Cavitation microstreaming

Liu, R. H., Lenigk, R., Druyor-Sanchez, R. L., Yang, J., Grodzinski, P., Hybridization enhancement using cavitation microstreaming, Anal. Chem. 2003, 75, 1911-1917. 

Liu RH, Lenigk R, Druyor-Sanchez RL, Yang J, Grodzinski P (2003) Hybridization enhancement using cavitation microstreaming. Anal Chem 75 1911-1917... 

Air bubbles can be introduced in a microfluidic mixer in order to enhance the mixing process. The surface of an air bubble in a liquid medium exposed to a sound field can act as a vibrating membrane. The membrane vibration causes a bulk fluid movement at the air-hquid interface. This effect, known as cavitation microstreaming, has been applied in microfluidic micromixers using a single bubble [181] (Fig. 19) or an array of bubbles [178, 179]. 

It has been generally observed that the mechanical effects due to cavitational events are more responsible for the microbial disinfection and the chemical and heat effects play only a supporting role [56]. Microstreaming resulting from stable cavitation has been shown to produce stresses, sufficient to disrupt cell membranes... 

On the other hand stable cavitation (bubbles that oscillate in a regular fashion for many acoustic cycles) induce microstreaming in the surrounding liquid which can also induce stress in any microbiological species present [5]. This type of cavitation may well be important in a range of applications of ultrasound to biotechnology [6]. An important consequence of the fluid micro-convection induced by bubble collapse is a sharp increase in the mass transfer at liquid-solid interfaces. In microbiology there are two zones where this ultrasonic enhancement of mass transfer will be important. The first is at the membrane and/or cellular wall and the second is in the cytosol i. e. the liquid present inside the cell. 

The effects of US on surviving cells may include structural changes and interactions with deoxyribonucleic acid (DNA) [83]. The biological effects observed in vitro include fragmentation of cell membranes caused by the collapse of cavitation bubbles, microstreaming near the boundary layer and formation of radicals, which promote chemical reactions leading to wall decomposition [84]. Carstensen et al. [85] found the extent of cell disruption to be inversely proportional to the cell concentration. 

An ultrasonic technique was applied to AGMD to enhance the permeability for the concentration of various aqueous solutions. In this study, an ultrasonic stimulation of resonance frequency of 20 kHz and power up to 90 W was applied to a flat-plate AGMD system of 1 p,m PTFE membrane with a temperamre difference of 55°C. A 200% improvement in the transmembrane flux was observed with an ultrasonic intensity of 5 W cm . Experimental investigation of the basic mechanism of ultrasonic enhancement of AGMD suggests that, for a continuous ultrasonic stimulation the dominating mechanisms are microstreaming and cavitation [70,130]. 

Fig. 20 Schematic diagram showing the lelatitmship between cavitational forces and cell siffface strength. Rigid structures are broken by bubble collapse, with little or no microstreaming effect. The a -axis strength factor, no numerical values are givtai) provides an indication on the shear forces required to dismpt mammalian or microbial cells. Reproduced with permission from...

The asymmetric collapse of imploding cavitational bubbles near the electrodesolution interface promotes turbulence and leads to microstreaming at the electrode surface and thereby to enhanced limiting currents. Thus ultrasonic irradiation tremendously decreases the depletion of electroactive species near the electrode. Typical values of the diffusion layer thickness under sonication are around 5 pm, compared to normal values of approximately 500 pm. Because the limiting current is given by... 

Many heterogeneous reactions are accelerated by the enhanced micromixing properties of cavitating sound fields. Oscillating and transient bubbles create intense microstreaming in the vicinity of suspended solids. Macromixing is induced by acoustic streaming and the oscillation of bubbles in the sound field. In most cases, a locally different mass-transport coefficient is observed. A tenfold increase in mass-transfer coefficients compared with silent reactions was measured [18]. 

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