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Acoustic streaming is the steady flow of a fluid that is caused by the propagation of sound through that fluid. The fluid flow in acoustic streaming is generated by a nonlinear, time-averaged effect that results from the spatial and temporal variations in a pressure field. When there is an oscillating body submerged in the fluid, such as a cavitation bubble, vorticity is generated on the boundary layer on its surface, resulting in microstreaming. Although the effects are generated at the microscale, microstreaming can have a profound influence on the fluid mechanics of ultrasound/acoustic processing systems, which are of high interest to sonochemistry, sonoprocessing, and acoustophoretic applications. The effects of microstreaming have been evaluated over the years using carefully controlled experiments that identify and quantify the fluid motion at a small scale. This mini-review article overviews the historical development of acoustic streaming, shows how microstreaming behaves, and provides an update on new numerical and experimental studies that seek to explore and improve our understanding of microstreaming.

• First cavitation-microstreaming-facilitated microfluidic chemical cell lysis and microbead-based DNA extraction. • Enhanced assay performance by agitation from bubbles oscillating at resonance frequency determined using the EMIS method. • Excellent performance of 77 % extraction efficiency and 1.85 purity on a par with that of a commercial kit. • Successful DNA extraction from as scarce as 18 cells and numbers of cells spanning 5 orders of magnitude. • Precise, rapid polycarbonate-chip fabrication (25 min) using optimized laser machining and solvent-assisted thermal bonding. We for the first time present a microfluidic cavitation-microstreaming-based cell lysis and DNA extraction method. Chemical lysis and DNA extraction have been demonstrated in a microfluidic format but the performance is limited by ineffective mass transport due to low Reynolds number. Here we propose to employ cavitation microstreaming for enhancing chemical lysis and magnetic-bead-based dynamic solid phase extraction (dSPE) of DNA. Cavitation microstreaming condition is optimized by exciting a microfluidic chip at its flexural resonance frequency (f r) measured via electrical impedance spectroscopy. Strong circulatory flows around bubbles excited at f r yields vigorous agitation, allowing fast lysis, and DNA extraction and purification. The microfluidic device is rapidly fabricated using CO 2 -laser machining and solvent-assisted thermal bonding of polycarbonate (∼25 min). Laser cutting conditions are experimentally determined to achieve a clean sidewall for negligible nonspecific binding and minimal burrs for unobstructed bonding. Solvent exposure and thermal bonding conditions are also experimentally determined for a leakage-free device with excellent dimensional integrity. Our method, although not fully optimized, exhibits an excellent DNA extraction performance, compared to a commercial kit and previous microfluidic methods. High extraction efficiency (76.9 %) and purity (A260/A280 = 1.85) are achieved for a relatively short assay time (∼25 min). Notably, DNA from as few as 18 cells is successfully extracted even from a highly diluted cell sample (0.18 cells/μl). PCR and electrophoresis results confirm the excellent quality of the extracted DNA. Considering these notable performances, and straightforward fabrication and operation, we anticipate our DNA extraction method will be widely used in microfluidic nucleic-acid analysis devices. [ABSTRACT FROM AUTHOR]

[Display omitted] • First EMIS-based rapid determination (∼4.5 min) of excitation frequency of cavitation-microstreaming micromixers. • Rational micromixer design for a longer lifetime and better mixing performance. • Optimization of adhesive-tape-assisted laser machining for clean, high-resolution, precise fabrication of micromixer chips. • Streaming patterns from oscillating bubbles generated using custom high-speed imaging setup. • Best mixing performance of the micromixers excited at the EMIS frequency, compared with those obtained at other frequencies. We for the first time propose and validate an electromechanical impedance spectroscopy (EMIS) technique to determine the resonance frequency (f r) of micromixers for effective cavitation microstreaming. Theoretical f r predictions of the oscillating bubble have been inaccurate because it was assumed that the bubble had a spherical shape even though bubbles are commonly trapped in low-profile air pockets, and thus their shapes are not spherical. Empirical excitation-frequency search has been employed but it was time-consuming and could overlook the exact value of f r. Strong electromechanical coupling between a piezoelectric transducer and a microfluidic chip (i.e., a mechanical structure) allows straightforward, rapid f r determination (∼4.5 min) using EMIS. To validate the EMIS method, we compare microstreaming patterns generated by bubbles excited at four different resonance frequencies using a high-speed imaging setup: 1) f r,B , the theoretical f r assuming a spherical bubble, 2) f r,Beq , the theoretical f r based on the concept of an "equivalent spherical bubble" having a surface area equal to the vibrating air-water interface area, 3) f r,Pf , the EMIS-based f r of a piezoelectric transducer, and 4) f r,Cf , the EMIS-based f r of the microfluidic chip bonded with the transducer. After confirming that f r,Cf yields the strongest cavitation microstreaming, the air-pocket shape is designed with consideration of the bubble stability and mixing effectiveness. A micromixer chip with the designed air pockets is excited at f r,Cf and exhibits rapid homogenization (37.2 s) of diluted ink and deionized water in the mixing chamber of a substantial volume (61.9 μL). Additionally, an experimentally validated, adhesive-tape-supported laser microfabrication technique allows rapid prototyping (∼10 min) of polymethyl methacrylate microfluidic chips with an excellent dimensional accuracy, machining resolution, and surface smoothness. We expect the proposed EMIS-based technique for resonance-frequency determination and the simple, rapid laser microfabrication method to be widely employed for bio-/chemical microfluidic applications in the near future. [ABSTRACT FROM AUTHOR]