Microstreaming plays an important role in various applications, ranging from sonochemistry and sonoprocessing in large systems to acoustophoresis in microfluidic lab-on-a-chip systems [18
]. In the following section, we overview some recent experimental developments in cavitation microstreaming, microstreaming that occurs specifically in microfluidics, and microstreaming in the context of particle separation and manipulation.
2.1. Cavitation Microstreaming
Cavitation microstreaming is the streaming induced by a bubble undergoing oscillations due to the influence of an acoustic field. This type of streaming plays an important role in both microfluidic-based applications and sonochemistry-type applications, where the production and collapse of bubbles is an important driving mechanism for efficient performance. This is because the streaming created may influence the growth of bubbles in an acoustic field (i.e., via rectified diffusion) and/or influence the location of nearby bubbles within the sound field itself.
Tho et al. [28
] used micro-PIV (particle image velocimetry) measurements and streak photography to study the flow field around both single and two oscillating bubbles that were resting on a solid boundary. They investigated several different modes of oscillation and, interestingly, found that the mode of oscillation varied primarily with the applied acoustic frequency. Translating modes were also observed to occur in a sequential order, changing from a translation along a single axis to an elliptical orbit and finally to a circular orbit. In regards to the streaming patterns, patterns ranging from symmetrical flow structures containing four vortices and circular vortexes centered on a bubble were observed.
Leong et al. [29
] used the techniques established by Tho et al. [28
] to relate the streaming velocities around cavitation bubbles to the enhancement in bubble growth rate within an acoustic field by a process known as rectified diffusion [30
]. As described by Church [31
] and Gould [32
], microstreaming around an oscillating bubble enhances the mass transfer effects and hence bubble growth rate. In the presence of different types of aqueous surfactant solutions, the authors [29
] found that different types of surfactants offered different magnitudes of streaming velocity enhancement depending on the electrostatics, head group size, and chain length. One interesting observation was the effect of surface oscillations, which were promoted by the presence of surfactant molecules. Surface oscillations resulted in a more chaotic type flow and produced streaming velocities that were orders of magnitude faster (see Figure 2
) than in the absence of surface oscillations.
Using microscopic observations, Marmottant et al. [33
] were able to capture details of the bubble motion during an ultrasound cycle. Fast frame recordings of a tracer particle embedded in the liquid around the particle enabled full resolution of the acoustic streaming flow induced by the bubble oscillation. When attached to a wall, the bubble is found to provide high efficiency as a “liquid pump” that drives and propels liquid with a characteristic velocity proportional to the square of the vibration amplitude. Interestingly, the viscosity of the fluid provides an important role in triggering a larger phase shift between the oscillation and translation, which is in contrast to more conventional consideration of streaming flows where velocities are assumed to be independent of viscosity.
As noted in some of the above studies, the confinement of cavitation bubbles, either attached to or between walls, enables more effective study of the microstreaming. Mekki-Berrada [34
] was able to analyse the microstreaming flow generated around either an isolated or a pair of interacting bubbles that were confined between two walls of a silicone microchannel that were anchored on micropits. Whilst isolated bubbles induce short-range microstreaming in the channel gap, a pair of bubbles were found to produce long-range microstreaming and large recirculatory motion that can be elegantly described as a butterfly-like shape (Figure 3
). By adjusting the distance between these bubbles, different streaming shapes could be observed.
High-intensity focused ultrasound (HIFU), whereby a sound field is established such that there is the formation of an intense cavitation focal point, is commonly used in sonochemistry applications. One interesting phenomenon observed in studies is that the sonochemiluminescence at the focal point of HIFU devices is actually lower than expected. Uemura et al. [35
] studied the influence of acoustic streaming on the generation of acoustic cavitation by analysing the flow in the sound field using PIV. Interestingly, it was found that acoustic cavitation bubbles in the focal area of a HIFU field become carried away by acoustic streaming as soon as they were generated in the focal area. This is the key reason as to why the sonochemiluminscence intensity is diminished in HIFU systems at the focal point. PIV can also be used to characterise acoustic streaming in focused fields, and Slama [36
] evaluated the effects of using different seeding particle sizes (5, 20, and 50 µm) to observe the behaviour. Larger particles are dominated by radiation forces, and streaming effects are not effectively characterised. Contrarily, smaller particles produce velocity measurements consistent with Computational Fluid Dynamics (CFD) simulations.
One of the most widespread and important uses of microstreaming currently in industry today is for the semiconductor industry. Particulate contaminants are deposited on silicon wafer surfaces by cleanroom personnel or equipment during production. These particles can cause critical defects if not removed. One of the main mechanisms for particle removal is acoustic streaming and microstreaming. However, despite this wide use of megasonics in the semiconductor industry, the physics of megasonic particle removal remains largely unexplained [37
]. Microstreaming in particular is of interest, as it is extremely powerful and generates strong localised currents that aid cleaning efficiency. The currents are most pronounced near bubbles that undergo volume resonance and/or are located along solid boundaries. The acoustic streaming patterns in sonic cleaning baths can be visualised using Ar-ion laser sheets directed into a tank to illuminate the air bubbles in the flow. Particle removal experiments have shown that wafers are cleaned due to both a combination of stable cavitation events (e.g., shock wave) and associated cavitation microstreaming motion that aid the detachment of contaminant particles from the wafer. Bulk acoustic streaming, in addition to microstreaming, provides an efficient transfer mechanism of detached particles away from the wafer surface through the creation of strong currents and boundary layer thinning.
Microstreaming in the context of large-scale systems is rather difficult to study due to the random and chaotic nature of the flow and interference caused by the presence of multiple bubbles. Instead, most studies to date are considered within a microfluidic regime, which is discussed in the next subsection.