at different depth locations show that the dick-shaped calibration dots and the spherical polystyrene particles have different intensity profiles. The calibration dots show local minimum of intensity at the correct depth location, while polystyrene particles show local maximum of intensity instead. Resolved particle trajectories show that the acoustic streaming flows cause particles to move with 3-D spiral shaped motions near the side of the triangular obstruction, while particles away from the obstruction shows planar motions.

Comparison of Acoustic Streaming Flow Patterns Induced by Solid, Liquid and Gas Obstructions

In this study, acoustic streaming flows inside micro-channels induced by three different types of obstruction—gaseous bubble, liquid droplet and solid bulge—are compared and investigated experimentally by particle tracking velocimetry (PTV) and numerically using the finite element method (FEM). The micro-channels are made by poly(dimethylsiloxane) (PDMS) using soft lithography with low-cost micro-machined mold. The characteristic dimensions of the media are 0.2 mm in diameter, and the oscillation generated by piezoelectric actuators has frequency of 12 kHz and input voltages of 40 V. The experimental results show that in all three obstruction types, a pair of counter-rotating vortical patterns were observed around the semi-circular obstructions. The gaseous bubble creates the strongest vortical streaming flow, which can reach a maximum of 21 mm/s, and the largest u component happens at Y/D = 0. The solid case is the weakest of the three, which can only reach 2 mm/s. The liquid droplet has the largest v components and speed at Y/D = 0.5 and Y/D = 0.6. Because of the higher density and incompressibility of liquid droplet compared to the gaseous bubble, the liquid droplet obstruction transfers the oscillation of the piezo plate most efficiently, and the induced streaming flow region and average speed are both the largest of the three. An investigation using numerical simulation shows that the differing interfacial conditions between the varying types of obstruction boundaries to the fluid may be the key factor to these differences. These results suggest that it might be more energy-efficient to design an acoustofluidic device using a liquid droplet obstruction to induce the stronger streaming flow.

Acoustic Streaming Generated by Sharp Edges: The Coupled Influences of Liquid Viscosity and Acoustic Frequency

Acoustic streaming can be generated around sharp structures, even when the acoustic wavelength is much larger than the vessel size. This sharp-edge streaming can be relatively intense, owing to the strongly focused inertial effect experienced by the acoustic flow near the tip. We conducted experiments with particle image velocimetry to quantify this streaming flow through the influence of liquid viscosity ν , from 1 mm 2 /s to 30 mm 2 /s, and acoustic frequency f from 500 Hz to 3500 Hz. Both quantities supposedly influence the thickness of the viscous boundary layer δ = ν π f 1 / 2 . For all situations, the streaming flow appears as a main central jet from the tip, generating two lateral vortices beside the tip and outside the boundary layer. As a characteristic streaming velocity, the maximal velocity is located at a distance of δ from the tip, and it increases as the square of the acoustic velocity. We then provide empirical scaling laws to quantify the influence of ν and f on the streaming velocity. Globally, the streaming velocity is dramatically weakened by a higher viscosity, whereas the flow pattern and the disturbance distance remain similar regardless of viscosity. Besides viscosity, the frequency also strongly influences the maximal streaming velocity.

Streaming flow on polythermal mountain glaciers: In-situ observations on Jarvis Glacier, Alaska

<p>Accelerated melting of glaciers and ice caps has raised serious concerns about sea level rise. As we work towards a solution to address these concerns, it has become a chief priority to rapidly improve predictions of future changes in global ice mass balance. Numerical simulations projecting ice loss have uncovered a strong sensitivity to mechanical and/or rheological weakening of the shear margins of streaming ice. To accurately project sea level rise, future models will require careful treatment of shear margins. This necessitates a deeper understanding of the flow dynamics at shear margins and how streaming flow relates to the constitutive flow law for ice.</p><p> </p><p>We developed an open source inexpensive tilt sensor (∼20% the cost of commercial sensors) for studying ice deformation and installed our tilt sensor systems in two boreholes drilled close to the shear margin of Jarvis Glacier, Alaska to obtain kinematic measurements of streaming ice. We used the collected tilt data to calculate borehole deformation by tracking the orientation of the sensors over time. The sensors' tilts generally trended down-glacier, with an element of cross-glacier flow in the borehole closer to the shear margin. We also evaluated our results against flow dynamic parameters derived from Glen's exponential flow law and explored the parameter space of the stress exponent <em>n</em> and enhancement factor <em>E</em>. Comparison with values from ice deformation experiments shows that the ice on Jarvis is characterized by higher <em>n</em> values than that is expected in regions of low stress, particularly at the shear margin (~3.4). The higher <em>n</em> values could be attributed to the observed high total strains coupled with potential dynamic recrystallization, causing anisotropic development and consequently sped up ice flow. Jarvis' <em>n</em> values place the creep regime of the ice between basal slip and dislocation creep. Tuning <em>E</em> towards a theoretical upper limit of 10 for anisotropic ice with single-maximum fabric reduces the <em>n</em> values by 0.2.</p>

Advances in Micromanipulation Actuated by Vibration-Induced Acoustic Waves and Streaming Flow

The use of vibration and acoustic characteristics for micromanipulation has been prevalent in recent years. Due to high biocompatibility, non-contact operation, and relatively low cost, the micromanipulation actuated by the vibration-induced acoustic wave and streaming flow has been widely applied in the sorting, translating, rotating, and trapping of targets at the submicron and micron scales, especially particles and single cells. In this review, to facilitate subsequent research, we summarize the fundamental theories of manipulation driven by vibration-induced acoustic waves and streaming flow. These methods are divided into two types: actuated by the acoustic wave, and actuated by the steaming flow induced by vibrating geometric structures. Recently proposed representative vibroacoustic-driven micromanipulation methods are introduced and compared, and their advantages and disadvantages are summarized. Finally, prospects are presented based on our review of the recent advances and developing trends.